15.17: Biological Evidence - Biology

15.17: Biological Evidence - Biology

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The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time. The presence of members of the plant family Proteaceae in Australia, southern Africa, and South America is best due to their appearance prior to the southern supercontinent Gondwana breaking up.

The great diversification of marsupials in Australia and the absence of other mammals reflect Australia’s long isolation. Australia has an abundance of endemic species—species found nowhere else—which is typical of islands whose isolation by expanses of water prevents species migration. Over time, these species diverge evolutionarily into new species that look very different from their ancestors that may exist on the mainland. The marsupials of Australia, the finches on the Galápagos, and many species on the Hawaiian Islands are all unique to their one point of origin, yet they display distant relationships to ancestral species on mainlands.

Molecular Biology

Like anatomical structures, the structures of the molecules of life reflect descent with modification. Evidence of a common ancestor for all of life is reflected in the universality of DNA as the genetic material and in the near universality of the genetic code and the machinery of DNA replication and expression. Fundamental divisions in life between the three domains are reflected in major structural differences in otherwise conservative structures such as the components of ribosomes and the structures of membranes. In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences—exactly the pattern that would be expected from descent and diversification from a common ancestor.

DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events that allow the free modification of one copy by mutation, selection, or drift (changes in a population’s gene pool resulting from chance), while the other copy continues to produce a functional protein.

Learning Objectives

Biogeography offers further clues about evolutionary relationships. The presence of related organisms across continents indicates when these organisms may have evolved. For example, some flora and fauna of the northern continents are similar across these landmasses but distinct from that of the southern continents. Islands such as Australia and the Galapagos chain often have unique species that evolved after these landmasses separated from the mainland. Finally, molecular biology provides data supporting the theory of evolution. In particular, the universality of DNA and near universality of the genetic code for proteins shows that all life once shared a common ancestor. DNA also provides clues into how evolution may have happened. Gene duplications allow one copy to undergo mutational events without harming an organism, as one copy continues to produce functional protein.

Evolution—It’s a Thing

This video defines evolution and discusses several varieties of evidence that support the Theory of Evolution:

A YouTube element has been excluded from this version of the text. You can view it online here:

MAP Biology

I’m making this site to argue against the modern pathologization of the male attraction and preference for young adolescent girls about 12-16 years old (teen schoolgirls, Lolitas, jailbait etc). I like to call this preference adolescentophilia. Due to the taboos over minor attraction many sexologists today aren’t objective about this subject and have been too eager to interpret data as evidence that adolescentophilia is biological maladaptive and abnormal. I aim to convince the reader that it’s biologically normal and evolutionary adaptive for men to find girls on the verge of reproductive age highly attractive.

Anticipated questions and comments

-I notice you’re not using the hebephilia (approx 11-14) and ephebophilia (approx 15-17) age categories. Why is this?

Because they’re stupid and artificial. I prefer to use the category “adolescent” meaning girls about 12-16. I think this is a more natural category and is close to how the word “adolescent” is used in primatology: females between the onset of puberty and their first birth.

-You’re making this site because you’re a predator. You want to lower the age of consent to 12 so you can molest little girls, don’t you?

Not really. I want to avoid any social and ethical issues about what the age of consent should be etc. Although there may be certain reasons for it to be illegal for men to have sex with young adolescent girls I believe it’s perfectly normal for men to find them highly attractive. What I’m arguing for is the acceptance of the normality of the male attraction to young adolescent girls not the lowering of the age of consent. Better understanding of male sexuality will in fact help us to prevent sexual abuse and rape.

-But you said you’re going to argue it’s adaptive for men to prefer adolescent girls. That must mean you think it should be legal to have sex with them.

No. Arguing that something is evolutionarily adaptive is not the same as arguing that it’s moral or should be legal. If you can’t separate those two things then maybe you shouldn’t read any further and should try reading up on the naturalistic and moralistic fallacies. If you can then please carry on.

-Who’s that girl in the header?

It’s 13 year old Jaroslava Schallerová from the film “Valerie and Her Week of Wonders”. It’s a film made in the 60s when it was still acceptable to appreciate the beauty of adolescent girls.

The Biological Basis of Morality

Do we invent our moral absolutes in order to make society workable? Or are these enduring principles expressed to us by some transcendent or Godlike authority? Efforts to resolve this conundrum have perplexed, sometimes inflamed, our best minds for centuries, but the natural sciences are telling us more and more about the choices we make and our reasons for making them

CENTURIES of debate on the origin of ethics come down to this: Either ethical principles, such as justice and human rights, are independent of human experience, or they are human inventions. The distinction is more than an exercise for academic philosophers. The choice between these two understandings makes all the difference in the way we view ourselves as a species. It measures the authority of religion, and it determines the conduct of moral reasoning.

The two assumptions in competition are like islands in a sea of chaos, as different as life and death, matter and the void. One cannot learn which is correct by pure logic the answer will eventually be reached through an accumulation of objective evidence. Moral reasoning, I believe, is at every level intrinsically consilient with -- compatible with, intertwined with -- the natural sciences. (I use a form of the word "consilience" -- literally a "jumping together" of knowledge as a result of the linking of facts and fact-based theory across disciplines to create a common groundwork of explanation -- because its rarity has preserved its precision.)

Every thoughtful person has an opinion on which premise is correct. But the split is not, as popularly supposed, between religious believers and secularists. It is between transcendentalists, who think that moral guidelines exist outside the human mind, and empiricists, who think them contrivances of the mind. In simplest terms, the options are as follows: I believe in the independence of moral values, whether from God or not, and I believe that moral values come from human beings alone, whether or not God exists.

Theologians and philosophers have almost always focused on transcendentalism as the means to validate ethics. They seek the grail of natural law, which comprises freestanding principles of moral conduct immune to doubt and compromise. Christian theologians, following Saint Thomas Aquinas's reasoning in Summa Theologiae, by and large consider natural law to be an expression of God's will. In this view, human beings have an obligation to discover the law by diligent reasoning and to weave it into the routine of their daily lives. Secular philosophers of a transcendental bent may seem to be radically different from theologians, but they are actually quite similar, at least in moral reasoning. They tend to view natural law as a set of principles so powerful, whatever their origin, as to be self-evident to any rational person. In short, transcendental views are fundamentally the same whether God is invoked or not.

For example, when Thomas Jefferson, following John Locke, derived the doctrine of natural rights from natural law, he was more concerned with the power of transcendental statements than with their origin, divine or secular. In the Declaration of Independence he blended secular and religious presumptions in one transcendentalist sentence, thus deftly covering all bets: "We hold these Truths to be self-evident, that all Men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the Pursuit of Happiness." That assertion became the cardinal premise of America's civil religion, the righteous sword wielded by Abraham Lincoln and Martin Luther King Jr., and it endures as the central ethic binding together the diverse peoples of the United States.

So compelling are such fruits of natural-law theory, especially when the Deity is also invoked, that they may seem to place the transcendentalist assumption beyond question. But to its noble successes must be added appalling failures. It has been perverted many times in the past -- used, for example, to argue passionately for colonial conquest, slavery, and genocide. Nor was any great war ever fought without each side thinking its cause transcendentally sacred in some manner or other.

So perhaps we need to take empiricism more seriously. In the empiricist view, ethics is conduct favored consistently enough throughout a society to be expressed as a code of principles. It reaches its precise form in each culture according to historical circumstance. The codes, whether adjudged good or evil by outsiders, play an important role in determining which cultures flourish and which decline.

The crux of the empiricist view is its emphasis on objective knowledge. Because the success of an ethical code depends on how wisely it interprets moral sentiments, those who frame one should know how the brain works, and how the mind develops. The success of ethics also depends on how accurately a society can predict the consequences of particular actions as opposed to others, especially in cases of moral ambiguity.

The empiricist argument holds that if we explore the biological roots of moral behavior, and explain their material origins and biases, we should be able to fashion a wise and enduring ethical consensus. The current expansion of scientific inquiry into the deeper processes of human thought makes this venture feasible.

The choice between transcendentalism and empiricism will be the coming century's version of the struggle for men's souls. Moral reasoning will either remain centered in idioms of theology and philosophy, where it is now, or shift toward science-based material analysis. Where it settles will depend on which world view is proved correct, or at least which is more widely perceived to be correct.

Ethicists, scholars who specialize in moral reasoning, tend not to declare themselves on the foundations of ethics, or to admit fallibility. Rarely do we see an argument that opens with the simple statement This is my starting point, and it could be wrong. Ethicists instead favor a fretful passage from the particular to the ambiguous, or the reverse -- vagueness into hard cases. I suspect that almost all are transcendentalists at heart, but they rarely say so in simple declarative sentences. One cannot blame them very much explaining the ineffable is difficult.

I am an empiricist. On religion I lean toward deism, but consider its proof largely a problem in astrophysics. The existence of a God who created the universe (as envisioned by deism) is possible, and the question may eventually be settled, perhaps by forms of material evidence not yet imagined. Or the matter may be forever beyond human reach. In contrast, and of far greater importance to humanity, the idea of a biological God, one who directs organic evolution and intervenes in human affairs (as envisioned by theism), is increasingly contravened by biology and the brain sciences.

The same evidence, I believe, favors a purely material origin of ethics, and it meets the criterion of consilience: causal explanations of brain activity and evolution, while imperfect, already cover most facts known about behavior we term "moral." Although this conception is relativistic (in other words, dependent on personal viewpoint), it can, if evolved carefully, lead more directly and safely to stable moral codes than can transcendentalism, which is also, when one thinks about it, ultimately relativistic.

Of course, lest I forget, I may be wrong.

THE argument of the empiricist has roots that go back to Aristotle's Nicomachean Ethics and, in the beginning of the modern era, to David Hume's A Treatise of Human Nature (1739-1740). The first clear evolutionary elaboration of it was by Charles Darwin, in The Descent of Man (1871).

Again, religious transcendentalism is bolstered by secular transcendentalism, to which it is fundamentally similar. Immanuel Kant, judged by history the greatest of secular philosophers, addressed moral reasoning very much as a theologian. Human beings, he argued, are independent moral agents with a wholly free will, capable of obeying or breaking moral law: "There is in man a power of self-determination, independent of any coercion through sensuous impulses." Our minds are subject to a categorical imperative, Kant said, of what our actions ought to be. The imperative is a good in itself alone, apart from all other considerations, and it can be recognized by this rule: "Act only on that maxim you wish will become a universal law." Most important, and transcendental, ought has no place in nature. Nature, Kant said, is a system of cause and effect, whereas moral choice is a matter of free will, absent cause and effect. In making moral choices, in rising above mere instinct, human beings transcend the realm of nature and enter a realm of freedom that belongs exclusively to them as rational creatures.

Now, this formulation has a comforting feel to it, but it makes no sense at all in terms of either material or imaginable entities, which is why Kant, even apart from his tortured prose, is so hard to understand. Sometimes a concept is baffling not because it is profound but because it is wrong. This idea does not accord, we know now, with the evidence of how the brain works.

In Principia Ethica (1903), G. E. Moore, the founder of modern ethical philosophy, essentially agreed with Kant. In his view, moral reasoning cannot dip into psychology and the social sciences in order to locate ethical principles, because those disciplines yield only a causal picture and fail to illuminate the basis of moral justification. So to reach the normative ought by way of the factual is is to commit a basic error of logic, which Moore called the naturalistic fallacy. John Rawls, in A Theory of Justice (1971), once again traveled the transcendental road. He offered the very plausible suggestion that justice be defined as fairness, which is to be accepted as an intrinsic good. It is the imperative we would follow if we had no starting information about our own future status in life. But in making such a suggestion Rawls ventured no thought on where the human brain comes from or how it works. He offered no evidence that justice-as-fairness is consistent with human nature, hence practicable as a blanket premise. Probably it is, but how can we know except by blind trial and error?

Had Kant, Moore, and Rawls known modern biology and experimental psychology, they might well not have reasoned as they did. Yet as this century closes, transcendentalism remains firm in the hearts not just of religious believers but also of countless scholars in the social sciences and the humanities who, like Moore and Rawls, have chosen to insulate their thinking from the natural sciences.

Many philosophers will respond by saying, Ethicists don't need that kind of information. You really can't pass from is to ought. You can't describe a genetic predisposition and suppose that because it is part of human nature, it is somehow transformed into an ethical precept. We must put moral reasoning in a special category, and use transcendental guidelines as required.

No, we do not have to put moral reasoning in a special category and use transcendental premises, because the posing of the naturalistic fallacy is itself a fallacy. For if ought is not is, what is? To translate is into ought makes sense if we attend to the objective meaning of ethical precepts. They are very unlikely to be ethereal messages awaiting revelation, or independent truths vibrating in a nonmaterial dimension of the mind. They are more likely to be products of the brain and the culture. From the consilient perspective of the natural sciences, they are no more than principles of the social contract hardened into rules and dictates -- the behavioral codes that members of a society fervently wish others to follow and are themselves willing to accept for the common good. Precepts are the extreme on a scale of agreements that range from casual assent, to public sentiment, to law, to that part of the canon considered sacred and unalterable. The scale applied to adultery might read as follows:

In transcendental thinking, the chain of causation runs downward from the given ought in religion or natural law through jurisprudence to education and finally to individual choice. The argument from transcendentalism takes the following general form: The order of nature contains supreme principles, either divine or intrinsic, and we will be wise to learn about them and find the means to conform to them. Thus John Rawls opens A Theory of Justice with a proposition he regards as irrevocable: "In a just society the liberties of equal citizenship are taken as settled the rights secured by justice are not subject to political bargaining or to the calculus of social interests." As many critiques have made clear, that premise can lead to unhappy consequences when applied to the real world, including a tightening of social control and a decline in personal initiative. A very different premise, therefore, is suggested by Robert Nozick in Anarchy, State, and Utopia (1974): "Individuals have rights, and there are things no person or group may do to them (without violating their rights). So strong and far-reaching are these rights that they raise the question of what, if anything, the state and its officials may do." Rawls would point us toward egalitarianism regulated by the state, Nozick toward libertarianism in a minimalist state.

The empiricist view, in contrast, searching for an origin of ethical reasoning that can be objectively studied, reverses the chain of causation. The individual is seen as predisposed biologically to make certain choices. Through cultural evolution some of the choices are hardened into precepts, then into laws, and, if the predisposition or coercion is strong enough, into a belief in the command of God or the natural order of the universe. The general empiricist principle takes this form: Strong innate feeling and historical experience cause certain actions to be preferred we have experienced them, and have weighed their consequences, and agree to conform with codes that express them. Let us take an oath upon the codes, invest our personal honor in them, and suffer punishment for their violation. The empiricist view concedes that moral codes are devised to conform to some drives of human nature and to suppress others. Ought is the translation not of human nature but of the public will, which can be made increasingly wise and stable through an understanding of the needs and pitfalls of human nature. The empiricist view recognizes that the strength of commitment can wane as a result of new knowledge and experience, with the result that certain rules may be desacralized, old laws rescinded, and formerly prohibited behavior set free. It also recognizes that for the same reason new moral codes may need to be devised, with the potential of being made sacred in time.

IF the empiricist world view is correct, ought is just shorthand for one kind of factual statement, a word that denotes what society first chose (or was coerced) to do, and then codified. The naturalistic fallacy is thereby reduced to the naturalistic problem. The solution of the problem is not difficult: ought is the product of a material process. The solution points the way to an objective grasp of the origin of ethics.

