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If men are opportunist-polygamists at heart, as I argued in the last chapter, then whence comes marriage? Although men are fickle (‘You’re afraid of commitment, aren’t you?’ says the stereotypical victim of a seducer), they are also interested in finding wives with whom to rear families and might well be very set on sticking by them despite their own infidelity (‘You’re never going to leave your wife for me, are you?’ says the stereotypical mistress).

The two goals are contradictory only because women are not prepared to divide themselves neatly into wives and whores. Woman is not the passive chattel that the tussles of despots, described in the last chapter, have implied. She is an active adversary in the sexual chess game, and she has her own goals. Women are and always have been far less interested in polygamy than men. But that does not mean they are not sexual opportunists. The eager-male/coy-female theory has a great deal of difficulty answering a simple question. Why are women ever unfaithful?

The Herod Effect

In the 1980s, a number of women scientists, led by Sarah Hrdy, now of the University of California at Davis, began to notice that the promiscuous behaviour of female chimpanzees and monkeys sat awkwardly alongside the Trivers theory that heavily female-biased parental investment leads directly to female choosiness. Hrdy’s own studies of langurs and the studies of macaques by her student Meredith Small seemed to reveal, instead of the stereotype of evolutionary theory, a very different kind of female: a female that sneaked away from the troop for assignations with males; a female that actively sought a variety of sexual partners; a female that was just as likely as a male to initiate sex. Far from being choosy, female primates seemed to be initiators of much promiscuity. Hrdy began to suggest that there was something wrong with the theory rather than the females. A decade later, it is suddenly clear what: a whole new light has been shed on the evolution of female behaviour by a group of ideas known as ‘sperm competition theory’.3

The solution to Hrdy’s concern lay in her own work. In her study of the langurs of Abu, in Rajasthan, Hrdy had discovered a grisly fact: the murder of baby monkeys by adult male monkeys was routine. Every time a male takes over a troop of females, he kills all the infants in the group. Exactly the same phenomenon was also discovered in lions a short time later: when a group of brothers wins a pride of females, the first thing they do is slaughter the innocents. In fact, as subsequent research revealed, infanticide by males is common in rodents, carnivores and primates. Even our closest relatives, the chimpanzees, are guilty. Most naturalists, reared on a diet of sentimental natural history programmes on television, were inclined to believe they were witnessing a pathological aberration, but Hrdy and her colleagues suggested otherwise. The infanticide, they said, was an ‘adaptation’ – an evolved strategy. By killing their stepchildren, the males would halt the females’ milk production and so bring forward the date on which the mother could conceive once more. An alpha male langur, or a pair of brother lions, have only a short time at the top and infanticide helps them to father the maximum number of offspring during that time.4

The importance of infanticide in primates gradually helped scientists to understand the mating systems of the five species of apes, because it suddenly provided a reason for females to be loyal to one male or a group of males, and vice versa: to protect their genetic investment in each other from murderous rival males. Broadly speaking, the social pattern of female monkeys and apes is determined by the distribution of their food, while the social pattern of males is determined by the distribution of females. Thus female orang-utans choose to live alone in strict territories, the better to exploit their scarce food resources. Males also live alone and try to monopolize the territories of several females. The females that live within his territory expect