are newborn apes, birth is exceptionally painful and dangerous in humans. Until the advent of modern medicine, women often died in childbirth, whereas I have never heard of such a fate befalling a female gorilla or chimpanzee. Once humans had evolved enough intelligence to associate conception with copulation, oestrous women could have chosen to avoid copulating at the time of ovulation, and could have thereby spared themselves the pain and peril of childbirth, but such women would have left fewer descendants than women who could not detect their ovulation. Thus, where male anthropologists saw concealed ovulation as something evolved by women for men (Theories 1 and 2), Nancy Burley sees it as a trick that women evolved to deceive themselves.
Which of these six theories for the evolution of concealed ovulation is correct? Not only are biologists uncertain; it is only in recent years that the question has begun to receive serious attention. This dilemma exemplifies a pervasive problem in establishing causation in evolutionary biology, as well as in history, psychology, and many other fields where one cannot manipulate variables to perform controlled experiments. Such experiments would afford the most convincing way to demonstrate cause or function. If we could remodel one tribe of people so that all women advertised their day of ovulation, we could then see whether cooperation within or between couples broke down, or whether the Women used their new knowledge to avoid becoming pregnant. In the absence of such experiments, we can never be certain what human society Would really be like today without concealed ovulation. If it is hard to determine the function of things happening today under our eyes, how much harder must it be to determine functions in the vanished past! We know that human bones and tools were different hundreds of thousands of years ago, when concealed ovulation may have been evolving. Probably human sexuality, including the function of concealed ovulation, may also have been different then, in ways now hard for us to picture. Interpretation of our past runs the constant risk of degenerating into mere 'paleopoetry' stories that we spin today, stimulated by a few bits of fossil bone, and expressing like Rohrschach tests our own personal prejudices, but devoid of any claim to validity about the past.
Nevertheless, having mentioned six plausible theories, I cannot just walk away from the problem without attempting some synthesis. Here again, we come up against another pervasive problem in establishing causation. It is rare for complex phenomena such as concealed ovulation to be influenced by only a single factor. It would be as silly to seek a single cause of concealed ovulation as to claim that there was a single root cause of the First World War. Instead, there were many independent factors in the period 1900–1914 pushing towards war, others pushing towards peace. War finally broke out when the net weight of factors tipped towards war. Yet that does not excuse going to the opposite extreme of 'explaining' complex phenomena by an unweighted laundry list encompassing every conceivable factor.
As a first step towards pruning down our laundry list of six theories, let's realize that, whatever factors caused our distinctive sexual habits to evolve in the distant past, they would not be persisting today if there were not some factors still sustaining them. But the factors responsible for their initial appearance need not have been the same as the ones now operative. In particular, although the factors behind Theories 3, 5, and 6 may have been major ones long ago, they do not seem to be so now. Only a minority of modern women uses sex either to obtain food or other resources from a number of men, or to confound paternity and induce many men simultaneously to support a woman's child. Postulates of their former role are paleopoetry, albeit plausible paleopoetry. Let's just content ourselves with trying to understand why concealed ovulation and frequent private copulation might make sense now. At least, our guesses can be guided by introspection about ourselves plus observations of others.
The factors behind Theories 1, 2, and 4 seem to me still operative today, and to be facets of the same paradox, the most distinctive feature of human social organization. That paradox is that a man and woman desirous for their child (and genes) to survive must cooperate with each other for a long time to rear their child,
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
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