A few investigators are now embarked on just such a foundational inquiry. Most agree that ethical codes have arisen by evolution through the interplay of biology and culture. In a sense these investigators are reviving the idea of moral sentiments that was developed in the eighteenth century by the British empiricists Francis Hutcheson, David Hume, and Adam Smith.

What have been thought of as moral sentiments are now taken to mean moral instincts (as defined by the modern behavioral sciences), subject to judgment according to their consequences. Such sentiments are thus derived from epigenetic rules -- hereditary biases in mental development, usually conditioned by emotion, that influence concepts and decisions made from them. The primary origin of moral instincts is the dynamic relation between cooperation and defection. The essential ingredient for the molding of the instincts during genetic evolution in any species is intelligence high enough to judge and manipulate the tension generated by the dynamism. That level of intelligence allows the building of complex mental scenarios well into the future. It occurs, so far as is known, only in human beings and perhaps their closest relatives among the higher apes.

A way of envisioning the hypothetical earliest stages of moral evolution is provided by game theory, particularly the solutions to the famous Prisoner's Dilemma. Consider the following typical scenario of the dilemma. Two gang members have been arrested for murder and are being questioned separately. The evidence against them is strong but not irrefutable. The first gang member believes that if he turns state's witness, he will be granted immunity and his partner will be sentenced to life in prison. But he is also aware that his partner has the same option, and that if both of them exercise it, neither will be granted immunity. That is the dilemma. Will the two gang members independently defect, so that both take the hard fall? They will not, because they agreed in advance to remain silent if caught. By doing so, both hope to be convicted on a lesser charge or escape punishment altogether. Criminal gangs have turned this principle of calculation into an ethical precept: Never rat on another member always be a stand-up guy. Honor does exist among thieves. The gang is a society of sorts its code is the same as that of a captive soldier in wartime, obliged to give only name, rank, and serial number.

In one form or another, comparable dilemmas that are solvable by cooperation occur constantly and everywhere in daily life. The payoff is variously money, status, power, sex, access, comfort, or health. Most of these proximate rewards are converted into the universal bottom line of Darwinian genetic fitness: greater longevity and a secure, growing family.

And so it has most likely always been. Imagine a Paleolithic band of five hunters. One considers breaking away from the others to look for an antelope on his own. If successful, he will gain a large quantity of meat and hide -- five times as much as if he stays with the band and they are successful. But he knows from experience that his chances of success are very low, much less than the chances of the band of five working together. In addition, whether successful alone or not, he will suffer animosity from the others for lessening their prospects. By custom the band members remain together and share equitably the animals they kill. So the hunter stays. He also observes good manners in doing so, especially if he is the one who makes the kill. Boastful pride is condemned, because it rips the delicate web of reciprocity.

Now suppose that human propensities to cooperate or defect are heritable: some people are innately more cooperative, others less so. In this respect moral aptitude would simply be like almost all other mental traits studied to date. Among traits with documented heritability, those closest to moral aptitude are empathy with the distress of others and certain processes of attachment between infants and their caregivers. To the heritability of moral aptitude add the abundant evidence of history that cooperative individuals generally survive longer and leave more offspring. Following that reasoning, in the course of evolutionary history genes predisposing people toward cooperative behavior would have come to predominate in the human population as a whole.

Such a process repeated through thousands of generations inevitably gave rise to moral sentiments. With the exception of psychopaths (if any truly exist), every person vividly experiences these instincts variously as conscience, self-respect, remorse, empathy, shame, humility, and moral outrage. They bias cultural evolution toward the conventions that express the universal moral codes of honor, patriotism, altruism, justice, compassion, mercy, and redemption.

The dark side of the inborn propensity to moral behavior is xenophobia. Because personal familiarity and common interest are vital in social transactions, moral sentiments evolved to be selective. People give trust to strangers with effort, and true compassion is a commodity in chronically short supply. Tribes cooperate only through carefully defined treaties and other conventions. They are quick to imagine themselves the victims of conspiracies by competing groups, and they are prone to dehumanize and murder their rivals during periods of severe conflict. They cement their own group loyalties by means of sacred symbols and ceremonies. Their mythologies are filled with epic victories over menacing enemies.

The complementary instincts of morality and tribalism are easily manipulated. Civilization has made them more so. Beginning about 10,000 years ago, a tick in geological time, when the agricultural revolution started in the Middle East, in China, and in Mesoamerica, populations increased tenfold in density over those of hunter-gatherer societies. Families settled on small plots of land, villages proliferated, and labor was finely divided as a growing minority of the populace specialized as craftsmen, traders, and soldiers. The rising agricultural societies became increasingly hierarchical. As chiefdoms and then states thrived on agricultural surpluses, hereditary rulers and priestly castes took power. The old ethical codes were transformed into coercive regulations, always to the advantage of the ruling classes. About this time the idea of law-giving gods originated. Their commands lent the ethical codes overpowering authority -- once again, no surprise, in the interests of the rulers.

Because of the technical difficulty of analyzing such phenomena in an objective manner, and because people resist biological explanations of their higher cortical functions in the first place, very little progress has been made in the biological exploration of the moral sentiments. Even so, it is astonishing that the study of ethics has advanced so little since the nineteenth century. The most distinguishing and vital qualities of the human species remain a blank space on the scientific map. I doubt that discussions of ethics should rest upon the freestanding assumptions of contemporary philosophers who have evidently never given thought to the evolutionary origin and material functioning of the human brain. In no other domain of the humanities is a union with the natural sciences more urgently needed.

When the ethical dimension of human nature is at last fully opened to such exploration, the innate epigenetic rules of moral reasoning will probably not prove to be aggregated into simple instincts such as bonding, cooperativeness, and altruism. Instead the rules will most probably turn out to be an ensemble of many algorithms, whose interlocking activities guide the mind across a landscape of nuanced moods and choices.

Such a prestructured mental world may at first seem too complicated to have been created by autonomous genetic evolution alone. But all the evidence of biology suggests that just this process was enough to spawn the millions of species of life surrounding us. Each kind of animal is furthermore guided through its life cycle by unique and often elaborate sets of instinctual algorithms, many of which are beginning to yield to genetic and neurobiological analyses. With all these examples before us, we may reasonably conclude that human behavior originated the same way.

MEANWHILE, the mélanges of moral reasoning employed by modern societies are, to put the matter simply, a mess. They are chimeras, composed of odd parts stuck together. Paleolithic egalitarian and tribalistic instincts are still firmly installed. As part of the genetic foundation of human nature, they cannot be replaced. In some cases, such as quick hostility to strangers and competing groups, they have become generally ill adapted and persistently dangerous. Above the fundamental instincts rise superstructures of arguments and rules that accommodate the novel institutions created by cultural evolution. These accommodations, which reflect the attempt to maintain order and further tribal interests, have been too volatile to track by genetic evolution they are not yet in the genes.

Little wonder, then, that ethics is the most publicly contested of all philosophical enterprises. Or that political science, which at its foundation is primarily the study of applied ethics, is so frequently problematic. Neither is informed by anything that would be recognizable as authentic theory in the natural sciences. Both ethics and political science lack a foundation of verifiable knowledge of human nature sufficient to produce cause-and-effect predictions and sound judgments based on them. Surely closer attention must be paid to the deep springs of ethical behavior. The greatest void in knowledge for such a venture is the biology of moral sentiments. In time this subject can be understood, I believe, by paying attention to the following topics:

* The definition of moral sentiments, first by precise descriptions from experimental psychology and then by analysis of the underlying neural and endocrine responses.

* The genetics of moral sentiments, most easily approached through measurements of the heritability of the psychological and physiological processes of ethical behavior, and eventually, with difficulty, through identification of the prescribing genes.

* The development of moral sentiments as products of the interactions of genes and the environment. Research is most effective when conducted at two levels: the histories of ethical systems as part of the emergence of different cultures, and the cognitive development of individuals living in a variety of cultures. Such investigations are already well along in anthropology and psychology. In the future they will be augmented by contributions from biology.

* The deep history of moral sentiments -- why they exist in the first place. Presumably they contributed to survival and reproductive success during the long periods of prehistoric time in which they genetically evolved.

From a convergence of these several approaches the true origin and meaning of ethical behavior may come into focus. If so, a more certain measure can then be taken of the strength and flexibility of the epigenetic rules composing the various moral sentiments. From that knowledge it should be possible to adapt ancient moral sentiments more wisely to the swiftly changing conditions of modern life into which, willy-nilly and largely in ignorance, we have plunged.

Then new answers might be found to the truly important questions of moral reasoning. How can the moral instincts be ranked? Which are best subdued and to what degree? Which should be validated by law and symbol? How can precepts be left open to appeal under extraordinary circumstances? In the new understanding can be located the most effective means for reaching consensus. No one can guess the exact form that agreements will take from one culture to the next. The process, however, can be predicted with assurance. It will be democratic, weakening the clash of rival religions and ideologies. History is moving decisively in that direction, and people are by nature too bright and too contentious to abide anything else. And the pace can be confidently predicted: change will come slowly, across generations, because old beliefs die hard, even when they are demonstrably false.

THE same reasoning that aligns ethical philosophy with science can also inform the study of religion. Religions are analogous to organisms. They have a life cycle. They are born, they grow, they compete, they reproduce, and, in the fullness of time, most die. In each of these phases religions reflect the human organisms that nourish them. They express a primary rule of human existence: Whatever is necessary to sustain life is also ultimately biological.

Successful religions typically begin as cults, which then increase in power and inclusiveness until they achieve tolerance outside the circle of believers. At the core of each religion is a creation myth, which explains how the world began and how the chosen people -- those subscribing to the belief system -- arrived at its center. Often a mystery, a set of secret instructions and formulas, is available to members who have worked their way to a higher state of enlightenment. The medieval Jewish cabala, the trigradal system of Freemasonry, and the carvings on Australian aboriginal spirit sticks are examples of such arcana. Power radiates from the center, gathering converts and binding followers to the group. Sacred places are designated, where the gods can be importuned, rites observed, and miracles witnessed.

The devotees of the religion compete as a tribe with those of other religions. They harshly resist the dismissal of their beliefs by rivals. They venerate self-sacrifice in defense of the religion.

The tribalistic roots of religion are similar to those of moral reasoning and may be identical. Religious rites, such as burial ceremonies, are very old. It appears that in the late Paleolithic period in Europe and the Middle East bodies were sometimes placed in shallow graves, accompanied by ocher or blossoms one can easily imagine such ceremonies performed to invoke spirits and gods. But, as theoretical deduction and the evidence suggest, the primitive elements of moral behavior are far older than Paleolithic ritual. Religion arose on a foundation of ethics, and it has probably always been used in one manner or another to justify moral codes.

The formidable influence of the religious drive is based on far more, however, than just the validation of morals. A great subterranean river of the mind, it gathers strength from a broad spread of tributary emotions. Foremost among them is the survival instinct. "Fear," as the Roman poet Lucretius said, "was the first thing on earth to make the gods." Our conscious minds hunger for a permanent existence. If we cannot have everlasting life of the body, then absorption into some immortal whole will serve. Anything will serve, as long as it gives the individual meaning and somehow stretches into eternity that swift passage of the mind and spirit lamented by Saint Augustine as the short day of time.

The understanding and control of life is another source of religious power. Doctrine draws on the same creative springs as science and the arts, its aim being the extraction of order from the mysteries and tumult of the material world. To explain the meaning of life it spins mythic narratives of the tribal history, populating the cosmos with protective spirits and gods. The existence of the supernatural, if accepted, testifies to the existence of that other world so desperately desired.

Religion is also mightily empowered by its principal ally, tribalism. The shamans and priests implore us, in somber cadence, Trust in the sacred rituals, become part of the immortal force, you are one of us. As your life unfolds, each step has mystic significance that we who love you will mark with a solemn rite of passage, the last to be performed when you enter that second world, free of pain and fear.

If the religious mythos did not exist in a culture, it would quickly be invented, and in fact it has been invented everywhere, thousands of times through history. Such inevitability is the mark of instinctual behavior in any species, which is guided toward certain states by emotion-driven rules of mental development. To call religion instinctive is not to suppose that any particular part of its mythos is untrue -- only that its sources run deeper than ordinary habit and are in fact hereditary, urged into existence through biases in mental development that are encoded in the genes.

Such biases are a predictable consequence of the brain's genetic evolution. The logic applies to religious behavior, with the added twist of tribalism. There is a hereditary selective advantage to membership in a powerful group united by devout belief and purpose. Even when individuals subordinate themselves and risk death in a common cause, their genes are more likely to be transmitted to the next generation than are those of competing groups who lack comparable resolve.

The mathematical models of population genetics suggest the following rule in the evolutionary origin of such altruism: If the reduction in survival and reproduction of individuals owing to genes for altruism is more than offset by the increased probability of survival of the group owing to the altruism, then altruism genes will rise in frequency throughout the entire population of competing groups. To put it as concisely as possible: the individual pays, his genes and tribe gain, altruism spreads.

LET me now suggest a still deeper significance of the empiricist theory of the origin of ethics and religion. If empiricism were disproved, and transcendentalism compellingly upheld, the discovery would be quite simply the most consequential in human history. That is the burden laid upon biology as it draws close to the humanities.

The matter is still far from resolved. But empiricism, as I have argued, is well supported thus far in the case of ethics. The objective evidence for or against it in religion is weaker, but at least still consistent with biology. For example, the emotions that accompany religious ecstasy clearly have a neurobiological source. At least one form of brain disorder is associated with hyperreligiosity, in which cosmic significance is given to almost everything, including trivial everyday events. One can imagine the biological construction of a mind with religious beliefs, although that alone would not disprove the logic of transcendentalism, or prove the beliefs themselves to be untrue.

Equally important, much if not all religious behavior could have arisen from evolution by natural selection. The theory fits -- crudely. The behavior includes at least some aspects of belief in gods. Propitiation and sacrifice, which are near-universals of religious practice, are acts of submission to a dominant being. They reflect one kind of dominance hierarchy, which is a general trait of organized mammalian societies. Like human beings, animals use elaborate signals to advertise and maintain their rank in the hierarchy. The details vary among species but also have consistent similarities across the board, as the following two examples will illustrate.

In packs of wolves the dominant animal walks erect and "proud," stiff-legged and deliberate, with head, tail, and ears up, and stares freely and casually at others. In the presence of rivals the dominant animal bristles its pelt while curling its lips to show teeth, and it takes first choice in food and space. A subordinate uses opposite signals. It turns away from the dominant individual while lowering its head, ears, and tail, and it keeps its fur sleek and its teeth covered. It grovels and slinks, and yields food and space when challenged.

In a troop of rhesus monkeys the alpha male is remarkably similar in mannerisms to a dominant wolf. He keeps his head and tail up, and walks in a deliberate, "regal" manner while casually staring at others. He climbs objects to maintain height above his rivals. When challenged he stares hard at the opponent with mouth open -- signaling aggression, not surprise -- and sometimes slaps the ground with open palms to signal his readiness to attack. The male or female subordinate affects a furtive walk, holding its head and tail down, turning away from the alpha and other higher-ranked individuals. It keeps its mouth shut except for a fear grimace, and when challenged makes a cringing retreat. It yields space and food and, in the case of males, estrous females.

My point is this: Behavioral scientists from another planet would notice immediately the parallels between animal dominance behavior on the one hand and human obeisance to religious and civil authority on the other. They would point out that the most elaborate rites of obeisance are directed at the gods, the hyperdominant if invisible members of the human group. And they would conclude, correctly, that in baseline social behavior, not just in anatomy, Homo sapiens has only recently diverged in evolution from a nonhuman primate stock.

