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By Root 427 0

which the details of sexual intercourse itself are relevant to an evolutionary argument. Baker and Bellis began by measuring how much sperm a man produces during an ejaculation and what happens to it. They discovered that the amount of sperm retained in the vagina varies according to the woman’s tendency to have an orgasm. If she has no orgasm, or if she has an orgasm more than a minute before the male ejaculates, very little sperm remains in the vagina at all. If she has an orgasm less than a minute before him or up to forty-five minutes after him, then most of the sperm stays in. It also depends on how long it was since she last had sex: the longer the period, the more sperm stays in – unless she has what the scientists call ‘a non-copulatory orgasm’ in between. The only thing that increases the chance of conception is a ‘high-retention’ (i.e., late) orgasm during sex.

So far none of this contained great surprises; these facts were unknown before Baker and Bellis did their work (which consisted of samples collected by selected couples and of a survey of four thousand people who replied to a questionnaire in a magazine), but they did not necessarily mean very much. But Baker and Bellis also did something rather brave. They asked their subjects about their extramarital affairs. They found that in faithful women about fifty-five per cent of the orgasms were of the high-retention (i.e., most fertile) type. In unfaithful women, only forty per cent of the copulations with the partner were of this kind, but seventy per cent of the copulations with the lover were of this fertile type. Moreover, whether deliberately or not, the unfaithful women were having sex with their lovers at times of the month when they were most fertile. These two effects combined meant that an unfaithful woman in their sample could have sex twice as often with her husband as with her lover, but was still slightly more likely to conceive a child by the lover than the husband.

Baker and Bellis interpret their results as evidence of an evolutionary arms race between males and females, a Red Queen game, but one in which the female sex is one evolutionary step ahead. The male is trying to increase his chances of being the father in every way. Many of his sperm do not even try to fertilize her eggs, but instead either attack other sperm or block their passage. By these and other means the male’s sexual behaviour is designed to maximize his chances of fertilizing an egg.

But the female has evolved a sophisticated set of techniques for preventing conception except on her own terms. In particular, by judicious orgasm she can virtually decide by which of two lovers she chooses to be impregnated. Of course, women did not know this before now and therefore do not set out to achieve it. But the astonishing thing is that, if the study by Baker and Bellis proves to be right, they are doing it anyway, perhaps quite unconsciously. This, of course, is typical of evolutionary explanations. Why do women have sex at all? Because they consciously want to. But why do they consciously want to? Because sex leads to reproduction and, being the descendants of those who reproduced, they are selected from among those who want things that lead to reproduction. This is merely a form of the same argument: the typical woman’s pattern of infidelity and orgasm is exactly what you would expect to find if she were unconsciously trying to get pregnant by a lover, while not leaving a husband.

Baker and Bellis do not claim to have found more than a tantalizing hint that this is so, but they have tried to measure the extent of cuckoldry in human beings. In a block of flats in Liverpool, they found by genetic tests that less than four in every five people were the sons of their ostensible fathers. The rest were apparently fathered by somebody else. In case this was something to do with Liverpool, they did the same tests in southern England and got the same result. We know from their earlier work that a small degree of adultery can lead to a larger degree of cuckoldry, through the orgasm effect.