by: Edward O. Wilson

THE CATEGORY OF HUMAN BEHAVIOR that provides the fullest test of the genetic fitness hypothesis to date is incest avoidance. A large amount of information concerning the phenomenon has become available at different levels of biology and culture. The behavior itself is universal, or nearly so. It is also relatively clear-cut in expression. Sexual activity in all societies is relatively uncommon between siblings and between parents and their offspring ; children produced by such activity are rare; and long-term unions made with the consensual purpose of having such children are almost nonexistent.

The current explanation of incest avoidance, which combines genetic and cultural evolution, is a straight forward sociobiological exercise. Inbreeding at the level of siblings and parents and children yields a high percentage of offspring with genetic defects. Humans tend to avoid this risk by unconscious obedience to the following epigenetic rule: If a boy and girl are brought together before one or the other is thirty months of age and then raised in close domestic proximity-use the same potty, so to speak- they are devoid of later sexual interest in each other, and the very thought of it arouses an acute aversion. This emotional incapacity; fortified in many societies by a rational understanding of the consequence of inbreeding, has led to the cultural incest taboos, which prohibit incest by custom and law.

The risk of defective children from incest--inbreeding depression as it is called by geneticists - is now well understood. On average, each person carries somewhere on his twenty-three pairs of chromosomes two sites that contain recessive lethal genes. The sites can be almost anywhere on the chromosomes. They also differ in exact number and location from one person to the next. Only one of the two homologous chromosome in the affected pair carries lethals at the site; the other homologous chromosome carries a normal gene, which overrides the effects of the lethal gene. The reason is the lethality itself. When both chromosomes carry a lethal gene at a particular site, the fetus is aborted or the child dies in infancy.

Consider a woman with a lethal gene at one such site. If she is impregnated by her brother, and if their parents themselves are unrelated, her child has one chance in eight of dying as a fetus or as an infant.. If she has lethal. genes at two such sites, her child has about one chance in four of dying.

There exist in addition a horde of other recessive genes that cause crippling ana-tomical and mental defects. The total effect is that early mortality of children born of incest is about twice that of outbred children, and among those that survive, genetic defects such as dwarfism, heart deformities, severe mental retardation, deaf-mutism, enlargement of the colon, and urinary tract abnormalities are ten times more common.

The destructive consequences of incest is a general phenomenon not just in humans but also in plants and animals. Almost all species vulnerable to moderate or severe inbreeding depression use some biologically programmed method to avoid incest. Among the apes, monkeys, and other nonhuman primates the method is two— layered. First, among all nineteen social species whose mating patterns have been studied, young individuals tend to practice the equivalent of human exogamy: Before reaching full adult size they leave the group in which they were born and join another. In the lemurs of Madagascar and in the majority of monkey species from both the Old and New Worlds, it is the males who emigrate. In red colobus monkeys, hamadryas baboons, gorillas, and chimpanzees of Africa, the females leave. In howler monkeys of Central and South America, both sexes depart. The restless young of these diverse primate species are not driven out of the group by aggressive adults. Their departure appears to be entirely voluntary.

Whatever its ultimate evolutionary origin, and however else it affects reproductive success, the emigration of young primates prior to reaching full sexual maturity greatly reduces the potential for inbreeding. But the barrier against inbreeding is reinforced by a second line of resistance. This is the avoidance of sexual activity by even those individuals who remain with their .natal group. . In all the social nonhu-man primate species whose sexual development has been carefully studied, includ-ing marmosets and tamarins of South America, Asian macaques, baboons, and chimpanzees, both adult males and females display the “Westermarck effect”: They spurn individuals with whom they were closely associated in early life. Mothers and sons almost never copulate, and brothers and sisters kept together mate much less frequently than do more distantly related individuals.

This elemental response was discovered, not in monkeys and apes, hut in human beings, by the Finnish anthropologist Edward A. Westermarck and first reported in his 1891 masterwork The History of Human Marriage.. The existence of the phenomenon has gained increasing support from many sources in the intervening years. None is more persuasive than the study of “minor marriage” in Taiwan by Arthur P. Wolf of Stanford University. Minor marriages, formerly widespread in southern China, are those in which unrelated infant girls are adopted by families, raised with the biological sons in an ordinary brother—sister relationship. and later married to the sons. The motivation for the practice appears to be to insure partners for sons when an unbalanced sex ratio and economic prospects combine to create a highly competitive marriage market.