Countless studies of animal species, whose instinctive behavior is unobscured by cultural elaboration, have shown that membership in dominance orders pays off in survival and lifetime reproductive success. That is true not just for the dominant individuals but for the subordinates as well. Membership in either class gives animals better protection against enemies and better access to food, shelter, and mates than does solitary existence. Furthermore, subordination in the group is not necessarily permanent. Dominant individuals weaken and die, and as a result some of the underlings advance in rank and appropriate more resources.

Modern human beings are unlikely to have erased the old mammalian genetic programs and devised other means of distributing power. All the evidence suggests that they have not. True to their primate heritage, people are easily seduced by confident, charismatic leaders, especially males. That predisposition is strong in religious organizations. Cults form around such leaders. Their power grows if they can persuasively claim special access to the supremely dominant, typically male figure of God. As cults evolve into religions, the image of the Supreme Being is reinforced by myth and liturgy. In time the authority of the founders and their successors is graven in sacred texts. Unruly subordinates, known as "blasphemers," are squashed.

The symbol-forming human mind, however, never remains satisfied with raw, apish feeling in any emotional realm. It strives to build cultures that are maximally rewarding in every dimension. Ritual and prayer permit religious believers to be in direct touch with the Supreme Being consolation from coreligionists softens otherwise unbearable grief the unexplainable is explained and an oceanic sense of communion with the larger whole is made possible.

Communion is the key, and hope rising from it is eternal out of the dark night of the soul arises the prospect of a spiritual journey to the light. For a special few the journey can be taken in this life. The mind reflects in certain ways in order to reach ever higher levels of enlightenment, until finally, when no further progress is possible, it enters a mystical union with the whole. Within the great religions such enlightenment is expressed by Hindu samadhi, Buddhist Zen satori, Sufi fana, and Pentecostal Christian rebirth. Something like it is also experienced by hallucinating preliterate shamans. What all these celebrants evidently feel (as I felt once, to some degree, as a reborn evangelical) is hard to put in words, but Willa Cather came as close as possible in a single sentence. In My Antonia her fictional narrator says, "That is happiness to be dissolved into something complete and great."

Of course that is happiness -- to find the godhead, or to enter the wholeness of nature, or otherwise to grasp and hold on to something ineffable, beautiful, and eternal. Millions seek it. They feel otherwise lost, adrift in a life without ultimate meaning. They enter established religions, succumb to cults, dabble in New Age nostrums. They push The Celestine Prophecy and other junk attempts at enlightenment onto the best-seller lists.

Perhaps, as I believe, these phenomena can all eventually be explained as functions of brain circuitry and deep genetic history. But this is not a subject that even the most hardened empiricist should presume to trivialize. The idea of mystical union is an authentic part of the human spirit. It has occupied humanity for millennia, and it raises questions of utmost seriousness for transcendentalists and scientists alike. What road, we ask, was traveled, what destination reached, by the mystics of history?

FOR many, the urge to believe in transcendental existence and immortality is overpowering. Transcendentalism, especially when reinforced by religious faith, is psychically full and rich it feels somehow right. By comparison, empiricism seems sterile and inadequate. In the quest for ultimate meaning the transcendentalist route is much easier to follow. That is why, even as empiricism is winning the mind, transcendentalism continues to win the heart. Science has always defeated religious dogma point by point when differences between the two were meticulously assessed. But to no avail. In the United States 16 million people belong to the Southern Baptist denomination, the largest favoring a literal interpretation of the Christian Bible, but the American Humanist Association, the leading organization devoted to secular and deistic humanism, has only 5,000 members.

Still, if history and science have taught us anything, it is that passion and desire are not the same as truth. The human mind evolved to believe in gods. It did not evolve to believe in biology. Acceptance of the supernatural conveyed a great advantage throughout prehistory, when the brain was evolving. Thus it is in sharp contrast to the science of biology, which was developed as a product of the modern age and is not underwritten by genetic algorithms. The uncomfortable truth is that the two beliefs are not factually compatible. As a result, those who hunger for both intellectual and religious truth face disquieting choices.

Meanwhile, theology tries to resolve the dilemma by evolving, sciencelike, toward abstraction. The gods of our ancestors were divine human beings. The Egyptians represented them as Egyptian (often with body parts of Nilotic animals), and the Greeks represented them as Greek. The great contribution of the Hebrews was to combine the entire pantheon into a single person, Yahweh (a patriarch appropriate to desert tribes), and to intellectualize his existence. No graven images were allowed. In the process, they rendered the divine presence less tangible. And so in biblical accounts it came to pass that no one, not even Moses approaching Yahweh in the burning bush, could look upon his face. In time the Jews were prohibited from even pronouncing his true full name. Nevertheless, the idea of a theistic God, omniscient, omnipotent, and closely involved in human affairs, has persisted to this day as the dominant religious image of Western culture.

During the Enlightenment a growing number of liberal Judeo-Christian theologians, wishing to accommodate theism to a more rationalist view of the material world, moved away from God as a literal person. Baruch Spinoza, the pre-eminent Jewish philosopher of the seventeenth century, visualized the deity as a transcendent substance present everywhere in the universe. Deus sive natura, "God or nature," he declared, they are interchangeable. For his philosophical pains he was banished from his synagogue under a comprehensive anathema, combining all the curses in the book. The risk of heresy notwithstanding, the depersonalization of God has continued steadily into the modern era. For Paul Tillich, one of the most influential Protestant theologians of the twentieth century, the assertion of the existence of God-as-person is not false it is just meaningless. Among many of the most liberal contemporary thinkers the denial of a concrete divinity takes the form of "process theology." Everything in this most extreme of ontologies is part of a seamless and endlessly complex web of unfolding relationships. God is manifest in everything.

Scientists, the roving scouts of the empiricist movement, are not immune to the idea of God. Those who favor it often lean toward some form of process theology. They ask this question: When the real world of space, time, and matter is well enough known, will that knowledge reveal the Creator's presence? Their hopes are vested in the theoretical physicists who pursue the final theory, the Theory of Everything, T.O.E., a system of interlocking equations that describe all that can be learned of the forces of the physical universe. T.O.E. is a "beautiful" theory, as Steven Weinberg has called it in his important book Dreams of a Final Theory -- beautiful because it will be elegant, expressing the possibility of unending complexity with minimal laws and symmetrical, because it will hold invariant through all space and time and inevitable, meaning that once it is stated, no part can be changed without invalidating the whole. All surviving subtheories can be fitted into it permanently, in the manner described by Einstein in his own contribution, the General Theory of Relativity. "The chief attraction of the theory," Einstein said, "lies in its logical completeness. If a single one of the conclusions drawn from it proves wrong, it must be given up to modify it without destroying the whole structure seems to be impossible."

The prospect of a final theory by the most mathematical of scientists might seem to signal the approach of a new religious awakening. Stephen Hawking, yielding to the temptation in A Brief History of Time (1988), declared that this scientific achievement "would be the ultimate triumph of human reason -- for then we would know the mind of God."

THE essence of humanity's spiritual dilemma is that we evolved genetically to accept one truth and discovered another. Can we find a way to erase the dilemma, to resolve the contradictions between the transcendentalist and empiricist world views?

Unfortunately, in my view, the answer is no. Furthermore, the choice between the two is unlikely to remain arbitrary forever. The assumptions underlying these world views are being tested with increasing severity by cumulative verifiable knowledge about how the universe works, from atom to brain to galaxy. In addition, the harsh lessons of history have taught us that one code of ethics is not always as good -- or at least not as durable -- as another. The same is true of religions. Some cosmologies are factually less correct than others, and some ethical precepts are less workable.

Human nature is biologically based, and it is relevant to ethics and religion. The evidence shows that because of its influence, people can readily be educated to only a narrow range of ethical precepts. They flourish within certain belief systems and wither in others. We need to know exactly why.

To that end I will be so presumptuous as to suggest how the conflict between the world views will most likely be settled. The idea of a genetic, evolutionary origin of moral and religious beliefs will continue to be tested by biological studies of complex human behavior. To the extent that the sensory and nervous systems appear to have evolved by natural selection, or at least some other purely material process, the empiricist interpretation will be supported. It will be further supported by verification of gene-culture coevolution, the essential process postulated by scientists to underlie human nature by linking changes in genes to changes in culture.

Now consider the alternative. To the extent that ethical and religious phenomena do not appear to have evolved in a manner congenial to biology, and especially to the extent that such complex behavior cannot be linked to physical events in the sensory and nervous systems, the empiricist position will have to be abandoned and a transcendentalist explanation accepted.

For centuries the writ of empiricism has been spreading into the ancient domain of transcendentalist belief, slowly at the start but quickening in the scientific age. The spirits our ancestors knew intimately fled first the rocks and trees and then the distant mountains. Now they are in the stars, where their final extinction is possible. But we cannot live without them. People need a sacred narrative. They must have a sense of larger purpose, in one form or another, however intellectualized. They will refuse to yield to the despair of animal mortality. They will continue to plead, in company with the psalmist, Now Lord, what is my comfort? They will find a way to keep the ancestral spirits alive.

If the sacred narrative cannot be in the form of a religious cosmology, it will be taken from the material history of the universe and the human species. That trend is in no way debasing. The true evolutionary epic, retold as poetry, is as intrinsically ennobling as any religious epic. Material reality discovered by science already possesses more content and grandeur than all religious cosmologies combined. The continuity of the human line has been traced through a period of deep history a thousand times as old as that conceived by the Western religions. Its study has brought new revelations of great moral importance. It has made us realize that Homo sapiens is far more than an assortment of tribes and races. We are a single gene pool from which individuals are drawn in each generation and into which they are dissolved the next generation, forever united as a species by heritage and a common future. Such are the conceptions, based on fact, from which new intimations of immortality can be drawn and a new mythos evolved.

Which world view prevails, religious transcendentalism or scientific empiricism, will make a great difference in the way humanity claims the future. While the matter is under advisement, an accommodation can be reached if the following overriding facts are realized. Ethics and religion are still too complex for present-day science to explain in depth. They are, however, far more a product of autonomous evolution than has hitherto been conceded by most theologians. Science faces in ethics and religion its most interesting and possibly most humbling challenge, while religion must somehow find the way to incorporate the discoveries of science in order to retain credibility. Religion will possess strength to the extent that it codifies and puts into enduring, poetic form the highest values of humanity consistent with empirical knowledge. That is the only way to provide compelling moral leadership. Blind faith, no matter how passionately expressed, will not suffice. Science, for its part, will test relentlessly every assumption about the human condition and in time uncover the bedrock of moral and religious sentiments.

The eventual result of the competition between the two world views, I believe, will be the secularization of the human epic and of religion itself. However the process plays out, it demands open discussion and unwavering intellectual rigor in an atmosphere of mutual respect.


Fetal development and hormones Edit

The influence of hormones on the developing fetus has been the most influential causal hypothesis of the development of sexual orientation. [5] [6] In simple terms, the developing fetal brain begins in a "female" typical state. The presence of the Y-chromosome in males prompts the development of testes, which release testosterone, the primary androgen receptor-activating hormone, to masculinize the fetus and fetal brain. This masculinizing effect pushes males towards male typical brain structures, and most of the time, attraction to females. It has been hypothesized that gay men may have been exposed to little testosterone in key regions of the brain, or had different levels of receptivity to its masculinizing effects, or experienced fluctuations at critical times. In women, it is hypothesized that high levels of exposure to testosterone in key regions may increase likelihood of same sex attraction. [5] Supporting this are studies of the finger digit ratio of the right hand, which is a robust marker of prenatal testosterone exposure. Lesbians on average, have significantly more masculine digit ratios, a finding which has been replicated numerous times in studies cross-culturally. [7] While direct effects are hard to measure for ethical reasons, animal experiments where scientists manipulate exposure to sex hormones during gestation can also induce lifelong male-typical behavior and mounting in female animals, and female-typical behavior in male animals. [5] [7] [6] [8]

Maternal immune responses during fetal development are strongly demonstrated as causing male homosexuality and bisexuality. [9] Research since the 1990s has demonstrated that the more male sons a woman has, there is a higher chance of later born sons being gay. During pregnancy, male cells enter a mother's bloodstream, which are foreign to her immune system. In response, she develops antibodies to neutralize them. These antibodies are then released on future male foetuses and may neutralize Y-linked antigens, which play a role in brain masculinization, leaving areas of the brain responsible for sexual attraction in the female-typical position, or attracted to men. The more sons a mother has will increase the levels of these antibodies, thus creating the observed fraternal birth order effect. Biochemical evidence to support this effect was confirmed in a lab study in 2017, finding that mothers with a gay son, particularly those with older brothers, had heightened levels of antibodies to the NLGN4Y Y-protein than mothers with heterosexual sons. [9] [10] J. Michael Bailey has described maternal immune responses as "causal" of male homosexuality. [11] This effect is estimated to account for between 15 and 29% of gay men, while other gay and bisexual men are thought to owe sexual orientation to genetic and hormonal interactions. [12] [9]

Socialization theories, which were dominant in the 1900s, favored the idea that children were born "undifferentiated" and were socialized into gender roles and sexual orientation. This led to medical experiments in which newborn and infant boys were surgically reassigned into girls after accidents such as botched circumcisions. These males were then reared and raised as females without telling the boys, which, contrary to expectations, did not make them feminine nor attracted to men. All published cases providing sexual orientation grew up to be strongly attracted to women. The failure of these experiments demonstrate that socialization effects does not induce feminine type behavior in males, nor make them attracted to men, and that the organizational effects of hormones on the fetal brain prior to birth have permanent effects. These are indicative of 'nature', not nurture, at least with regards to male sexual orientation. [5]

The sexually dimorphic nucleus of the preoptic area (SDN-POA) is a key region of the brain which differs between males and females in humans and a number of mammals (e.g., sheep/rams, mice, rats), and is caused by sex differences in hormone exposure. [5] [7] The INAH-3 region is bigger in males than in females, and is thought to be a critical region in sexual behavior. Dissection studies found that gay men had significantly smaller sized INAH-3 than heterosexual males, which is shifted in the female typical direction, a finding first demonstrated by neuroscientist Simon LeVay, which has been replicated. [7] Dissection studies are rare, however, due to lack of funding and brain samples. [5]

Long-term studies of domesticated sheep lead by Charles Roselli have found that 6-8% of rams have a homosexual preference through their life. Dissection of ram brains also found a similar smaller (feminized) structure in homosexually oriented rams compared to heterosexually oriented rams in the equivalent brain region to the human SDN, the ovine sexually dimorphic nucleus (oSDN). [13] The size of the sheep oSDN has also been demonstrated to be formed in utero, rather than postnatally, underscoring the role of prenatal hormones in masculinization of the brain for sexual attraction. [8] [5]

Other studies in humans have relied on brain imaging technology, such as research lead by Ivanka Savic which compared hemispheres of the brain. This research found that straight men had right hemispheres 2% larger than the left, described as modest but "highly significant difference" by LeVay. In heterosexual women, the two hemispheres were the same size. In gay men, the two hemispheres were also the same size, or sex atypical, while in lesbians, the right hemispheres were slightly larger than the left, indicating a small shift in the male direction. [14]

A model proposed by evolutionary geneticist William R. Rice argues that a misexpressed epigenetic modifier of testosterone sensitivity or insensitivity that affected development of the brain can explain homosexuality, and can best explain twin discordance. [15] Rice et al. propose that these epimarks normally canalize sexual development, preventing intersex conditions in most of the population, but sometimes failing to erase across generations and causing reversed sexual preference. [15] On grounds of evolutionary plausibility, Gavrilets, Friberg and Rice argue that all mechanisms for exclusive homosexual orientations likely trace back to their epigenetic model. [16] Testing this hypothesis is possible with current stem cell technology. [17]

