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FOUR

THE SCIENCE OF ADULTERY

Cold-blooded analysis of adultery views life as an evolutionary contest whose winners are those individuals leaving the largest number of surviving offspring. This view helps one understand why humans reinvented adultery after the other two chimps had bypassed it.

People have many reasons to lie when asked whether they have committed adultery. Consequently, it is notoriously difficult to get accurate scientific information about this important subject. One of the few existing sets of hard facts emerged as a totally unexpected by-product of a medical study, performed nearly half-a-century ago for a different reason. That study's findings have never been revealed until now.

I recently learned those facts from the distinguished medical scientist who ran the study. (Since he does not wish to be identified in this connection, I shall refer to him as Dr X.) In the late 1940s Dr X was studying the genetics of human blood groups, which are molecules that we acquire only by inheritance. Each of us has dozens of blood-group substances on our red blood cells, and we inherit each substance either from our mother or from our father. The study's research plan was straightforward: go to the obstetrics ward of a highly respectable US hospital; collect blood samples from 1,000 newborn babies and their mothers and fathers; identify the blood groups in all the samples; and then use standard genetic reasoning to deduce the inheritance patterns.

To Dr X's shock, the blood groups revealed nearly ten per cent of those babies to be the fruits of adultery! Proof of the babies' illegitimate origin was that they had one or more blood groups lacking in both alleged parents. There could be no question of mistaken maternitythe blood samples were drawn from an infant and its mother soon after the infant emerged from the mother. A blood group present in a baby but absent in its undoubted mother could only have come from its father. Absence of the blood group from the mother's husband as well showed conclusively that the baby had been sired by some other man, extramaritally. The true incidence of extramarital sex must have been considerably higher than ten per cent, since many other blood-group substances now used in paternity tests were not yet known in the 1940s, and since most bouts of intercourse do not result in conception. At the time that Dr X made his discovery, research on American sexual habits was virtually taboo. He decided to maintain a prudent silence, never published his findings, and it was only with difficulty that I got his permission to mention his results without betraying his name. However, his results have more recently been confirmed by several similar genetic studies whose results did get published. Those studies variously showed between about five and thirty per cent of American and British babies to have been adulterously conceived. Again, the proportion of the tested couples of whom at least one practised adultery must have been higher, for the same two reasons as in Dr X's study.

We can now answer the question posed at the end of the last chapter: whether extramarital sex is for humans a rare aberration, a frequent exception to a 'normal' pattern of marital sex, or so frequent as to make a sham of marriage. The middle alternative proves to be the correct one. Most fathers really are raising their own children, and human marriage is not a sham. We are not just promiscuous chimpanzees pretending to be otherwise. Yet it is also clear that extramarital sex is an integral, albeit unofficial, part of the human mating system. Adultery has also been observed in many other animal species whose societies resemble ours in being based on male and female co-parents with a lasting bond. Since such lasting bonds do not characterize common chimpanzee or pygmy chimpanzee society, it is meaningless to talk of adultery in chimps. We must have reinvented it after our chimp-like ancestors had rendered it obsolete. Therefore, we cannot discuss human sexuality, and its role in our rise to humanity, without carefully considering the science of adultery. Most of our information about adultery's incidence has come from researchers asking people about their sex lives, rather than from blood-grouping their babies. Since the 1940s, the myth that marital infidelity is rare in the US has been publicly exploded by a long succession of surveys, beginning with the Kinsey report. Nevertheless, even though this is the supposedly liberated 1990s, we are still profoundly ambivalent about adultery. It is thought of as exciting; no television soap opera could attract many viewers without it. It has few rivals as a basis of humour. Yet, as Freud pointed out, we often use humour to deal with things that are intensely painful. Thus, throughout history, adultery has also had few rivals as a cause of murder and human misery. In writing about this subject, it is impossible to remain completely serious, but it is also impossible not to be revolted at the sadistic institutions by which societies have attempted to deal with extramarital sex.