Across four decades, from 1937 to 1993. \Wolf studied the histories of 14,200 Taiwanese women contracted for minor marriage during the late nineteenth and early twentieth centuries. The statistics (14,200 separate women) were then also supplemented by personal .interviews with many of these “little daughters-in-law,” or sim-pua, as they are fondly known in the Hokkien language, as well as with their friends and relatives.

What Wolf had hit upon was a controlled--- if unintended—experiment in the psychological origins of a major piece of human social behavior. The sim-pua and their husbands were not biologically related, thus taking away all of the conceive-able factors due to close genetic similarity. . Yet they were raised in a proximity as intimate as that experienced by brothers and sisters in Taiwancse households.

The results unequivocally favor the Westermarck hypothesis. When the future wife was adopted before thirty months of age. she usually resisted later marriage with her de facto brother. The parents often had to coerce the couple to consum-mate the marriage, in some cases by threat of physical punishment. The marriages ended in divorce three times more often than “major marriages” in the same communities. They produced nearly 40 percent fewer children, and a third of the women were reported to have committed adultery, as opposed to about 10 percent of wives in major marriages.

In a meticulous series of cross-analyses, Wolf identified the key inhibiting factor as close coexistence during the first thirty months of life of either or both of the partners. The longer and closer the association during this critical period, the stronger the later effect. Wolf’s data allow the reduction or elimination of other imaginable factors that might base play a role, including the experience of adoption, financial al status of the host family, health, age at marriage, sibling rivalry, and the natural aversion to incest that could have arisen from confusing the pair with true, genetic siblings.

A parallel unintended experiment has been performed iii Israeli kibbutzim, where children are raised in creches as brothers and sisters in conventional families. The anthropologist Joseph Shepher and his co-workers reported in 1971 that among 2,769 marriages of young adults reared in this environment, none was between members of the same kibbutz peer group who had lived together since birth. There was not even a single known case of heterosexual activity, despite the fact that the kibbutz adults were not especially opposed to it.

From these examples, and a great deal of additional anecdotal evidence gleaned from other societies, it is evident that the human brain is programmed to follow a simple rule of thumb: have no sexual interest in those whom you knew intimately during the earliest years of your life.

The Westermarck effect is also consistent with the principle of graded effect in psychology. The evidence from across many societies shows that the more intimate the association during the critical period of early childhood, the less likely is it that heterosexual activity will occur. Hence mother-son incest, which is inhibited by the intense bonding during the infancy of the son, is by far the rarest kind. Next in scarcity is sibling incest, then sexual abuse of girls by their biological fathers (I say abuse because consent is seldom given freely by the daughters), and finally sexual abuse of girls by their stepfathers.

Yet, while the evidence makes a tidy and persuasive picture, we are still far from a full explanation of incest avoidance. There is no conclusive proof that the Westermarck effect originated from genetic evolution by natural selection. Certainly all the signs point that way. Incest avoidance diminishes inbreeding and thereby’ increases the production of healthy offspring. Given even a small amount of genetic variability in sexual responsiveness to childhood associates, the differences in fitness based on it would have been strong enough, in population genetics theory at least, to spread the Westermarck effect throughout the population from a very low incidence to widespread occurrence in as few as ten generations . Further evidence is the occurrence of the effect in other primates, including our closest living relatives the chimpanzees, where it is unquestionably genetic, not cultural, in origin. Still, no attempt has been made to measure heritability in the human response or to discover the genes underwriting it.

A second shortcoming on the research front is that we do not know the exact psychological source of the Westermarck effect. The stimuli from child-mates that trigger the inhibition have not been pinpointed. It is not known whether they occur during play, eating together, unavoidable aggressive exchanges, or other events more subtle and perhaps only subliminally sensed. The critical stimuli could be anything, large or small, visual, auditory, or olfactory, and not necessarily understood in any ordinary adult sense. The essence of instinct as interpreted by bio-logists is that it is evoked by simple cues that need only be associated in real life with the object to which it is directed. A scent or a single touch at a critical moment can unleash complex behavior, or inhibit it.