Genetic influences Edit

Multiple genes have been found to play a role in sexual orientation. Scientists caution that many people misconstrue the meanings of genetic and environmental. [4] Environmental influence does not automatically imply that the social environment influences or contributes to the development of sexual orientation. Hypotheses for the impact of the post-natal social environment on sexual orientation are weak, especially for males. [4] There is, however, a vast non-social environment that is non-genetic yet still biological, such as prenatal development, that likely helps shape sexual orientation. [4] : 76

Twin studies Edit

A number of twin studies have attempted to compare the relative importance of genetics and environment in the determination of sexual orientation. In a 1991 study, Bailey and Pillard conducted a study of male twins recruited from "homophile publications", and found that 52% of monozygotic (MZ) brothers (of whom 59 were questioned) and 22% of the dizygotic (DZ) twins were concordant for homosexuality. [18] 'MZ' indicates identical twins with the same sets of genes and 'DZ' indicates fraternal twins where genes are mixed to an extent similar to that of non-twin siblings. In a study of 61 pairs of twins, researchers found among their mostly male subjects a concordance rate for homosexuality of 66% among monozygotic twins and a 30% one among dizygotic twins. [19] In 2000, Bailey, Dunne and Martin studied a larger sample of 4,901 Australian twins but reported less than half the level of concordance. [20] They found 20% concordance in the male identical or MZ twins and 24% concordance for the female identical or MZ twins. Self reported zygosity, sexual attraction, fantasy and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt. Other researchers support biological causes for both men and women's sexual orientation. [21]

A 2008 study of all adult twins in Sweden (more than 7,600 twins) [22] found that same-sex behaviour was explained by both heritable genetic factors and unique environmental factors (which can include the prenatal environment during gestation, exposure to illness in early life, peer groups not shared with a twin, etc.), although a twin study cannot identify which factor is at play. Influences of the shared environment (influences including the family environment, rearing, shared peer groups, culture and societal views, and sharing the same school and community) had no effect for men, and a weak effect for women. This is consistent with the common finding that parenting and culture appears to play no role in male sexual orientation, but may play some small role in women. The study concludes that genetic influences on any lifetime same-sex partner were stronger for men than women, and that "it has been suggested individual differences in heterosexual and homosexual behavior result from unique environmental factors such as prenatal exposure to sex hormones, progressive maternal immunization to sex-specific proteins, or neurodevelopmental factors", although does not rule out other variables. The use of all adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twins may influence the results:

Biometric modeling revealed that, in men, genetic effects explained .34–.39 of the variance [of sexual orientation], the shared environment .00, and the individual-specific environment .61–.66 of the variance. Corresponding estimates among women were .18–.19 for genetic factors, .16–.17 for shared environmental, and .64–.66 for unique environmental factors. Although wide confidence intervals suggest cautious interpretation, the results are consistent with moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment (social and biological) on same-sex sexual behavior. [22]

Chromosome linkage studies Edit

Chromosome linkage studies of sexual orientation have indicated the presence of multiple contributing genetic factors throughout the genome. In 1993, Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families. [23] Hamer et al. found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. In another finding, thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly dubbed the "gay gene" in the media, causing significant controversy. Sanders et al. in 1998 reported on their similar study, in which they found that 13% of uncles of gay brothers on the maternal side were homosexual, compared with 6% on the paternal side. [24]

A later analysis by Hu et al. replicated and refined the earlier findings. This study revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28. [25] Two other studies (Bailey et al., 1999 McKnight and Malcolm, 2000) failed to find a preponderance of gay relatives in the maternal line of homosexual men. [24] One study by Rice et al. in 1999 failed to replicate the Xq28 linkage results. [26] Meta-analysis of all available linkage data indicates a significant link to Xq28, but also indicates that additional genes must be present to account for the full heritability of sexual orientation. [27]

Mustanski et al. (2005) performed a full-genome scan (instead of just an X chromosome scan) on individuals and families previously reported on in Hamer et al. (1993) and Hu et al. (1995), as well as additional new subjects. In the full sample they did not find linkage to Xq28. [28]

Results from the first large, comprehensive multi-center genetic linkage study of male sexual orientation were reported by an independent group of researchers at the American Society of Human Genetics in 2012. [29] The study population included 409 independent pairs of gay brothers, who were analyzed with over 300,000 single-nucleotide polymorphism markers. The data strongly replicated Hamer's Xq28 findings as determined by both two-point and multipoint (MERLIN) LOD score mapping. Significant linkage was also detected in the pericentromeric region of chromosome 8, overlapping with one of the regions detected in the Hamer lab's previous genomewide study. The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in each of these regions contributes to development of the important psychological trait of male sexual orientation". Female sexual orientation does not seem to be linked to Xq28, [25] [30] though it does appear moderately heritable. [29]

In addition to sex chromosomal contribution, a potential autosomal genetic contribution to the development of homosexual orientation has also been suggested. In a study population composed of more than 7000 participants, Ellis et al. (2008) found a statistically significant difference in the frequency of blood type A between homosexuals and heterosexuals. They also found that "unusually high" proportions of homosexual males and homosexual females were Rh negative in comparison to heterosexuals. As both blood type and Rh factor are genetically inherited traits controlled by alleles located on chromosome 9 and chromosome 1 respectively, the study indicates a potential link between genes on autosomes and homosexuality. [31] [32]

The biology of sexual orientation has been studied in detail in several animal model systems. In the common fruit fly Drosophila melanogaster, the complete pathway of sexual differentiation of the brain and the behaviors it controls is well established in both males and females, providing a concise model of biologically controlled courtship. [33] In mammals, a group of geneticists at the Korea Advanced Institute of Science and Technology bred a female mice specifically lacking a particular gene related to sexual behavior. Without the gene, the mice exhibited masculine sexual behavior and attraction toward urine of other female mice. Those mice who retained the gene fucose mutarotase (FucM) were attracted to male mice. [34]

In interviews to the press, researchers have pointed that the evidence of genetic influences should not be equated with genetic determinism. According to Dean Hamer and Michael Bailey, genetic aspects are only one of the multiple causes of homosexuality. [35] [36]

In 2017, Scientific Reports published an article with a genome wide association study on male sexual orientation. The research consisted of 1,077 homosexual men and 1,231 heterosexual men. A gene named SLITRK6 on chromosome 13 was identified. [37] The research supports another study which had been done by the neuroscientist Simon LeVay. LeVay's research suggested that the hypothalamus of gay men is different from straight men. [38] The SLITRK6 is active in the mid-brain where the hypothalamus is. The researchers found that the thyroid stimulating hormone receptor (TSHR) on chromosome 14 shows sequence differences between gay and straight men. [37] Graves' disease is associated with TSHR abnormalities, with previous research indicating that Graves' disease is more common in gay men than in straight men. [39] Research indicated that gay people have lower body weight than straight people. It had been suggested that the overactive TSHR hormone lowered body weight in gay people, though this remains unproven. [40] [41]

In 2018, Ganna et al. performed another genome-wide association study on sexual orientation of men and women with data from 26,890 people who had at least one same-sex partner and 450,939 controls. The data in the study was meta-analyzed and obtained from the UK Biobank study and 23andMe. The researchers identified four variants more common in people who reported at least one same-sex experience on chromosomes 7, 11, 12, and 15. The variants on chromosomes 11 and 15 were specific to men, with the variant on chromosome 11 located in an olfactory gene and the variant on chromosome 15 having previously been linked to male-pattern baldness. The four variants were also correlated with mood and mental health disorders major depressive disorder and schizophrenia in men and women, and bipolar disorder in women. However, none of the four variants could reliably predict sexual orientation. [42]

In August 2019, a genome-wide association study of 493,001 individuals concluded that hundreds or thousands of genetic variants underlie homosexual behavior in both sexes, with 5 variants in particular being significantly associated. Some of these variants had sex-specific effects, and two of these variants suggested links to biological pathways that involve sex hormone regulation and olfaction. All the variants together captured between 8 and 25% of the variation in individual differences in homosexual behavior. These genes partly overlap with those for several other traits, including openness to experience and risk-taking behavior. Additional analyses suggested that sexual behavior, attraction, identity, and fantasies are influenced by a similar set of genetic variants. They also found that the genetic effects that differentiate heterosexual from homosexual behavior are not the same as those that differ among nonheterosexuals with lower versus higher proportions of same-sex partners, which suggests that there is no single continuum from heterosexual to homosexual preference, as suggested by the Kinsey scale. [43]

Epigenetics studies Edit

A study suggests linkage between a mother's genetic make-up and homosexuality of her sons. Women have two X chromosomes, one of which is "switched off". The inactivation of the X chromosome occurs randomly throughout the embryo, resulting in cells that are mosaic with respect to which chromosome is active. In some cases though, it appears that this switching off can occur in a non-random fashion. Bocklandt et al. (2006) reported that, in mothers of homosexual men, the number of women with extreme skewing of X chromosome inactivation is significantly higher than in mothers without gay sons. 13% of mothers with one gay son, and 23% of mothers with two gay sons, showed extreme skewing, compared to 4% of mothers without gay sons. [44]

Birth order Edit

Blanchard and Klassen (1997) reported that each additional older brother increases the odds of a man being gay by 33%. [45] [46] This is now "one of the most reliable epidemiological variables ever identified in the study of sexual orientation". [47] To explain this finding, it has been proposed that male fetuses provoke a maternal immune reaction that becomes stronger with each successive male fetus. This maternal immunization hypothesis (MIH) begins when cells from a male fetus enter the mother's circulation during pregnancy or while giving birth. [48] Male fetuses produce H-Y antigens which are "almost certainly involved in the sexual differentiation of vertebrates". These Y-linked proteins would not be recognized in the mother's immune system because she is female, causing her to develop antibodies which would travel through the placental barrier into the fetal compartment. From here, the anti-male bodies would then cross the blood/brain barrier (BBB) of the developing fetal brain, altering sex-dimorphic brain structures relative to sexual orientation, increasing the likelihood that the exposed son will be more attracted to men than women. [48] It is this antigen which maternal H-Y antibodies are proposed to both react to and 'remember'. Successive male fetuses are then attacked by H-Y antibodies which somehow decrease the ability of H-Y antigens to perform their usual function in brain masculinization. [45]

In 2017, researchers discovered a biological mechanism of gay people who tend to have older brothers. They think Neuroligin 4 Y-linked protein is responsible for a later son being gay. They found that women had significantly higher anti-NLGN4Y levels than men. In addition, mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons. The results suggest an association between a maternal immune response to NLGN4Y and subsequent sexual orientation in male offspring. [10]

The fraternal birth order effect, however, does not apply to instances where a firstborn is homosexual. [49] [50]

Female fertility Edit

In 2004, Italian researchers conducted a study of about 4,600 people who were the relatives of 98 homosexual and 100 heterosexual men. Female relatives of the homosexual men tended to have more offspring than those of the heterosexual men. Female relatives of the homosexual men on their mother's side tended to have more offspring than those on the father's side. The researchers concluded that there was genetic material being passed down on the X chromosome which both promotes fertility in the mother and homosexuality in her male offspring. The connections discovered would explain about 20% of the cases studied, indicating that this is a highly significant but not the sole genetic factor determining sexual orientation. [51] [52]

Pheromone studies Edit

Research conducted in Sweden [53] has suggested that gay and straight men respond differently to two odors that are believed to be involved in sexual arousal. The research showed that when both heterosexual women and gay men are exposed to a testosterone derivative found in men's sweat, a region in the hypothalamus is activated. Heterosexual men, on the other hand, have a similar response to an estrogen-like compound found in women's urine. [54] The conclusion is that sexual attraction, whether same-sex or opposite-sex oriented, operates similarly on a biological level. Researchers have suggested that this possibility could be further explored by studying young subjects to see if similar responses in the hypothalamus are found and then correlating these data with adult sexual orientation. [ citation needed ]

Studies of brain structure Edit

A number of sections of the brain have been reported to be sexually dimorphic that is, they vary between men and women. There have also been reports of variations in brain structure corresponding to sexual orientation. In 1990, Dick Swaab and Michel A. Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men. [55] In 1992, Allen and Gorski reported a difference related to sexual orientation in the size of the anterior commissure, [56] but this research was refuted by numerous studies, one of which found that the entirety of the variation was caused by a single outlier. [57] [58] [59]

Research on the physiologic differences between male and female brains are based on the idea that people have male or a female brain, and this mirrors the behavioral differences between the two sexes. Some researchers state that solid scientific support for this is lacking. Although consistent differences have been identified, including the size of the brain and of specific brain regions, male and female brains are very similar. [60] [61]

Sexually dimorphic nuclei in the anterior hypothalamus Edit

LeVay also conducted some of these early researches. He studied four groups of neurons in the hypothalamus called INAH1, INAH2, INAH3 and INAH4. This was a relevant area of the brain to study, because of evidence that it played a role in the regulation of sexual behaviour in animals, and because INAH2 and INAH3 had previously been reported to differ in size between men and women. [38]

He obtained brains from 41 deceased hospital patients. The subjects were classified into three groups. The first group comprised 19 gay men who had died of AIDS-related illnesses. The second group comprised 16 men whose sexual orientation was unknown, but whom the researchers presumed to be heterosexual. Six of these men had died of AIDS-related illnesses. The third group was of six women whom the researchers presumed to be heterosexual. One of the women had died of an AIDS-related illness. [38]

The HIV-positive people in the presumably heterosexual patient groups were all identified from medical records as either intravenous drug abusers or recipients of blood transfusions. Two of the men who identified as heterosexual specifically denied ever engaging in a homosexual sex act. The records of the remaining heterosexual subjects contained no information about their sexual orientation they were assumed to have been primarily or exclusively heterosexual "on the basis of the numerical preponderance of heterosexual men in the population". [38]

LeVay found no evidence for a difference between the groups in the size of INAH1, INAH2 or INAH4. However, the INAH3 group appeared to be twice as big in the heterosexual male group as in the gay male group the difference was highly significant, and remained significant when only the six AIDS patients were included in the heterosexual group. The size of INAH3 in the homosexual men's brains was comparable to the size of INAH3 in the heterosexual women's brains. [ citation needed ]

William Byne and colleagues attempted to identify the size differences reported in INAH 1–4 by replicating the experiment using brain sample from other subjects: 14 HIV-positive homosexual males, 34 presumed heterosexual males (10 HIV-positive), and 34 presumed heterosexual females (9 HIV-positive). The researchers found a significant difference in INAH3 size between heterosexual men and heterosexual women. The INAH3 size of the homosexual men was apparently smaller than that of the heterosexual men, and larger than that of the heterosexual women, though neither difference quite reached statistical significance. [58]

Byne and colleagues also weighed and counted numbers of neurons in INAH3 tests not carried out by LeVay. The results for INAH3 weight were similar to those for INAH3 size that is, the INAH3 weight for the heterosexual male brains was significantly larger than for the heterosexual female brains, while the results for the gay male group were between those of the other two groups but not quite significantly different from either. The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation. [58]

LeVay has said that Byne replicated his work, but that he employed a two-tailed statistical analysis, which is typically reserved for when no previous findings had employed the difference. LeVay has said that "given that my study had already reported a INAH3 to be smaller in gay men, a one tailed approach would have been more appropriate, and it would have yielded a significant difference [between heterosexual and homosexual men]". [62] : 110

J. Michael Bailey has criticized LeVay's critics – describing the claim that the INAH-3 difference could be attributable to AIDS as "aggravating", since the "INAH-3 did not differ between the brains of straight men who died of AIDS and those who did not have the disease". [63] : 120 Bailey has further criticized the second objection that was raised, that being gay might have somehow caused the difference in INAH-3, and not vice-versa, saying "the problem with this idea is that the hypothalamus appears to develop early. Not a single expert I have ever asked about LeVay's study thought it was plausible that sexual behavior caused the INAH-3 differences." [63] : 120

The SCN of homosexual males has been demonstrated to be larger (both the volume and the number of neurons are twice as many as in heterosexual males). These areas of the hypothalamus have not yet been explored in homosexual females nor bisexual males nor females. Although the functional implications of such findings still have not been examined in detail, they cast serious doubt over the widely accepted Dörner hypothesis that homosexual males have a "female hypothalamus" and that the key mechanism of differentiating the "male brain from originally female brain" is the epigenetic influence of testosterone during prenatal development. [64]

A 2010 study by Garcia-Falgueras and Swaab stated that "the fetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed or organized into our brain structures when we are still in the womb. There is no indication that social environment after birth has an effect on gender identity or sexual orientation." [65]

Ovine model Edit

The domestic ram is used as an experimental model to study early programming of the neural mechanisms which underlie homosexuality, developing from the observation that approximately 8% of domestic rams are sexually attracted to other rams (male-oriented) when compared to the majority of rams which are female-oriented. In many species, a prominent feature of sexual differentiation is the presence of a sexually dimorphic nucleus (SDN) in the preoptic hypothalamus, which is larger in males than in females.