What makes a married person decide to seek or avoid adultery? Scientists have theories to explain many other things, so it should not be surprising that there is also a theory of extramarital sex (abbreviated to EMS, and not to be confused with premarital sex or PMS, in turn not to be confused with premenstrual syndrome, also PMS). With many species of animals the problem of EMS never arises, because they do not opt for marriage in the first place. For instance, a female Barbary macaque in heat copulates promiscuously with every adult male in her troop and averages one copulation per seventeen minutes. However, some mammals and most bird species do opt for 'marriage'. That is, a male and a female form a lasting pair-bond to devote care or protection to their joint offspring. Once there is marriage, there is also the possibility of what socio-biologists euphemistically term 'the pursuit of a mixed reproductive strategy' (abbreviated to MRS). In plain English, that means being married while simultaneously seeking extramarital sex. Married animals vary enormously in the degree to which they mix their reproductive strategies. There appears to be no recorded instance of EMS in the little apes called gibbons, while snow geese indulge regularly. Human societies similarly vary, but I suspect that none comes close to the faithful gibbons. To explain all this variation, sociobiologists have found it useful to apply the reasoning of game theory. That is, life is considered an evolutionary contest whose winners are those individuals leaving the largest number of surviving offspring. Contest rules are set by the ecology and reproductive biology of the particular species. The problem is then to figure out which strategy is most likely to win the contest: rigid fidelity, pure promiscuity, or a mixed strategy. But I must make one thing clear right at the outset. While this sociobiological approach certainly proves useful for understanding adultery in animals, its relevance for human adultery is an explosive issue and one to which I shall return. The first thing one realizes is that the best game strategy differs between males and females of the same species. This is because of two profound differences between the reproductive biology of males and females, in the minimum necessary reproductive effort, and in the risk of being cuckolded. Let's consider these differences, which are painfully familiar to humans.

For men, the minimum effort needed to sire an offspring is the act of copulation, a brief expenditure of time and energy. The man who sires a baby by one woman one day is biologically capable of siring a baby by another woman the same day. For women, however, the minimum effort consists of copulation plus pregnancy plus (throughout most of human history) several years spent nursinga huge commitment of time and energy. Thus, a man potentially can sire far more offspring than can a woman. A nineteenth-century visitor who spent a week at the court of the Nizam of Hyderabad, a polygamous Indian potentate, reported that four of the Nizam's wives gave birth within eight days, and that nine more births were anticipated for the following week. The record lifetime number of offspring for a man is 888, sired by Emperor Moulay Ismail the Bloodthirsty of Morocco, while the corresponding record for a woman is only sixty-nine (a nineteenth-century Moscow woman specializing in triplets). Few women have topped twenty children, whereas some men easily do so in polygynous societies. As a result of this biological difference, a man stands to gain much more from EMS or polygamy than does a womanif one's sole criterion is the number of offspring born. (To female readers about to stop reading in outrage, or to male readers about to cheer, I warn you nowkeep reading, there is much more to the question of EMS.) For human EMS the statistical evidence is naturally hard to come by, but for human polygamy it is readily available. In the sole polyandrous society for which I could find data, the Tre-ba of Tibet, women with two husbands average fewer children, not more children, than women with one husband. In contrast, nineteenth-century American Mormon men realized big benefits from polygyny: men with one wife averaged only seven children, but men with two wives averaged sixteen children, and those with three wives averaged twenty. Polygynous Mormon men as a group averaged 2.4 wives and fifteen children, while polygynous Mormon church leaders in particular averaged five wives and twenty-five children. Similarly, among the polygynous Temne people of Sierra Leone, a man's average number of children increases from 1.7 to seven as his number of wives increases from one to five. The other sexual asymmetry relevant to the best game strategy involves confidence that one really is the biological parent of one's putative offspring. A cuckolded animal, deceived into rearing offspring not its own, has thereby lost the evolutionary game while advancing the victory of another player, the real parent. Barring a switch of babies in the hospital nursery, women cannot be cuckolded; they see their baby emerge from their bodies. Nor can there be cuckoldry of males in animal species practising external fertilization (that is, fertilization of eggs outside the female's body). For instance, some male fish watch a female shed eggs, then immediately deposit sperm on the eggs and scoop them up to care for them, secure in their paternity. However, men and other Wale animals practising internal fertilizationfertilization of eggs inside the female's bodycan readily be cuckolded. All that the putative father knows for sure is that his sperm went into the mother, and eventually an offspring came out. Only observation of the female throughout her whole fertile period can absolutely exclude the possibility that some other male's sperm also entered and did the actual fertilizing.

