Human testis size also seems to me an outcome of that same basic paradox of human social organization. While our testes are larger than a gorilla's, because we have frequent sex for fun, they are still smaller than a chimpanzee's, because we are more monogamous. The oversized human penis may have evolved as an arbitrary sexual display symbol, as arbitrary as a lion's mane or a woman's enlarged breasts. Why were lionesses not the ones to develop enlarged breasts, lions an oversized penis, and men a mane? If they had, those permuted signals could have functioned equally well. That it did not come out that way may bejust an accident of evolution, a result of each species' and sex's relative ease of evolving those various structures. But there is still something basic missing from our discussion so far. I have talked about an idealized form of human sexuality: monogamous couples (plus a few polygynous households), husbands confident in their paternity of their wives' children, and husbands helping their wives with child-rearing rather than neglecting the kids in order to philander. As justification for discussing this fictitious ideal, I maintain that actual human practice is much closer to this ideal than to baboon or chimpanzee practice. But the ideal is still fictitious. Any social system with rules of conduct is open to the risk of individuals cheating when they find the advantages of cheating to outweigh the burden of sanctions. The question is thus a quantitative one. Does cheating become so regular that the whole system collapses, or does cheating occur but not so often as to destroy the system, or is cheating vanishingly rare? As translated for human sexuality, that question becomes one of whether ninety, thirty or one per cent of human babies are fathered extramaritally. That question and its consequences will be the subject of the next chapter.
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 maternity—the 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.