Roselli et al. discovered an ovine SDN (oSDN) in the preoptic hypothalamus that is smaller in male-oriented rams than in female-oriented rams, but similar in size to the oSDN of females. Neurons of the oSDN show aromatase expression which is also smaller in male-oriented rams versus female-oriented rams, suggesting that sexual orientation is neurologically hard-wired and may be influenced by hormones. However, results failed to associate the role of neural aromatase in the sexual differentiation of brain and behavior in the sheep, due to the lack of defeminization of adult sexual partner preference or oSDN volume as a result of aromatase activity in the brain of the fetuses during the critical period. Having said this, it is more likely that oSDN morphology and homosexuality may be programmed through an androgen receptor that does not involve aromatisation. Most of the data suggests that homosexual rams, like female-oriented rams, are masculinized and defeminized with respect to mounting, receptivity, and gonadotrophin secretion, but are not defeminized for sexual partner preferences, also suggesting that such behaviors may be programmed differently. Although the exact function of the oSDN is not fully known, its volume, length, and cell number seem to correlate with sexual orientation, and a dimorphism in its volume and of cells could bias the processing cues involved in partner selection. More research is needed in order to understand the requirements and timing of the development of the oSDN and how prenatal programming effects the expression of mate choice in adulthood. [66]

Childhood gender nonconformity Edit

Childhood gender nonconformity, or behaving like the other sex, is a strong predictor of adult sexual orientation that has been consistently replicated in research, and is thought to be strong evidence of a biological difference between heterosexual and non-heterosexuals. A review authored by J. Michael Bailey states: "childhood gender nonconformity comprises the following phenomena among boys: cross-dressing, desiring to have long hair, playing with dolls, disliking competitive sports and rough play, preferring girls as playmates, exhibiting elevated separation anxiety, and desiring to be—or believing that one is—a girl. In girls, gender nonconformity comprises dressing like and playing with boys, showing interest in competitive sports and rough play, lacking interest in conventionally female toys such as dolls and makeup, and desiring to be a boy". This gender nonconformist behavior typically emerges at preschool age, although is often evident as early as age 2. Children are only considered gender nonconforming if they persistently engage in a variety of these behaviors, as opposed to engaging in a behavior on a few times or on occasion. It is also not a one-dimensional trait, but rather has varying degrees. [67]

Children who grow up to be non-heterosexual were, on average, substantially more gender nonconforming in childhood. This is confirmed in both retrospective studies where homosexuals, bisexuals and heterosexuals are asked about their gender typical behavior in childhood, and in prospective studies, where highly gender nonconforming children are followed from childhood into adulthood to find out their sexual orientation. A review of retrospective studies that measured gender nonconforming traits estimated that 89% of homosexual men exceeded heterosexual males level of gender nonconformity, whereas just 2% of heterosexual men exceeded the homosexual median. For female sexual orientation, the figures were 81% and 12% respectively. A variety of other assessments such as childhood home videos, photos and reports of parents also confirm this finding. [67] Critics of this research see this as confirming stereotypes however, no study has ever demonstrated that this research has exaggerated childhood gender nonconformity. J. Michael Bailey argues that gay men often deny that they were gender nonconforming in childhood because they may have been bullied or maltreated by peers and parents for it, and because they often do not find femininity attractive in other gay males and thus would not want to acknowledge it in themselves. [68] Additional research in Western cultures and non-Western cultures including Latin America, Asia, Polynesia, and the Middle East supports the validity of childhood gender nonconformity as a predictor of adult non-heterosexuality. [67]

This research does not mean that all non-heterosexuals were gender nonconforming, but rather indicates that long before sexual attraction is known, non-heterosexuals, on average, are noticeably different from other children. There is little evidence that gender nonconforming children have been encouraged or taught to behave that way rather, childhood gender nonconformity typically emerges despite conventional socialization. [67] Medical experiments in which infant boys were sex reassigned and reared as girls did not make them feminine nor attracted to males. [5]

Boys who were surgically reassigned female Edit

Between the 1960s and 2000, many newborn and infant boys were surgically reassigned as females if they were born with malformed penises, or if they lost their penises in accidents. [4] : 72–73 Many surgeons believed such males would be happier being socially and surgically reassigned female. In all seven published cases that have provided sexual orientation information, the subjects grew up to be attracted to females. Six cases were exclusively attracted to females, with one case 'predominantly' attracted to females. In a review article in the journal Psychological Science in the Public Interest, six researchers including J. Michael Bailey state this establishes a strong case that male sexual orientation is partly established before birth:

This is the result we would expect if male sexual orientation were entirely due to nature, and it is opposite of the result expected if it were due to nurture, in which case we would expect that none of these individuals would be predominantly attracted to women. They show how difficult it is to derail the development of male sexual orientation by psychosocial means.

They further argue that this raises questions about the significance of the social environment on sexual orientation, stating, "If one cannot reliably make a male human become attracted to other males by cutting off his penis in infancy and rearing him as a girl, then what other psychosocial intervention could plausibly have that effect?" It is further stated that neither cloacal exstrophy (resulting in a malformed penis), nor surgical accidents, are associated with abnormalities of prenatal androgens, thus, the brains of these individuals were male-organized at birth. Six of the seven identified as heterosexual males at follow up, despite being surgically altered and reared as females, with researchers adding: "available evidence indicates that in such instances, parents are deeply committed to raising these children as girls and in as gender-typical a manner as possible." Bailey et al. describe these sex reassignments as 'the near-perfect quasi-experiment' in measuring the impact of 'nature' versus 'nurture' with regards to male homosexuality. [4]

'Exotic becomes erotic' theory Edit

Daryl Bem, a social psychologist at Cornell University, has theorized that the influence of biological factors on sexual orientation may be mediated by experiences in childhood. A child's temperament predisposes the child to prefer certain activities over others. Because of their temperament, which is influenced by biological variables such as genetic factors, some children will be attracted to activities that are commonly enjoyed by other children of the same gender. Others will prefer activities that are typical of another gender. This will make a gender-conforming child feel different from opposite-gender children, while gender-nonconforming children will feel different from children of their own gender. According to Bem, this feeling of difference will evoke psychological arousal when the child is near members of the gender which it considers as being 'different'. Bem theorizes that this psychological arousal will later be transformed into sexual arousal: children will become sexually attracted to the gender which they see as different ("exotic"). This proposal is known as the "exotic becomes erotic" theory. [69] Wetherell et al. state that Bem "does not intend his model as an absolute prescription for all individuals, but rather as a modal or average explanation." [70]

Two critiques of Bem's theory in the journal Psychological Review concluded that "studies cited by Bem and additional research show that [the] Exotic Becomes Erotic theory is not supported by scientific evidence." [71] Bem was criticized for relying on a non-random sample of gay men from the 1970s (rather than collecting new data) and for drawing conclusions that appear to contradict the original data. An "examination of the original data showed virtually all respondents were familiar with children of both sexes", and that only 9% of gay men said that "none or only a few" of their friends were male, and most gay men (74%) reported having "an especially close friend of the same sex" during grade school. [71] Further, "71% of gay men reported feeling different from other boys, but so did 38% of heterosexual men. The difference for gay men is larger, but still indicates that feeling different from same-sex peers was common for heterosexual men." Bem also acknowledged that gay men were more likely to have older brothers (the fraternal birth order effect), which appeared to contradict an unfamiliarity with males. Bem cited cross-cultural studies which also "appear to contradict the EBE theory assertion", such as the Sambia tribe in Papua New Guinea, which ritually enforced homosexual acts among teenagers yet once these boys reached adulthood, only a small proportion of men continued to engage in homosexual behaviour - similar to levels observed in the United States. [71] Additionally, Bem's model could be interpreted as implying that if one could change a child's behavior, one could change their sexual orientation, but most psychologists doubt this would be possible. [72]

Neuroscientist Simon LeVay said that while Bem's theory was arranged in a "believable temporal order", [62] : 65 that it ultimately "lacks empirical support". [62] : 164 Social psychologist Justin Lehmiller stated that Bem's theory has received praise "for the way it seamlessly links biological and environmental influences" and that there "is also some support for the model in the sense that childhood gender nonconformity is indeed one of the strongest predicators of adult homosexuality", but that the validity of the model "has been questioned on numerous grounds and scientists have largely rejected it." [72]

General Edit

Sexual practices that significantly reduce the frequency of heterosexual intercourse also significantly decrease the chances of successful reproduction, and for this reason, they would appear to be maladaptive in an evolutionary context following a simple Darwinian model (competition amongst individuals) of natural selection—on the assumption that homosexuality would reduce this frequency. Several theories have been advanced to explain this contradiction, and new experimental evidence has demonstrated their feasibility. [73]

Some scholars [73] have suggested that homosexuality is indirectly adaptive, by conferring a reproductive advantage in a non-obvious way on heterosexual siblings or their children, a hypothesised instance of kin selection. By way of analogy, the allele (a particular version of a gene) which causes sickle-cell anemia when two copies are present, also confers resistance to malaria with a lesser form of anemia when one copy is present (this is called heterozygous advantage). [74]

Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved do not drive them to complete rejection of heterosexuality. [75]

In a 2008 study, its authors stated that "There is considerable evidence that human sexual orientation is genetically influenced, so it is not known how homosexuality, which tends to lower reproductive success, is maintained in the population at a relatively high frequency." They hypothesized that "while genes predisposing to homosexuality reduce homosexuals' reproductive success, they may confer some advantage in heterosexuals who carry them". Their results suggested that "genes predisposing to homosexuality may confer a mating advantage in heterosexuals, which could help explain the evolution and maintenance of homosexuality in the population". [76] However, in the same study, the authors noted that "nongenetic alternative explanations cannot be ruled out" as a reason for the heterosexual in the homosexual-heterosexual twin pair having more partners, specifically citing "social pressure on the other twin to act in a more heterosexual way" (and thus seek out a greater number of sexual partners) as an example of one alternative explanation. The study acknowledges that a large number of sexual partners may not lead to greater reproductive success, specifically noting there is an "absence of evidence relating the number of sexual partners and actual reproductive success, either in the present or in our evolutionary past". [76]

The heterosexual advantage hypothesis was given strong support by the 2004 Italian study demonstrating increased fecundity in the female matrilineal relatives of gay men. [51] [52] As originally pointed out by Hamer, [77] even a modest increase in reproductive capacity in females carrying a "gay gene" could easily account for its maintenance at high levels in the population. [52]

Gay uncle hypothesis Edit

The "gay uncle hypothesis" posits that people who themselves do not have children may nonetheless increase the prevalence of their family's genes in future generations by providing resources (e.g., food, supervision, defense, shelter) to the offspring of their closest relatives. [78]

This hypothesis is an extension of the theory of kin selection, which was originally developed to explain apparent altruistic acts which seemed to be maladaptive. The initial concept was suggested by J. B. S. Haldane in 1932 and later elaborated by many others including John Maynard Smith, W. D. Hamilton and Mary Jane West-Eberhard. [79] This concept was also used to explain the patterns of certain social insects where most of the members are non-reproductive.

Vasey and VanderLaan (2010) tested the theory on the Pacific island of Samoa, where they studied women, straight men, and the fa'afafine, men who prefer other men as sexual partners and are accepted within the culture as a distinct third gender category. Vasey and VanderLaan found that the fa'afafine said they were significantly more willing to help kin, yet much less interested in helping children who are not family, providing the first evidence to support the kin selection hypothesis. [80] [81]

The hypothesis is consistent with other studies on homosexuality, which show that it is more prevalent amongst both siblings and twins. [80] [81]

Vasey and VanderLaan (2011) provides evidence that if an adaptively designed avuncular male androphilic phenotype exists and its development is contingent on a particular social environment, then a collectivistic cultural context is insufficient, in and of itself, for the expression of such a phenotype. [82]

Anatomical Edit

Some studies have found correlations between physiology of people and their sexuality these studies provide evidence which suggests that:

  • Gay men and straight women have, on average, equally proportioned brain hemispheres. Lesbian women and straight men have, on average, slightly larger right brain hemispheres. [83]
  • The suprachiasmatic nucleus of the hypothalamus was found by Swaab and Hopffman to be larger in gay men than in non-gay men [84] the suprachiasmatic nucleus is also known to be larger in men than in women. [85][86]
  • Gay men report, on average, slightly longer and thicker penises than non-gay men. [87]
  • The average size of the INAH 3 in the brains of gay men is approximately the same size as INAH 3 in women, which is significantly smaller, and the cells more densely packed, than in heterosexual men's brains. [38]
  • The anterior commissure is larger in women than men and was reported to be larger in gay men than in non-gay men, [56] but a subsequent study found no such difference. [88]
  • The functioning of the inner ear and the central auditory system in lesbians and bisexual women are more like the functional properties found in men than in non-gay women (the researchers argued this finding was consistent with the prenatal hormonal theory of sexual orientation). [89]
  • The startle response (eyeblink following a loud sound) is similarly masculinized in lesbians and bisexual women. [90]
  • Gay and non-gay people's brains respond differently to two putative sex pheromones (AND, found in male armpit secretions, and EST, found in female urine). [53][91][92]
  • The amygdala, a region of the brain, is more active in gay men than non-gay men when exposed to sexually arousing material. [93] between the index and ring fingers have been reported to differ, on average, between non-gay and lesbian women. [94][95][96][97][98][99][100][101][102][103]
  • Gay men and lesbians are significantly more likely to be left-handed or ambidextrous than non-gay men and women [104][105][106] Simon LeVay argues that because "[h]and preference is observable before birth. [107] [t]he observation of increased non-right-handness in gay people is therefore consistent with the idea that sexual orientation is influenced by prenatal processes," perhaps heredity. [38]
  • A study of over 50 gay men found that about 23% had counterclockwise hair whorl, as opposed to 8% in the general population. This may correlate with left-handedness. [108]
  • Gay men have increased ridge density in the fingerprints on their left thumbs and little fingers. [108]
  • Length of limbs and hands of gay men is smaller compared to height than the general population, but only among white men. [108]