An extreme solution to this simple asymmetry is the one formerly adopted by southern India's

Nayar society. Among the Nayar, women freely took many lovers simultaneously or in sequence, and husbands accordingly had no confidence in paternity. To make the best of a bad situation, a Nayar man did not live with his wife or care for his supposed children, but he instead lived with his sisters and cared for his sisters' children. At least, those nieces and nephews were sure to share one-quarter of his genes.

Bearing in mind these two basic facts of sexual asymmetry, we can now examine what is the best game strategy, and when EMS pays. Let's examine three game plans of increasing complexity:

Game Plan 1.

A man should always seek EMS, because he has so little to lose and so much to gain.

Consider the hunter-gatherer conditions prevailing throughout most of human evolution, under which a woman could at best rear about four children in the course of her life. Through one dalliance, her otherwise faithful husband could increase his lifetime reproductive output from four to five: an enormous increase of twenty-five per cent, for only a few minutes' work.

What is wrong with this dazzlingly naive reasoning?

Game Plan 2.

A moment's reflection should expose a basic flaw of Game Plan 1; it considers only the potential benefits of EMS to a man and ignores his potential costs. Obvious costs would include the risk of detection and injury or murder by the husband of the woman sought as

EMS partner; the risk that one's own wife will desert; the risk of being cuckolded by one's wife while one is off seeking EMS; and the risk that one's legitimate children will suffer through one's neglect of them. Thus, according to Game Plan 2 the would-be Casanova, like a sophisticated investor, should seek to maximize his gains while minimizing his losses. What reasoning could be more impeccably judicious?

Game Plan 3.

The man silly enough to be satisfied with Game Plan 2 has obviously never approached a lady with an offer of EMS or PMS. Worse yet, the silly man has never even thought about the statistics of human heterosexual intercourse, which dictate that, for every bout of

EMS by a man, there must be one bout of EMS (or at least PMS) by a woman.

Game Plans 1 and 2 share the flaw that they ignore considerations of the woman's strategy, without which any male strategy is doomed to failure. Hence Game Plan 3 must combine a male strategy and a female strategy. But, since one husband suffices to realize a woman's maximum reproductive potential, what could possibly attract a woman to EMS or PMS? This question puzzles the current generation of theoretical sociobiologists with a purely intellectual interest in EMS, just as it has taxed the ingenuity of would-be male adulterers throughout human history.

To proceed further with our theoretical exploration of Game Plan 3, we need rigorous empirical data on EMS. As surveys of people's sexual habits are notoriously unreliable, let's first turn to some recently published studies of birds that nest as mated pairs in large colonies. These, rather than our closest relatives the apes, are the animals whose mating system most closely resembles our own. Compared to us, birds have the disadvantage that one cannot ask them about their motives for EMS, but this is no great loss, as our answers are often lies anyway. The great virtue of colonial birds for EMS research is that one can band the birds in a colony, then sit nearby for hundreds of hours and determine exactly who does what with whom. I am unaware of equivalent information for a large human population. Important recent observations of adultery among birds were made on five species of herons, gulls, and geese. All five nest in dense colonies composed of nominally monogamous male/female pairs. One parent alone is incapable of rearing a chick, as an unguarded nest is likely to be destroyed while the parent is off gathering food, nor is a male capable of feeding or guarding two families simultaneously. Consequently, among the ground rules of sexual strategy for these colonial birds are the following: polygamy is forbidden; copulation with or by an unmated female is pointless, unless she soon acquires a mate to care for the resulting offspring; but surreptitious fertilization by one male of another male's mate is a viable strategy.