A further complication in the story of human incest avoidance is the existence of a third barrier, incest taboos, the culturally transmitted sets of rules that prohibit sexual activity among very close relatives. Many societies permit or even encourage marriages between first cousins, especially when the bonding serves group cohesion and consolidates wealth, but forbid it between siblings and half siblings.

The taboos, being conscious inventions and not simple instinctive responses, vary enormously in detail from one society to the next. In many cultures they are interwoven with the structures of kinship classification and exogamous marriage con-tracts. In preliterate societies incest is commonly thought to be connected with cannibalism, vampirism, and malign witchcraft, each of which is punishable on its own account . Modern societies enact laws to discourage incest. During the Commonwealth and Protectorate period of England, from 1650 to the Restoration a decade later, it was punishable by death. In Scotland until 1887, it was nominally a capital offense, although transgressions seldom drew more than life imprisonment. In the United States incest has been generally treated as a felony punishable by fine, imprisonment, or both. The sexual abuse of children is considered all the more abhorrent when it is in addition incestuous.

History, as ever true for human mores generally, records exceptions. Societies with some degree of permissiveness have included the Incas, Hawaiians, Thais, ancient Egyptians, Nkole (Uganda), Btinyoro (Uganda), Canda (Uganda), Zande (Sudan), and Dahomeyans of West Africa. In each case the practice is (or in most instances was, having been discontinued) surrounded by ritual and limited to royalty or other groups of high status. In all the incestuous arrangements the male also consorted with other women, fathering outbred children in addition to ‘pure” progeny. The ruling families are or were patrilineal. The strategy yielding maximum genetic fitness for a high-ranking male is to mate with his own sister, producing children who share with him 75 percent of their genes by Common descent, instead of the usual 50 percent, and also to mate with women who are genetically unrelated and more likely to give birth to normal children. Less easily explained are the common and well—documented cases of brother-sister marriages among commoners of Roman Egypt, from about 30 B.C. to A.D. 324. Papyrus texts from the period reveal beyond reasonable doubt that at least some of the siblings engaged iii full and most unabashed sexual relations.

Incest taboos have led us, once again, to the borderland between the natural and social sciences. The question they raise is as follows: What is the relationship between the Wcstermarck effect, which is biological, and the incest taboos, which are cultural?

The issue can be drawn more sharply by distinguishing the two principal hypotheses that compete for the explanation of human incest avoidance. The first is Westermarck’s, which I will now summarize in updated language: People avoid incest because of a hereditary cpigenetic rule of human nature that they have now translated into taboos. The opposing hypothesis is that of Sigmund Freud. There is no Wcsterinarek effect, the great theoretician insisted when he learned of it. Just the opposite: Heterosexual lust among members of the same family is primal and compelling, and not forestalled by any instinctive inhibition. In order to prevent such incest, and the consequent disastrous ripping apart of family bonds, societies invent taboos. One result, which Freud developed as part of his grand scheme for psychology, is the Oedipus complex, the unresolved desire of a son for sexual gratification with his mother and his simultaneous hatred for the father, who is seen as a rival. The first choice of object in mankind,” be wrote in 1917, “is regularly an incestuous one, directed to the mother and sister of men, and the most stringent prohibitions are required to prevent this sustained infantile tendency from being

carried into effect.

Labeling the idea of the Westernarck effect “preposterous,” Freud carried the day from the very start. Tb e findings of psychoanalysis, he asserted, make the phenom-enon untenable. He also drew heavily on a rebuttal by James Frazer. The British anthropologist, classicist, and author of The Golden Bough, If the Westermarck effect really existed, Frazer reasoned, no taboos would be required. “It is not easy to see why any deep human instinct should need to be reinforced by law” That logic prevailed in textbooks and scholarly reviews for most of the rest of the twentieth century.

Wcstermarck’s response to Frazeu was simple, equally logical, and supported by growing amounts of evidence, but ignored in the triumphant onrush of of psycho-analytic theory. Individual humans, Westeumarck said, reason as follows: I am sexually indifferent to my parents and siblings. Yet occasionally I wonder what it would be like to have sex with them. The thought is repugnant! Incest is forced and unnatural. It would alter or break other bonds I have formed with them and must maintain on a day-to-day basis for my own welfare. Incest by others is by extension also repugnant to my mind, and evidently to that of others too, and so the rare cases in which it occurs should be condemmed as imoral.