J. Michael Bailey has argued that the early childhood gender nonconforming behavior of homosexuals, as opposed to biological markers, are better evidence of homosexuality being an inborn trait. He argues that gay men are "punished much more than rewarded" for their childhood gender nonconformity, and that such behavior "emerges with no encouragement, and despite opposition", making it "the sine qua non of innateness". [109]

Whether genetic or other physiological determinants form the basis of sexual orientation is a highly politicized issue. The Advocate, a U.S. gay and lesbian newsmagazine, reported in 1996 that 61% of its readers believed that "it would mostly help gay and lesbian rights if homosexuality were found to be biologically determined". [110] A cross-national study in the United States, the Philippines, and Sweden found that those who believed that "homosexuals are born that way" held significantly more positive attitudes toward homosexuality than those who believed that "homosexuals choose to be that way" or "learn to be that way". [111] [112]

Equal protection analysis in U.S. law determines when government requirements create a “suspect classification" of groups and therefore eligible for heightened scrutiny based on several factors, one of which is immutability. [113]

Evidence that sexual orientation is biologically determined (and therefore perhaps immutable in the legal sense) would strengthen the legal case for heightened scrutiny of laws discriminating on that basis. [114] [115] [116]

The perceived causes of sexual orientation have a significant bearing on the status of sexual minorities in the eyes of social conservatives. The Family Research Council, a conservative Christian think tank in Washington, D.C., argues in the book Getting It Straight that finding people are born gay "would advance the idea that sexual orientation is an innate characteristic, like race that homosexuals, like African-Americans, should be legally protected against 'discrimination' and that disapproval of homosexuality should be as socially stigmatized as racism. However, it is not true." On the other hand, some social conservatives such as Reverend Robert Schenck have argued that people can accept any scientific evidence while still morally opposing homosexuality. [117] National Organization for Marriage board member and fiction writer Orson Scott Card has supported biological research on homosexuality, writing that "our scientific efforts in regard to homosexuality should be to identify genetic and uterine causes. so that the incidence of this dysfunction can be minimized. [However, this should not be seen] as an attack on homosexuals, a desire to 'commit genocide' against the homosexual community. There is no 'cure' for homosexuality because it is not a disease. There are, however, different ways of living with homosexual desires." [118]

Some advocates for the rights of sexual minorities resist what they perceive as attempts to pathologise or medicalise 'deviant' sexuality, and choose to fight for acceptance in a moral or social realm. [117] The journalist Chandler Burr has stated that "[s]ome, recalling earlier psychiatric "treatments" for homosexuality, discern in the biological quest the seeds of genocide. They conjure up the specter of the surgical or chemical "rewiring" of gay people, or of abortions of fetal homosexuals who have been hunted down in the womb." [119] LeVay has said in response to letters from gays and lesbians making such criticisms that the research "has contributed to the status of gay people in society". [117]

Top Programs Methodology

Many different characteristics of school programs were considered when deciding what schools to place on this list. Schools that offer students hands-on skills with laboratory classes, seminars and internships, or field study were highly-rated. Other factors that were considered included the facilities at a school, its partnerships, and career placement opportunities.


Schools on this list may have state-of-the-art lab equipment to help with learning. For example Michigan State University features equipment for doing single nucleotide polymorphism analysis, as well as mitochondrial DNA research in their PCR laboratory.


Many of the schools such as Delaware State University have built extensive partnerships into their programs to help students progress with learning and to develop deeper skills to be prepared to work in the field of forensic biology.

Graduate Outcomes

Schools with a stated track record of placing students in jobs or master’s degree programs after graduation were also highly considered for this list. For example, students at Ohio Northern University generally go on to work at places such as the Armed Forces DNA Identification Laboratories, private forensic testing laboratories, or to pursue advanced degrees at schools such as Clemson University and Ohio State University.

Where’s the Biological Evidence?

Biological evidence cannot give specifics about how animals were originally designed. The ultimate authority is the Bible, which was written by men who were inspired of God.

What is your biological evidence that tigers and snakes were designed to be plant stalkers? I know all you scriptural arguments, In the bible we read there is a city of Jerico at such a place, we dig, we find it. We read a Hebrew was a big shot in Egypt, we dig, we find it. The bible states credible findable facts. What evidence do you see that supports your idea that animals were intended and designed to be plant only eaters? Put your self in the place of the modern lost bioligist, what would convince him you are not just presenting a doctrine of your own making? (by the way I want you to be correct, I just don't think the scriptures absolutely support you, though they do to a degree)

—R. J., US

Biological evidence cannot give specifics about how animals were originally designed. The ultimate authority is the Bible , which was written by men who were inspired of God. The Bible says:

These biblical statements need to be embraced and not ignored by the biological community.

If we refuse to accept the biblical statements, then we give up the foundation of our belief system. The non-Christian doesn’t want us to use the Bible but wants us to reject it and accept his belief system. They accept biological “evidence” as their supreme authority.

We need to be discerning and realize when others want us to give up the Bible and instead use biological (or other) “evidence” as the final authority. We need to refuse to do so, and then show the falsity in doing so ( Proverbs 26:5 ) so that the non- Christian will not assume his position is the only correct one. Why would we want to give up our starting point and accept theirs? It should be the other way around—they need to give up their reliance on man-made ideas about the past and instead use the Bible !

In the bible we read there is a city of Jerico at such a place, we dig, we find it. We read a Hebrew was a big shot in Egypt, we dig, we find it. The bible states credible findable facts.

Archaeological findings do not provide an absolute authority. As confirmation of the Word of God, they are important, but they are secondary to God and His Word. On the other hand, lack of archaeological evidence (for example, not finding the cursed fig tree Mark 11:13-21 ) will not invalidate the scriptures. The Scriptures are the Word of God , which is truth and will therefore have evidence of its veracity.

What evidence do you see that supports your idea that animals were intended and designed to be plant only eaters?

Again, we accept that all animals were originally designed to eat green plants because this is what the Bible teaches in Genesis 1:30 . We don’t look to the “evidence” to see if animals were originally vegetarian, and then confirm this with the Bible . That said, there have been examples of meat-eating animals that have survived on non-meat diets (e.g. the lion that refused to eat meat, the vulture that refused to eat meat, and so on). Just because an animal has a carnivorous diet today, doesn’t mean its ancestors only ate meat, or that it can’t survive on a vegetarian diet today.

The un-petition

I am so glad that AiG is standing strong and growing fast. What I read from Ken's blog about the petitions is actually encouraging. You would not be receiving so much opposition if you did not have any potential. and this also shows just how “dangerous” evangelism is to lost souls—they might be saved. Thank you! Thank you! Thank you!

All of us were “lost,” before God graciously saved us, so we can sympathize with the biologist (as a matter of fact, some of us are biologists!). However, as Christians, we now have a different foundation (the Bible ) from the non- Christian , even though we both study the same “evidence.” When Jesus came in human form to teach mankind, did He give up the Word of God as the authority? By no means! He used it. So, we must not ignore God ’s Word, and rely only on the “evidence.”

Instead, we should ask the lost biologist to put the Bible in the position it deserves—the supreme authority in all matters on which it touches, including biology. We can also challenge his position, if it is unbiblical, so that he realizes biology makes little sense without the Bible . For example:

  • Why do we observe animals varying within their respective kinds and not changing into other kinds?
  • Why do we observe life giving rise to life (Law of Biogenesis), not abiogenesis, which molecules-to-man evolution requires?
  • Why are laws of nature uniform in the first place?

What would convince him you are not just presenting a doctrine of your own making?

The doctrine of animals originally being vegetarian is obviously not a doctrine of our own making, as it was given thousands of years ago by the Creator to His creation ( Genesis 1:30 ).

We can present what the Bible says to the non-Christian, but it is not up to us to convince him to change his mind. Conviction comes from the Holy Spirit ( 2 Thessalonians 2:13 ), who opens hearts. When we witness to others, we can:

  1. Pray that God would open that person’s heart.
  2. Question the false foundation that the person accepts and show that it is faulty (such questions above help). This will take time.
  3. Provide the correct biblical foundation as taught in Genesis and the rest of the Bible.
  4. Share the gospel and show how it relates. The point isn’t to just convince someone that creation is true but to present them with the good news of salvation in Christ.

(by the way I want you to be correct,

—R. J., US

Thanks, but you’re not really rooting for us–you’re rooting for the Word of God to be correct.

I just don't think the scriptures absolutely support you, though they do to a degree)

Of course, our opinions can’t take precedence over the authority of the Scriptures themselves. Like all sinners, our opinions are not the final authority. Additionally, science models can change, but the scriptures are not in error nor will they change. I want to encourage you to re-read Genesis 1–11 .

Biological influences on criminal behaviour: how good is the evidence?

The perception that crime, especially violent crime, has become one of the most serious problems facing society has led to determined efforts by many researchers to find the causes of criminal behaviour. Researchers have focused on biological causes, believing that a biological basis of criminality exists and that an understanding of the biology will be useful in predicting which people are predisposed to become criminals. In the 1960s it was proposed that males with an extra Y chromosome were predisposed to violent criminal behaviour later work found no support for this hypothesis. 1 Recently, two approaches, one genetic, the other biochemical, have received widespread publicity. I would argue that currently neither approach provides convincing evidence that criminal behaviour can be understood in terms of genetics or biochemistry.

Before these two approaches are discussed, the many family, twin, and adoption studies that have concluded that a biological basis exists for antisocial behaviour should be noted. 2 3 4 At least two recent reviews, however, have suggested that the support for these conclusions, especially those concerned with violent …

Engineering further containment: genetic containment

Life on Earth is underpinned by nucleic acids. The two natural backbones, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are biopolymers made from a limited set of possible nucleotide precursors variants of a common chemical structure: a phosphate group that is linked to a nitrogenous base via a 5-carbon sugar linker. The fact that modern, naturally occurring genetic systems are all based on DNA and RNA does not mean that other chemical forms of genetic information storage are not conceivable. With the aim of finding novel building blocks for the design of artificial nucleic acids, each of the three components of natural nucleotides has been iteratively replaced by alternative chemical structures rendering what has been defined as ‘xeno–nucleotides’ [66]. Modifications to the canonical structure of nucleotides have resulted in numerous variants presenting: unnatural nucleobases, alternative base-pairings, novel cyclic or even acyclic sugar backbones, and altered phosphate groups or alternative leaving groups. In depth reviews regarding the different chemistries developed so far can be found in [67–69].

A subset of the available chemistries has even been successfully polymerized into Xenobiotic Nucleic Acids (XNAs). Some of these XNAs allowed duplex formation [67] and information storage [70,71], and were even evolved into functional molecules such as aptamers and XNAzymes [70–73]. These results suggest that DNA and RNA are not, in principle, the only genetic polymers capable of sustaining life, and so life based on XNAs may also be thinkable.

To date, no life on Earth has been reported to use XNA for genetic information storage and it is unclear whether biology would be able to access the information within it. If not, then the biocontainment of a GEM carrying XNA as genetic material could be feasible. Prevention of horizontal gene transfer to nature could arise from the genetic isolation of XNA-based GEMs. As replication, transcription or incorporation of an XNA into the genome of GEM's natural counterparts would be prevented, a ‘genetic firewall’ could be raised between nature and genetic engineering.

Before even envisaging a GEM built upon XNA, a simpler first approach to develop and test a genetic firewall would be to create an XNA-based replication unit (e.g. episome) that can be maintained in a bacterial cell independently from its DNA genome (Figure 3). Any desired trait could be engineered into the XNA episome while the DNA genome would only be used for chassis maintenance. In order to set up an XNA-episome replication system in vivo there are several characteristics that the XNA, the episome itself and the host cell must fulfil (adapted from [74]):

Regarding the potential XNA,

1) to ensure cell survival, the XNA cannot be toxic in any way, be it as precursor, triphosphate, polymer or degradation products.

2) to allow an initial transference of information from natural nucleic acids and vice versa, a dedicated XNA synthetase and XNA reverse-transcriptase should be available.

3) to attain orthogonality (complete isolation from the DNA/RNA genetic system), the XNA should not be interpreted by the host cell replication or transcription machinery.

Regarding the EPISOME,

4) to guarantee maintenance in controlled culture conditions, one selectable function indispensable for growth must be encoded in the XNA episome (like antibiotic resistance is encoded in commercial plasmids):

4.1) if the selectable marker encodes an mRNA that is destined to be translated into an essential protein (e.g. detoxification of an antibiotic), then a dedicated RNA polymerase is required.

4.1) if the selectable marker is a functional XNA, this molecule would need to perform a metabolic action vital to the host cell (e.g. XNA molecule that binds and inactivates a small toxic RNA). Such functional XNA must be developed and crucially, any function must be restricted to XNA–the same sequence in a different chemistry cannot rescue selection.

5) to replicate the XNA episome across generations, a dedicated XNA replicase, XNA helicase and XNA single-strand binding protein should be encoded in the episome.

6) to retain orthogonality, XNA enzymes should not accept natural nucleic acids as substrates.

Regarding the HOST CELL (e.g.E. coli),

7) to prevent replication in non-controlled conditions, XNA precursors cannot be made in the cell. Cells must be auxotrophic for the XNA building blocks.

8) to provide cells with XNA precursors, a natural or engineered mechanism for the uptake should exist.

An orthogonal replication system for genetic containment

An XNA genetic element (in orange) is maintained by the external provision of xenobiotic nucleoside triphosphates (xNTPs) or cell-permeable precursors (yellow) and replicated by means of an engineered XNA-dependent XNA polymerase (or XNA replicase) and accessory proteins (in purple). Selection of the synthetic episome across generations occurs by encoding a vital gene product or functional XNA in the episome itself. In either case, an XNA-directed RNA polymerase is necessary (in purple). Prevention of cross-talk with the natural DNA genetic system (in green) is essential to create a stable XNA system and to establish an effective genetic firewall (dotted line).

An XNA genetic element (in orange) is maintained by the external provision of xenobiotic nucleoside triphosphates (xNTPs) or cell-permeable precursors (yellow) and replicated by means of an engineered XNA-dependent XNA polymerase (or XNA replicase) and accessory proteins (in purple). Selection of the synthetic episome across generations occurs by encoding a vital gene product or functional XNA in the episome itself. In either case, an XNA-directed RNA polymerase is necessary (in purple). Prevention of cross-talk with the natural DNA genetic system (in green) is essential to create a stable XNA system and to establish an effective genetic firewall (dotted line).

There is a remaining point, regarding the destiny of xeno-nucleotides in vivo that has not been included in the list because it does not represent an essential item to establish an XNA episome, but requires attention. Natural nucleic acids are synthesized using nucleoside triphosphates as monomers. However, only the nucleoside monophosphate ends up being incorporated into the DNA or RNA molecule while the remaining pyrophosphate, necessary for the activation of the monomer, is released as a secondary product. As the pyrophosphate is not included in the final biopolymer it is known as the ‘leaving group’. Pyrophosphate is further hydrolysed to release two molecules of phosphate. In nature, no exception to the usage of pyrophosphate as the leaving group has ever been reported. It has been pointed out that it would be difficult to install xeno-nucleoside triphosphates in a cell environment without interfering with DNA and RNA metabolism, cell energy supply [adenosine triphosphate (ATP) levels] or substrate-level phosphorylation (due to the free available phosphate) [74]. One way to overcome this problem would be to use a different type of leaving group in the precursors for the enzymatic synthesis of the XNA. Different chemistries for alternative leaving groups, based on L-amino acids, phosphono-L-Ala or iminodiacetate, have been suggested and studied [75], however the enzymes capable of efficiently incorporating the nucleoside monophosphates out of the alternative nucleotides have yet to be developed.