The first study involved great blue herons and great egrets at Hog Island, Texas. In these species the male builds a nest and stays there to court visiting females. Eventually a male and female accept each other and copulate about twenty times. The female then lays eggs and goes off to spend most of the daylight hours feeding, while the male remains to guard the nest and eggs. During the first day or two after pairing, the male often resumes courting any passing female as soon as his mate leaves to feed, but EMS does not result. Instead, the male's halfway-unfaithful behaviour seems to constitute 'divorce insurance' that reserves a back-up mate for him in case his own mate deserts (she does desert him in up to twenty per cent of the pairings reported). The passing 'back-up' females pursue the courtship out of ignorance. They are seeking a mate and have no way of knowing that the male is already mated, until his spouse returns (which she does at frequent intervals) and drives them off. Eventually, the male gains complete confidence that he will not be deserted, and he ceases to court any passing females.

In the second study, of little blue herons in Mississippi, behaviour that might have originated as divorce insurance took a more serious turn. Sixty-two cases of EMS were documented, mostly between a female on her nest and a male from the neighbouring nest while the female's mate was busy finding food. Most females initially resisted but then ceased resisting, and some females engaged in more EMS than marital sex. To reduce his own risk of being cuckolded, the adulterous male did his feeding as quickly as possible, returned often to his own nest to guard his mate, and travelled no further than neighbouring nests to seek EMS. EMS was usually timed to occur when the chosen female had not yet completed egg-laying and could still be fertilized. However, adulterous copulations were quicker than marital copulations (eight versus twelve seconds), hence possibly less effective at fertilizing, and nearly half of all nests involved in EMS were subsequently abandoned.

Among herring gulls in Lake Michigan, thirty-five per cent of mated males were observed to engage in EMS. This percentage is nearly the same as the value of thirty-two per cent reported for young American husbands in a study published by Playboy Press in 1974, but there is a big difference between gulls and humans in female behaviour. Whereas Playboy Press reported EMS for twenty-four per cent of young American wives, every mated female gull virtuously rejected adulterous male advances and never solicited the neighbouring male in her own mate's absence. Instead, all cases of male EMS involved unmated female gulls practising PMS. To decrease his own risk of being cuckolded, the male spent more time chasing intruders away from his nest when his mate was fertile than when she was not fertile. As for how the male induced his mate to remain faithful during the time that he was off seeking EMS, his secretlike that of some married men similarly pursuing a mixed reproductive strategyconsisted of feeding her diligently and copulating often whenever she was receptive.

Our final set of rigorous data involves snow geese breeding in Manitoba. Just as I explained in the case of little blue herons, EMS in snow geese mainly involves a male approaching an initially resisting female on a neighbouring nest in the absence of her mate. The mate's absence is usually due to the fact that he himself is off seeking EMS. It may seem as if the male thereby loses as much as he gains, but a male goose is not so dumb. As long as the female is still laying eggs, her mate remains to guard her. (A nesting female is propositioned fifty times less often in her mate's presence than in his absence.) Only after the female has finished laying does her mate go off on EMS quests, with his paternity assured at home. Such bird studies illustrate the value of a scientific approach to adultery. They have revealed a series of sophisticated strategies by which adulterous male birds try to have it both ways, so as to obtain confidence of paternity at home while sowing their seed abroad. The strategies include wooing unmated females for 'divorce insurance', as long as one feels unsure of one's wife's fidelity; guarding one's fertile spouse; feeding her copiously and copulating with her often, to induce her to remain faithful in one's absence; and coveting one's neighbour's spouse at a time when she is fertile and one's own spouse is no longer fertile. However, not even these applications of the scientific method in all its power sufficed to clarify what, if anything, female birds gain from EMS. One possible answer is that female herons weighing desertion of their mates may use EMS to shop around for a new mate. Another is that some unmated female gulls in colonies with a deficit of males may get fertilized by PMS, and then try to rear the chicks with the help of another, similar female.

The chief limitation of these colonial bird studies is that the females often seem to be unwilling participants in EMS. For further understanding of a more active female role, we have no choice but to turn to human studies, riddled as they are with problems of cultural variation, observer bias, and dubiously reliable survey responses.

