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were not doing so in the hope of helping to feed children they had already fathered but as direct payment for an affair. It was not easy to discover. Hill found that he was gradually forced to drop questions about adultery from his studies because the Aché, under missionary influence, became increasingly squeamish about discussing the subject. The chiefs and the headmen were especially reluctant to talk about it, which is hardly surprising in view of the fact that they were the ones having the most affairs. None the less, by relying on gossip, Hill was able to piece together the pattern of adultery in the Aché. As expected, he found that high-ranking men were involved most, which is consistent with the idea of having your paternal-genetic cake and eating it. However, unlike in birds, it was not just the wives of low-ranking men who indulged. It is true that Aché adulterers frequently ply their mistresses with gifts of meat, but Hill thinks the most important motive is that Aché women are constantly preparing for the possibility that they will be deserted by their husbands; they are building up alternative relationships. And they are more likely to be unfaithful if the marriage is going badly. That is, of course, a double-edged sword: the marriage could break up if the affair is discovered.27

Whatever the motive for women, Hill and others believe that adultery has been much underemphasized as an influence in the evolution of the human mating system. In hunter-gatherer societies, the male opportunist streak would have been far more easily satisfied by adultery than by polygamy. In only two known hunter-gatherer societies is polygamy either common or extreme. In the rest, it is rare to find a man with more than one wife and very rare to find a man with more than two. The two exceptions prove the rule. One is among the Indians of the Pacific north-west of America, who depended upon abundant and reliable supplies of salmon, and were more like farmers than hunter-gatherers in their ability to pile up surpluses. The other is certain tribes of Australian Aboriginals, which practise gerontocratic polygamy: men do not marry until they are forty and by the age of sixty-five they have usually accumulated up to thirty wives. But this peculiar system is far from what it seems. Each old man has younger assistant men, whose help, protection and economic support he purchases by, among other things, turning a blind eye to their affairs with his wives. The old man looks the other way when the helpful nephew carries on with one of his junior wives.28

Polygamy is rare in hunter-gatherer society, but wherever adultery has been looked for, it is common. By analogy with monogamous, colonial birds, therefore, one would expect to find human beings practising either mate guarding, or frequent copulation. Richard Wrangham has speculated that human beings practise mate guarding in absentia. Men keep an eye on their wives by proxy. If the husband is away hunting all day in the forest, he can ask his mother, or his neighbour, whether his wife got up to anything during the day. In the African pygmies Wrangham studied, gossip was rife and a husband’s best chance of deterring his wife’s affairs was to let her know that he kept abreast of the gossip. Wrangham goes on to observe that this is impossible without language. So he speculates that the sexual division of labour, the institution of child-rearing marriages and the invention of language – three of the most fundamental human characteristics that we share with no other ape – all depended on each other.29

Why the Rhythm Method Does not Work

What happened before language allowed proxy mate guarding? Here, anatomy provides an intriguing clue. Perhaps the most startling difference between the physiology of a woman and that of a chimpanzee is that it is impossible for anybody, including the woman herself, to determine precisely when in the menstrual cycle she is fertile. Whatever doctors, old wives’ tales and the Roman Catholic Church may say, human ovulation is invisible and unpredictable. Chimpanzees