15 Best Biology Extended Essay Topics

It’s hard to come up with interesting and unique biology extended essay topics for a school assignment. And if you don’t have the time to brainstorm ideas you might not come up with a well-developed idea before your assignment is due. We’ve created this list of great biology extended essay assignments on a variety of topics for free use:

  1. Describe the research histories of curable diseases which were at one point incurable. What kinds of studies were needed to come to this point?
  2. Is there such a thing as a healthy diet or healthy way of eating that most people are ignoring but would be beneficial to a majority of the population?
  3. Taking an in-depth look of human brain and its evolution, how have certain phobias developed from genuine fears meant to protect people from dangers?
  4. Should more sleep research be conducted to find biological remedies that can be healthier alternatives to dangerous or addictive medications?
  5. What are some of the effects of the photosynthesis process when done without using natural sunlight? Is this better or worse for farmers and the community?
  6. Is there any biological evidence in support of alternative medicines for the treatment of incurable diseases? Why won’t the health community research this further?
  7. What biological evidence supports the idea of finding a cure for aging? Would these studies have any benefit for regenerative tissue research?
  8. Do genetics play a role in determining sexual preference? If so, is there in supporting evidence to suggest that parents may one day want to control this?
  9. Should people be asked to join a national blood bank registry to protect them should injuries require transfusions as well others who require the process?
  10. What kind of influence do human genetics play on our lifespan? What about our health and/or psychological characteristics and traits?
  11. Analyze and evaluate the current standards for storing cow’s milk as it is intended for sale and mass consumption? What risks do people face?
  12. Is there such a thing as biological hypnosis where patients can heal when placed under controlled hypnosis by a professional?
  13. Are there any special mental ties between twins? What biological studies have been conducted in the last half century to prove or disprove this belief?
  14. Does biology present any potential solutions to treat eating disorders? How can this be applied to regular health regimes in people who have developed disorders?
  15. What are the major connections between the body and brain? How does one affect the other and what does this say about current medicinal practices?

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Biological Evidence Management for DNA Analysis in Cases of Sexual Assault

Biological evidence with forensic interest may be found in several cases of assault, being particularly relevant if sexually related. Sexual assault cases are characterized by low rates of disclosure, reporting, prosecution, and conviction. Biological evidence is sometimes the only way to prove the occurrence of sexual contact and to identify the perpetrator. The major focus of this review is to propose practical approaches and guidelines to help health, forensic, and law enforcement professionals to deal with biological evidence for DNA analysis. Attention should be devoted to avoiding contamination, degradation, and loss of biological evidence, as well as respecting specific measures to properly handle evidence (i.e., selection, collection, packing, sealing, labeling, storage, preservation, transport, and guarantee of the chain custody). Biological evidence must be carefully managed since the relevance of any finding in Forensic Genetics is determined, in the first instance, by the integrity and quantity of the samples submitted for analysis.

1. Introduction

Biological evidence with forensic interest may be found in several cases of assault, being particularly relevant for sexually related ones. Sexual aggression constitutes a serious social and public health problem that calls for an urgent forensic medical examination (FME), particularly in acute cases, that is, when the elapsed time between the assault and the FME is less than 72 hours, in the generality of cases [1–6].

In these cases a large number of forensic areas are involved (e.g., clinical forensic medicine, genetics, and toxicology) aiming to obtain the proof and elaboration of a final forensic report [1].

From the forensic intervention perspective, despite some published protocols and guidelines, few countries have officially adopted guidelines for evidence management, namely, in acute sexual assault (ASA) cases. Even when guidelines are adopted they may vary within the same country, between different regions and different institutions. However, to standardize the FME of ASA victims and the credibility of forensic practices, which are essential during judicial proceedings, clear guidelines developed by the scientific community are required [2, 6]. These guidelines will aid in optimizing forensic intervention and reduce unnecessary variations in the procedures, as well as improving collaboration among several entities and professionals, while enabling a well-timed and comprehensive forensic evaluation. An essential part of these guidelines should concern management of biological evidence for DNA analytical studies.

This work will focus on the management of forensic evidence, more specifically the biological samples. Indeed, examiners performing FME in ASA cases must have knowledge and training in collecting and handling evidence, always respecting guidelines and legal obligations. This is true regardless of the value of other forensic procedures (e.g., forensic interview, forensic medical history, photo documentation, or physical examination) that may be required. Examiners should also be aware of the scope and limitations of laboratory analysis as well as the consequences of contamination or degradation of any evidence [7]. Moreover, the interpretation of the findings related to evidence should also receive careful and thorough consideration, as there are multiple variables that may influence the quality of evidence [8, 9]. All these variables should be taken into consideration and discussed in any recommendations or guidelines, as well as in the expert (medical or laboratorial) reports.

Because of its utility in proving the occurrence of sexual contact and the identification of the suspects, biological evidence for DNA studies is nowadays considered the most important evidence for legal proof in courts of law [4, 10–12]. The proper handling procedures during selection, collection, packaging, labeling, storing, and transportation of evidence to the laboratory are key steps aiming to achieve final valid and reliable results [8, 9]. Oversights or faults in these procedures can call into question the production of the proof, namely, regarding evidence preservation (loss or contamination) and chain of custody [13].

In this study, we aimed to review and update forensic procedures already implemented in various forensic institutions. These are based on Portuguese and international forensic expertise and evidence gathered through the review of scientific literature and institutional guidelines. However, it is important to note that the application of these guidelines is highly dependent on the available local resources and should be mainly regarded to promote the quality and safety of forensic practices and fill some existing gaps.

It is hoped that this work can be a useful tool (not only for forensic practitioners) to help the mission of forensic expertise regarding ASA, promoting the ability of professionals to detect, collect, and properly appraise biological forensic evidence.

2. Forensic Evidence

In every crime against people, as in sexual assault, the contact between the perpetrator and the victim, or his/her environment, or both always leaves evidence which is transferred from the perpetrator to the victim, to the scene, and vice versa [8, 14, 15].

Forensic evidence, in the broadest sense, is any item or information about a suspected crime, which is considered to be relevant to an investigation in order to find the truth of the facts. It may be useful to (1) orient police investigation (2) provide a reliable identification of the perpetrator (3) exonerate a suspect or an accused from a crime (4) support or contradict a victim’s, witness’, or suspect’s statement and, consequently, promote police to conduct further investigations (5) provide information about the crime scene and (6) provide proof that attests to the occurrence of the alleged event.

Typically crime scene evidence could be found on any place where a criminal offence was committed, on anything worn or carried by the victim during the time the offence or within or on the body of any person associated with the offence.

Evidence may be found at the victim’s body or clothes, in condoms or bed clothes, or at the crime scene [2, 11, 16–19]. Therefore, the examiner should rapidly inform police to isolate and protect the crime scene and should collect first the more urgent samples [20]. Nevertheless, professionals must be aware that in ASA cases the victim’s body may be the most important part of the crime scene [5].

Two types of evidence can be considered: (a) Direct evidence: it establishes the fact without needing further investigations. The most important one is the eyewitness or victim statement nevertheless, their statement can be prone to many inaccuracies and may be contradicted or supported by other types of evidence (e.g., biological evidence for DNA testing) [6]. (b) Circumstantial evidence (or indirect evidence): it needs to be identified and matched with a control or reference sample collected from the victim, suspect, and/or the crime scene or database. Although it is more objective than direct evidence, it must be handled carefully aiming to avoid risk of destruction, contamination, or loss. It is the majority of the evidence analyzed in the forensic laboratories and can be divided into two basic classes. (1) Physical evidence: it includes items of nonbiological origin, such as finger and foot prints, shoe/tire impressions, fibers, paint, soil, dirt, glass, headlamps or arson debris, explosives and gunshot residues, and figured injuries (e.g., bite marks, scratches) [6, 21]. They are very useful to identify the crime scene and should be collected when available [8, 14, 22]. (2) Biological evidence: it includes items from a biological origin, usually from the victim or the perpetrator (e.g., semen, vaginal fluid, oral fluid, sweat, blood, and other body fluids, hair, cells of the alleged perpetrator under a victim’s finger nails, or epithelial cells of the alleged victim present on the penis of the perpetrator) [19, 23], and botanical elements (e.g., pollen, plants, and wood). It is considered the most important type of evidence (especially semen) since it is very useful to prove that physical/sexual contact occurred and to identify a perpetrator through DNA studies [5, 9–11, 24].

3. Biological Evidence

The collection of biological evidence for DNA studies is particularly useful in ASA cases to establish the occurrence of sexual contact and to proceed with suspect identification. In fact, the presence of semen on a prepubertal child’s body, clothes, or vicinity during the FME usually confirms the diagnosis of the sexual contact and is generally accepted in a court of law as proof [3, 17–20, 25, 26]. Nevertheless, this interpretation should not be regarded as an irrefutable proof, especially for incest or intrafamily cases, since a secondary transfer of sperm cells from adult clothing/bedsheets to babies’ or children’s clothing during laundry washes was previously evidenced [27, 28]. Moreover, a complete genetic profile of the father can be obtained despite the fact there was absolutely no sexual abuse involved [27, 28].

The following considerations should be taken in order to help forensic examiner to define the best practices in each case and to interpret the findings: (a) Semen (spermatozoa suspension in the seminal fluid): it is rarely present in oral, anorectal, and vaginal cavities 6, 24, and 72 hours after sexual contact, respectively [22, 29]. In vaginal cavity the half-life time depends on the age of the victim (pre- or postpubertal) and if the semen is localized in the cervix, the half-life may be much higher than 72 hours [22, 29]. In postpubertal girls spermatozoa may remain motile in the vaginal secretions for 6 to 12 hours and in the cervix for as long as 5 days [30] nonmotile spermatozoa may be found in stains of vaginal secretions from 12 to 48 hours after ejaculation [22, 29]. The half-life of semen in the prepubertal girls is comparatively shorter due to the absence of cervical mucus [22, 29]. Dried secretions on clothing remain quite stable, so that semen may be detected for longer than 1 year [22, 31]. These half-lives represent mere estimations, since several variables (that should be described in the forensic medical report) must be considered when documenting the presence or not of semen in sexual assault cases [22, 29, 31]: (1) the type of practice and circumstances (e.g., where evidence is deposited ejaculation occurred in the skin, oral, anal, or vaginal mucosa or in the cervix condom use the perpetrator is azoospermic or vasectomized) (2) the time between sexual contact and evidence collection (3) victim’s gender, age, and activities (e.g., urinating, defecating, vomiting, brushing teeth, bathing, eating, drinking, smoking, spitting, running, and walking) after sexual contact. (b) Observation of spermatozoa under an optical microscope (e.g., using stains such as the Kernechtrot Picroindigocarmine (KPIC or Christmas Tree stain), Giemsa, hematoxylin/eosin, and methylene blue/eosin) or by phase contrast microscopy (no stains): these are considered for diagnosis of sexual contact and the concomitant observation of motile spermatozoa allows estimating the time of the assault. However, since these techniques do not lead to the identification of the perpetrator and biological material is lost to perform smears, some authors do not recommend this procedure. The absence of spermatozoa may occur if the suspects are azoospermic or vasectomized or if semen stains are dry [24, 32]. Under an optical microscope, the Florence Iodine (FI) test is used for seminal fluid identification by detecting the presence of choline through the addition of an iodine based reagent, which produces characteristic brown choline periodide crystals. In a recent study, Hardinge and colleagues [33] observed that prostate-specific antigen (PSA) is much more sensitive but less specific than the FI test to confirm the presence of seminal fluid. (c) Seminal acid phosphatase (AP): this enzyme is present in semen and for positivity, the presence of spermatozoa is not needed since it is a prostatic enzyme. In postpubertal girls’ vagina or cervix, the possibility to register elevated AP levels ranges from 24 hours [24] to 72 hours after ejaculation [22, 29]. AP levels are elevated for a much shorter time in mouth (perhaps only 6 hours) and in the rectum (less than 24 hours), but only estimates are available [34]. On the other hand, in spite of an elevated level of AP being a specific indicator of recent sexual intercourse and ejaculation, its use as evidence is somewhat limited due to the existence of an isoenzyme in low levels in postpubertal vaginal fluid and female urine [22]. The presence and concentration of AP in prepubertal girls is unknown. Analytical techniques to quantify AP (e.g., Brentamine Fast Blue reaction) should be regarded as guide and if result is negative, DNA studies (autosomal STRs and Y-STRs) must proceed [35]. Indeed, the results of the Brentamine colorimetric reaction may be difficult to interpret due to the interference of fabric colors and therefore may lead to false negative results. (d) Prostate-specific antigen (PSA): it is a serine protease produced by prostatic epithelial cells found in many tissues (e.g., seminal fluid, prostatic fluid, male serum, male urine, apocrine sweat glands, and breast milk from lactating women). Although PSA is not tissue and gender specific, in ASA cases, the interpretation of the results should not pose a significant problem due to its low concentrations in nonprostatic fluids [36–38]. PSA can be found up to 48 hours in postpubertal girls’ vagina or cervix [24]. PSA is considered one of the most sensitive methods for semen detection and can be applied for azoospermic individuals. Similar to AP, PSA should be regarded as guide and if result is negative, DNA studies (autosomal STRs and Y-STRs) must be performed.

Other aspects should be considered: (a) Oral fluid: it constitutes the second biological evidence commonly found in ASA cases, often observed by the Phadebas test, which detects α-amylase activity. Nevertheless, it should be taken into account that α-amylase can be present in body fluids other than saliva. This biological material transports epithelial cells from buccal mucosa which contains DNA [3, 4, 23, 39, 40]. It is very useful since the perpetrator commonly licks, bites, or kisses the victim, and his/her oral fluid may prevail on the victim’s skin (e.g., neck, thorax, and abdomen). Cigarette filters, bottles, or cans of soft drinks are likely to lead to the identification of the perpetrator. Stamps and envelopes are less likely to provide DNA that could lead to a perpetrator because they are usually now self-adhesive and therefore few people lick them anymore. (b) Some studies argue that perpetrator’s DNA may be detected in the victim’s oral cavity up to 1 hour after intense kissing [41]. Nevertheless, collection within this period is very difficult to accomplish, since the victims are presented later for FME and usually wash their mouth. Thus the collection of oral fluid must be performed as soon as possible for victim’s hygiene and comfort but also for avoiding loss or destruction of this sensitive evidence that normally presents low amounts of DNA. (c) Head or pubic hair, and/or epithelial cells of the victim or perpetrator transferred between them during the sexual contact or a fight, should also be collected with utmost care due to the low amount of DNA present [2, 4, 5, 42–46]. It should be born in mind that the pubic hair transferred during intercourse, victim being in the dorsal decubitus position, is minimal even if samples are collected during a short time afterwards, as previously demonstrated [45]. (d) The fingernail hyponychium is an isolated area where evidence may accumulate and can provide a valuable source of evidential material for investigation. During the course of a sexual assault, trace amounts of skin (especially if the victim scratched the perpetrator), body fluids, hairs, fibers, and vegetation may collect under the nails of either the victim or perpetrator [42, 43, 47]. The persistence of foreign DNA did not tend to last beyond 6 h [42].

4. Evidence Preservation

Evidence preservation aims to avoid its destruction, contamination, or loss.