Surveys comparing men with women in various cultures scattered around the world typically purport to find the following differences: men are more interested in EMS than are women; men are more interested than women in seeking a variety of sexual partners for the sake of variety 'itself; women's motives for EMS are more likely to be marital dissatisfaction and/or a desire for a lasting new relationship; and men are less selective in taking on a casual female sexual partner than vice versa. For example, among the New Guinea highlanders with whom I work, the Men say they seek EMS because sex with their own wife (or even wives, m the case of polygynous men) inevitably becomes boring, while the women who seek EMS do so mainly because their husband cannot satisfy them sexually (for example, because of old age). In the questionnaires that several hundred young Americans filled out for a computer dating service, women expressed stronger partner preferences than men did in almost every respect: intelligence, status, dancing ability, religion, race, etc. The only category in which men were more selective than women was physical attractiveness. After a date the men and women then filled out 'debriefing' questionnaires, with the result that two-and-a-half times as many men as women expressed a strong romantic attraction to their computer-selected partner. Thus, the women were choosier, the men more undiscriminating, in their reactions to partners.

Obviously, we are on shaky ground if we expect an honest answer when we ask people their attitudes about EMS. However, people also express their attitudes in laws and in their behaviour. In particular, some widespread hypocritical and sadistic features of human societies stem from two fundamental difficulties that men face in seeking EMS. Firstly, a man who pursues an MRS is trying to have it both ways: he wishes to obtain sex with other men's wives, while denying sex with his own wife (or wives) to other men. Some men therefore inevitably gain at the expense of other men. Secondly, as we have discussed, there is a realistic biological basis for men's widespread paranoia about being cuckolded.

Adultery laws provide a clear example of how men have dealt with these dilemmas. Until recently, essentially all such lawsHebraic, Egyptian, Roman, Aztec, Moslem, African, Chinese, Japanese, and otherswere asymmetrical. They existed to secure a married man's confidence in his paternity of his children, and for no other purpose. Consequently these laws define adultery by the marital status of the participating woman; that of the participating man is irrelevant. EMS by a married woman is considered an offence against her husband, who is commonly entitled to damages, often including violent revenge or else divorce with refund of the bride price. EMS by a married man is not considered an offence against his wife. Instead, if his partner in adultery is married, the offence is against her husband; if she is unmarried, the offence is against her father or brothers (because her value as a prospective bride is reduced).

No criminal law against male infidelity even existed until a French law of 1810, and that law only forbade a married man to keep a concubine in his conjugal house against his wife's wishes. Viewed from the perspective of human history, the absence or near-symmetry of modern Western adultery laws is a novelty that only appeared in the last 150 years. Even today, prosecutors, judges, and juries in the US and England often reduce a homicide charge to manslaughter of the lowest degree, or else acquit altogether, when a husband kills an adulterous wife or her lover caught in the act.

Perhaps the most elaborate system to uphold confidence of paternity was that maintained by Chinese emperors of the T'ang Dynasty. For each of the emperor's hundreds of wives and concubines, a team of court ladies kept records on dates of menstruation, so that the emperor could copulate with that wife on a date likely to result in fertilization. Dates of copulation were also recorded, and as an auxiliary form of record-keeping, were commemorated by an indelible tattoo on the woman's arm and by a silver ring on her left leg. It goes without saying that equal thoroughness was applied to excluding men other than the emperor from the harem. Men of other cultures have resorted to less complicated but even more repulsive means of ensuring paternity. These measures limit sexual access to wives, or else to daughters or sisters who would command a high bride price if delivered as proven virgin goods. Relatively mild measures include close chaperoning or virtual imprisonment of women. Similar purposes are served by the code of 'honour and shame' widespread in Mediterranean countries. (Translation: EMS for me but not for you; only the latter is a shame to my honour.) Stronger measures include the barbaric mutilations euphemistically and misleadingly termed 'female circumcision'. These consist of removal of the clitoris or most of the external female genitalia to reduce female interest in sex, marital or extramarital. Men bent on total certainty invented infibulation, suturing a woman's labia majora nearly shut, so as to make intercourse impossible. An infibulated wife can be de-infibulated for childbirth or for re-insemination after each child is weaned, and can be re-infibulated when the husband takes a long trip. Female circumcision and infibulation are still practised in twenty-three countries today, from Africa through Saudi Arabia to Indonesia. When adultery laws, imperial records, and coercive restraint still fail to ensure paternity, murder is available as a last resort. The role of sexual jealousy as one of the commonest causes of homicide emerges from studies in many American cities and in many other countries. Usually, the murderer is a husband while the victim is his adulterous wife or her lover, or else the lover kills the husband. The table on the following page gives some actual numbers for murders committed in Detroit in 1972. Until the formation of centralized political states provided soldiers with loftier motives, sexual jealousy also loomed large in human history as a cause of war. It was the seduction (abduction, rape) by Paris of Mefielaus's wife Helen that provoked the Trojan War. In the modern New Guinean highlands, only disputes over ownership of pigs rival disputes over sex in triggering war.