Reasonable as that explanation may be, and supported by evidence, it is neverthe-less easy to see why Freud and a host of other influential social theorists reacted so vehemently to the Westermarck effect. It imperiled a foundation piece of modernist thought, calling into question what had come to be regarded as a major intellectual advance of the era. Wolf has expressed the difficulty with precision: Freud saw all too clearly that if Westernmarck was right, he was wrong. The possibility that early childhood association suppressed sexual attraction had to be denied lest the basis of the Oedipus complex crumble and with it his conception of personality dynamics, his explanation of neuroses, and his grand view of the origins of law, art, and civilization.”

The Westermarck effect rocks other boats as well. There is the matter of whether social regulation in general exists to repress human nature or to express it. And from that comes the not so trivial question of what incest taboos imply about the origins of morality. Orthodox social theory holds that morality is largely a convent-ion of obligation and duty constructed from mode and custom. The alternative view, favored by Westermarck in his writings on ethics, is that moral concepts are derived from innate emotions.

In the clash of ethical theory at least, the matter of incest avoidance can be settled empirically. Either Westermarck or Freud was factually right. The evidence now leans strongly to Westermarck. Yet there is more to incest taboos than the mere grafting of cultural conventions onto personal preference. It is also possible for people to observe the effects of inbreeding directly. They are capable of recogniz-ing in at least a vague way that deformed children are a frequent product of incest-uotis unions. William H. Durham, a colleague of Arthur Wolf’s at Stanford University, searched the ethnographic records of sixty societies chosen at random from around the world for references to any form of understanding of the consccinences of incest, he found that twenty showed some degree of such awareness. The Tlingit Amcrindians of the Pacific Northwest, for example, grasped in a straighltforward manner that defective children are often produced from matings of very close kin. Other societies not only knew that much, but also developed folk theories to also explain it. The Lapps of Scandinavia spoke of “bad blood” created by incest.

The Tikopian Polynesians thought that mara, the doom generated by partners in incest, is transmitted to their young. The Kapauku of New Guinea, in a similar theory, believed that the act of incest causes a deterioration of the vital substances of the transgressors, which is then passed on to their children. The Toradja of Sulawesi, Indonesia, were more cosmic in their interpretation. They said that whenever people mate who have certain conflicting characteristics, as between close kin, nature is thrown into confusion.

Curiously, while fifty-six of Durham’s sixty societies bad incest motifs in one or more of their myths, oniy five contained accounts of evil effects. A somewhat largei number ascribed beneficial results, in particular the creation of giants and heroes. But even here incest was viewed as something special if not abnormal.

In summary, the factual picture emerging from research on human incest avoid-ance is one of multiple, successive barriers. Up front is the Westermarck effect, the ancient sexual desensitization found in all other primates thus far, and thus likely to be universal in humans. Next there is the dispersal of the young at sexual maturity, also a universal primate trait, manifested in humans by adolescent restlessness and the formal practices of exogamous marriage. The deeper psychological motivations of the dispersal behaviors and the epigenetic rules composing them remain unknown. Finally, there are the cultural incest taboos, which enhance the Westermarck effect and dispersal. The taboos seem likely to have arisen from the Westermarck effect but also, in a minority of societies, from a direct perception of the destructive effects of inbreeding.

By translating the Westermarck effect into incest taboos, humans appear to pass from pure instinct to pure rational choice. But do they really? What is rational choice anyway? I suggest that rational choice is the casting about among alternative mental scenarios to hit upon the ones which, in a given context, satisfy the strongest epigenetic rules. It is these rules and this hierarchy of their relative strengths by which human beings have successfully survived and reproduced for hundreds of millennia. The incest avoidance case may illustrate the manner in which the co-evolution of genes and culture has woven not just part but all of the rich fabric of human social behavior.



                                                             The Unity of Knowledge

                                                                     By: Edward O. Wilson

                                                             Copyright @ 1998 by Edward O. Wilson

                                                             Chapter Eight, Fitness of Human Nature,

                                                                                  (pgs. 173 - 180)


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