4.1. Destruction

To avoid the destruction of evidence, the professional to whom the case was reported should inform the victim or any person who reported the incidence/offence about practices that the victim should refrain from until FME can be completed [3–5, 26, 48, 49]: (a) shower or wash any part of the body, including mouth, hands, and head hair (b) brush teeth (c) clean or cut fingernails (d) comb or cut paint hair (e) perform vaginal irrigation (f) urinate, defecate, or vomit (and if this is imperative, do it in a clean container with a lid) (g) eat, drink, chew, or smoke (h) run or perform any kind of sport activities or the same (i) change, wash, or destroy clothing worn during the event (j) change or destroy sanitary pads worn during the event (k) touch the crime scene (including emptying garbage can or flushing the toilet). Moreover in order to prevent DNA degradation, the forensic examiner must correctly select the type of material used for collection and storage (e.g., paper versus plastic containers—please see Section 5.3) and ensure complete drying of the sample prior to packaging [9, 13, 50].

4.2. Contamination

For DNA studies, one of the greatest laboratory barriers is the contamination of genetic material from other sources (e.g., from the examiner and other biological evidence). Contamination may occur during the sexual contact (e.g., if there is more than one perpetrator), during the period between the sexual contact and the FME, during the FME, and in the laboratory [51–53]. In order to avoid it, examiners should take special precautions to prevent cross-contamination between evidences [7, 29, 50]. For this purpose, it is important [4, 54] (a) to work under aseptic conditions to avoid microbial contamination (b) to always use disposable supplies to ensure individual protection (e.g., gowns, powder-free gloves, mask, or other protective clothing) and to avoid direct contact with the samples (c) to ensure that the room where FME takes place is regularly cleaned before and after patient use (d) to avoid sneezing, coughing, or talking over the samples (e) not to drink, eat, and/or smoke when handling samples (f) to store swabs or other samples separately, ensuring that they are contact-free (e.g., while drying or during storage), particularly in circumstances where reference samples were also collected at the same time [49] (g) to avoid contamination with evidence of other body areas, since the specific location of each biological sample is crucial to the investigation [29] gloves should be changed regularly between the collections of each item of evidence (h) not to touch the cotton-tipped swabs.

4.3. Loss

In many ASA cases, the evidence is recovered in very low amounts. Consequently, two issues must be weighed: the number of swabs to be performed during the collection for each evidence and the pertinence (or not) of doing semen smears for spermatozoa observation under optical microscope.

The number of swabs performed per body area is important for financial reasons but also due to evidence concentration in each swab. Hochmeister and Ferrel [49] consider that one swab per item of evidence is more than enough. Others advise to collect at least two swabs for the same item of evidence [22, 29, 55, 56]. The medical examiner should justify the adopted procedure in the FME report and consider the following objectives in the decision making process: (1) ability to conduct independent analysis for counterproof (2) collection of all biological evidence available and (3) facility to collect evidence. Therefore, it is important to highlight how each technique meets these objectives [6]: (a) One swab: it is a rapid technique but does not guarantee that the entire evidence is collected for laboratory analysis. It is particularly useful in the presence of evidence with limited quantity. Moreover, if counterproof is required, two situations may be possible: (1) half the cotton swab could be preserved in the laboratory allowing one to perform a new analysis that will begin from extraction of DNA (2) the entire swab is used in the first forensic analysis (most common situation) and counterproof analysis must be made from DNA previously extracted, ensuring that both analyses begin from the same DNA sample. (b) Two swabs simultaneously: in this case, biological material will be divided into two swabs, which could reduce the success of the laboratory analysis. Furthermore, nothing can guarantee that the two swabs, even used together, have the same evidence quantity, which for some authors seems to be relevant for legal issues. The evidence is rapidly collected and allows the use of the second swab for counterproof [29, 55, 56]. In anogenital area this technique is only performed in adult or postpubertal victims. It should be considered when there is enough biological material available (e.g., direct ejaculation within vaginal cavity occurred). (c) Two swabs successively or “double swab technique”: it is the application of two successive swabs, the first being wet (aiming to collect the majority of the evidence) and the second being a dry swab passed through the same place, the order of the swabs being annotated. This technique aims to collect the largest quantity of evidence available. It is not rapid and there is no guarantee of equality of the two swabs (the second swab may have much lower concentration of the evidence), reducing usefulness in counterproof. In spite of these limitations, this technique has been widely reported in the literature for the collection of various different biological samples (e.g., perpetrator’s oral fluid or epithelial cells on the victim’s skin) due to good outcomes [23, 39, 47].

In the majority of cases, semen smears for spermatozoa observation under optical microscope should not be performed, except in very specific circumstances, which should be detailed in the FME report. The following reasons justify their uselessness [49]: (a) Many variables impact the semen motility and its observation by the examiner does not give a precise estimation of the time of the sexual contact. (b) Due to the increasing number of vasectomized individuals, a semen smear has become a less effective screening tool to prove sexual contact. (c) DNA analysis will be performed in the forensic laboratory regardless of the examiner’s findings on a smear. (d) Precious DNA evidence studies may be wasted by preparing a smear.

5. Evidence Management

Good evidence management must properly ensure procedures in the sequence ranging between selecting and collection, packing, sealing, labeling, and insertion into the kit, its storage, preservation and transportation, and reception by the forensic laboratory, always ensuring the compliance of chain of custody.

5.1. Evidence Selection

The details of the sexual assault history and the physical exam should guide the examiner for evidence collection [3, 4]. During the physical examination, an alternate light source may assist the detection of some findings (that may need special techniques for visualization such as injuries) which may be invisible to the naked eye [57, 58]. Lamps are also an effective alternative to chemical-based screening tests.

Semen is very fluorescent in nature and the fluorescence can be observed on dark as well as light textiles when illuminated with an intense UV light, without the need for using colored goggles. To detect semen the standard Wood’s lamp (wavelength 360 nm), often used during SAS examinations, has been shown to be ineffective since several creams and ointments fluoresce in a similar manner to semen [59]. Instead, other light sources, with appropriate filters [59], may be used with the understanding that relatively fresh semen might be more easily observed with the naked eye than with an alternative light source [29]. Application has been possible on skin surfaces and vaginal, anal, and pharyngeal mucosa. The Polilight has also been considered a useful light source to detect biological samples such as semen, oral fluid, and bloodstains (e.g., on clothing) [60–62].

5.2. Evidence and Reference Sample Collection

In ASA cases, biological fluids collected on cotton-tipped swabs are the most important items of trace evidence. However, all evidence must be collected since, in most cases, it is not possible to collect it later on even if evidence is still intact, the chain of custody may already be “broken” and evidence will be compromised and therefore should not be analyzed since it will not be admissible in a court of law. For this reason, it is advisable to collect any evidence relevant to the case even though only some samples may be subjected to laboratory analysis [7, 50].

The technique and materials used to collect evidence depend on the type of evidence and its support. For DNA analysis, swabs are usually preferred to collect semen and other fluids, but different techniques exist for hair collection, for example. The presence of inhibitors is another limitation that sometimes examiners have to face [16]. Indeed, substances such as indigo dye present in denim affect the PCR amplification and therefore compromise the DNA results [63, 64].

5.2.1. Swab Techniques

Depending on the purpose, swabs of different design, shape, and size are commercially available and should be judiciously selected. Synthetic swabs (e.g., flocked nylon) are now available and are proved to be more efficient at releasing cells during the extraction [26] than cotton-tipped ones. Generally, the collection should be done by gently (to prevent exfoliation of the victim’s own epithelial cells) rubbing in a circular motion for 15 seconds, a restricted area of the mucosa or skin, from the periphery to the center and rotating the swab. In the following, specific collection procedures are briefly outlined according to surface type examined [3, 46]: (a) For dry surfaces (e.g., skin) the swabs should be slightly moistened with 1-2 drops of sterile distilled water, “damp swab.” Phosphate buffered saline (PBS) is also advocated (e.g., penile swab) since it prevents cells rupturing or shriveling up due to osmosis. Therefore, visualization of spermatozoa or vaginal epithelial cells from swabs is more prone to be successful, especially if the number is reduced. PBS does not affect subsequent DNA analysis. The same is not true for certain saline solutions and tap water due to electrolytes content and pH [65]. (b) For mucosa/epithelium (e.g., oral) or other wet surfaces, a dry swab should be used. (c) To collect evidence from underneath fingernails, a damp, small, and thin tip swab should be used to be able to reach under the fingernails.

All collected swabs should be air-dried at room temperature for a minimum of one hour [22]. To accelerate drying, a cold hair-dryer (not heat to dry swabs) or a swab dryer may be used [22] and the chosen procedure should be described in the FME report. Nevertheless, it should be highlighted that the use of a cold hair-dryer to dry swabs is controversy since it could promote cross-contamination of samples by having biological material fly inside the hood or over the area where swabs are being dried and on the hair-dryer itself. Some of this material could end up on subsequent swabs to be dried. Therefore it is mandatory that each swab is dried separately and the hood/area/dryer must be thoroughly cleaned between samples to prevent contamination.

5.2.2. Other Techniques

Other techniques to collect biological evidence (e.g., loose hairs) or physical evidence may be used, such as disposable plastic tweezers, combs, or scrapers (the latter for fingernail evidence). All specimens should be collected separately and packaged inside a little paper bag or in a bindle (piece paper folded in order to hold evidence at the center, avoiding loss in the folds of the envelope or bag the sheet is folded in half and then folded again into three equal parts).

In case of stains on substrates that can be brought in the laboratory (e.g., clothes), collection should be performed in the laboratory and not on-site. Clothes or other items should be collected separately in appropriate paper bags [3].

Disposable plastic pipettes may be useful to collect liquid remains and, in this case, the material must be packed in a tube or another suitable container.

Tape adapted to evidence collection (e.g., on clothing or other support) could be repetitively applied at the suspected sites and then placed directly in the DNA extraction tube [66].

If a broken fingernail is collected as evidence, the cut should be performed away from the broken area.

5.2.3. Reference

Reference biological sample from the suspect and the victim should be collected to perform comparative DNA testing and then correctly labeled to avoid confusion with the evidence [67]. An oral fluid reference sample is usually performed using a foam swab [4]. Most frequently, appropriate reference swabs (e.g., serrated) are used to vigorously rub the oral mucosa (inner cheek) in order to collect some mucosal cells, as rapidly and painlessly as possible.

If oral-genital contact is suspected, blood or hair sample may be preferred to act as reference sample since oral sample might be contaminated with the perpetrator’s DNA. A blood sample should be collected by venipuncture and deposited onto an appropriate support (e.g., clean white cotton fabric, cellulose, and FTA paper (promotes long term storage of the blood/DNA at room temperature)). If it is suspected that a blood transfusion or bone marrow transplantation has been performed, the blood sample is not advisable and a hair or oral sample should be collected instead [68]. To collect a hair sample, at least 7 hairs should be pulled out in order to keep the roots intact, where the DNA is concentrated [69]. Nevertheless, it should be noted that the amount of DNA in each hair (e.g., on average 200 ng of DNA compared to 10 ng in fallen hairs) depends on the anatomic place of collection (e.g., head, beard, or pubis) and varies between individuals [68]. Moreover, the hair melanin is an important inhibitor of PCR DNA amplification, and therefore roots are preferable. Hair chemical treatments may also decrease the DNA yields.

5.3. Labeling, Packaging, and Storage of Evidence

Evidence must be properly labeled and packaged, in order to ensure that evidence is not lost, damaged, or contaminated until handled by the laboratory personnel and to guarantee that reliable results and the chain of custody compliance to evidence be admissible in a court of law. Therefore, all professionals involved need to respect strict procedures [7, 48, 70, 71] that will be briefly discussed: (a) Each evidence must be dried before packaging. If a damp swab or other biological evidence is placed in a plastic or glass container, it will create a favorable environment to the development of bacteria and fungus, thus accelerating the degradation of DNA. If drying is not possible, evidence should be frozen (e.g., hygienic pads or tampons with blood). (b) Paper foldable racks, packages, or bags are preferred for biological evidence instead of plastic or glass containers, since paper allows remaining humidity to evaporate. Plastic or glass containers could be used to package physical evidence. (c) For debris such as hairs, leaves, and fibers, a bindle can be used and then put into a paper package (double packaged). (d) All evidence should be individually sealed. (e) Self-sealing envelopes or suitable adhesive should be used. If the glue is to be moistened, this procedure should be accomplished with tap water or soaked gauze and not saliva, to prevent the contamination of DNA. (f) Evidence should be clearly labeled with at least case number, victim’s examiner’s names, collection date and time, sample type, evidence number, and the location from which the evidence was collected on the victim’s body. Ideally a barcode should be used. (g) The request forms should be carefully filled. In Figure 1 we present an example of a request form for Forensic Genetics in case of sexual assault. (h) The examiner should put the request form together with samples inside the respective kit before sealing it. (i) Packages should not be stapled and must be signed across the seal in order to detect possible fraud. A biohazard label must be affixed to the package if needed. (j) Each kit should be kept in a suitable and secure place with adequate environmental conditions (e.g., DNA samples are either stored in a refrigerator at 4°C or a freezer at −20°C to reduce microorganisms’ growth rate and to avoid DNA degradation). (k) The kit should be shipped to the laboratory as soon as possible. (l) Regardless of the transportation means, it is important to ensure that samples are not exchanged/switched from time of collection to receipt in the laboratory. (m) All individuals handling samples must sign appropriate chain of custody report to track documentation regarding date, time, and names.

6. Discussion

Biological evidence is very important, especially in ASA, since it may prove the existence of sexual contact and lead to the identification of a perpetrator. Knowing and respecting the best practices of evidence management is essential to ensure that evidence (sometimes found in low quantities) is not lost, destroyed, or contaminated and to guarantee reliable results and the admissibility of evidence in the court of law. Carelessness or ignorance of proper handling procedures can result in a sample unsuitable for analysis and in the acquittal of a perpetrator. With this work we intended to summarize and harmonize FME procedures with regard to evidence management for DNA analysis, specifically the selection, collection, packaging, storage, preservation, and transportation of the evidence to the laboratories. Knowing and respecting the good practices of evidence management is essential to ensure that it is not lost, destroyed, or contaminated and to guarantee reliable results and the admissibility of evidence in court of law. Carelessness or ignorance of proper handling procedures for biological evidence can result in an unfit sample for analysis and in the acquittal of the perpetrator. The victim is entitled to a fair judicial decision.

Finally, it is important to highlight all steps that any forensic medical expert should be aware of in the management of evidence for DNA analysis: (a) Sexual assault history and the physical observation which should guide the examiner for evidence collection (e.g., victim’s activities between the sexual contact and the examination, victim’s gender and age, and type of evidence). (b) Performing a proper collection, avoiding loss or contamination (specially cross-contamination). (c) Drying under suitable conditions. (d) Individualized packaging. (e) Sealing of containers. (f) Labeling and signing of packages. (g) Correct and complete filling out of forms requesting laboratory analysis. (h) Storing the evidence along with the request form into a kit box or appropriate envelope or bag, guaranteeing the adequate conditions of conservation and security. (i) Sealing of kits. (j) Labeling and signing kits each kit must be assigned a number and must contain many labels printed with this number. (k) Adequate environmental and secure storage of kits. (l) Delivering to the forensic laboratory all kits and the sealed bag with clothing, accompanied with the chain of custody forms signed by all individuals.

Conflict of Interests

The authors report no declarations of interest.


Ricardo Dinis-Oliveira acknowledges Fundação para a Ciência e a Tecnologia (FCT) for his Investigator Grant (IF/01147/2013). Authors are also thankful to Professor Laura Cainé and Dr. Maria João Porto of the National Institute of Legal Medicine and Forensic Sciences of Portugal, for the relevant collaboration in preparing the Forensic Genetics request form for sexual assault cases.


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