Asymmetric adultery laws, tattooing of wives after insemination, virtual imprisonment of women, genital mutilation of womenthese behavioural habits are unique to the human species, defining humanity as much as does invention of the alphabet. More exactly, they are new means to the old evolutionary goal of males promoting their genes. Some of our other means to this goal are ancient ones shared with many animals, including jealous murder, infanticide, rape, inter-group warfare, and adultery itself. Human male infibulators stitch the vagina closed; some male animals achieve the same result by cementing a female's vagina after copulating with her.

Sociobiologists have had considerable success at understanding the marked differences among animal species in the details of these practices. As a result of recent research, it is no longer controversial to conclude that natural selection caused animals to evolve behavioural patterns, as well as anatomical structures, that tend to maximize the number of their descendants. Few scientists doubt that natural selection moulded human anatomy. However, no theory has caused such bitter divisions among my fellow biologists today as the claim that natural selection likewise moulded our social behaviour. Most of the human behaviour discussed in this chapter is considered barbaric by modern Western society. Some biologists are outraged not only by the behaviours themselves, but also by sociobiological explanations for the evolution of the behaviours. To 'explain' a behaviour seems uncomfortably close to defending it.

Like nuclear physics and all other knowledge, sociobiology is available for abuse. People have never lacked pretexts to justify the abuse or killing of other people, but ever since Darwin formulated his theory of evolution, evolutionary reasoning has also been abused as such a pretext. Sociobiological discussions of human sexuality can be seen as seeking to justify men's abuse of women, analogous to the biological justifications advanced for whites' treatment of blacks or Nazis' treatment of Jews. In the critiques that some biologists have directed at sociobiology, two fears recur: that a demonstrated evolutionary basis for a barbaric behaviour would seem to justify it; and that a demonstrated genetic basis for the behaviour would imply the futility of attempts at change.

In my view, neither fear is warranted. As for the first, one can seek to understand how something arose, regardless of whether one considers that something admirable or abominable. Most books analysing the motives of murderers are not written in an effort to justify murder, but instead to understand its causes as a way of preventing it. As for the second fear, we are not mere slaves to our evolved characteristics, not even to our genetically acquired ones. Modern civilization is fairly successful at thwarting ancient behaviours like infanticide, and one of the main objectives of modern medicine is to thwart the effects of our harmful genes and microbes, despite our having come to understand why it is natural for those genes and microbes to tend to kill us. The case against infibulation does not collapse even if the practice can be shown to be genetically advantageous to male infibulators. Instead, we condemn it because we hold the mutilation of one person by another to be ethically loathsome.

While sociobiology is therefore useful for understanding the evolutionary context of human social behaviour, this approach still should not be pushed too far. The goal of all human activity cannot be reduced to the leaving of descendants. Once human culture was firmly in place, it acquired new goals. Many people debate today whether to have children, and many decide that they prefer to devote their time and energy to other activities. We shall reach a simiiar perspective in later chapters for other attributes as uniquely human as our sexuality, including our art and our abuse of drugs. For these activities too, one can identify animal precursors and discern original roles in promoting survival and gene transmission, but these activities also proceeded to take on a life of their own. Hence I claim only that evolutionary reasoning is valuable for understanding the origin of such human practices, and not that it is necessarily the only way to understand their current forms.

In short, we evolved, like other animals, to win the reproduction game. That contest has a single aim, to leave as many descendants as possible. Much of the legacy of that game strategy is still with us. But we have also chosen to pursue ethical goals, which can conflict with the goals and methods of the sexual contest. Having that choice among goals represents one of our most radical departures from other animals.


FEMALES, AS MALES SEE THEM | The rise and fall of the third chimpanzee | FIVE HOW WE PICK OUR MATES AND SEX PARTNERS