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in sexual variety. A female human being does not have to share her sexual favours with many males to prevent infanticide, but she may have a good reason to share them with one well-chosen male apart from her husband. This is because her husband is, almost by definition, usually not the best male there is – else how would he have ended up married to her? His value is that he is monogamous and will therefore not divide his child-rearing effort among several families. But why accept his genes? Why not have his parental care and some other male’s genes?

In describing the human mating system, it is hard to be precise. People are immensely flexible in their habits depending on their racial origin, religion, wealth and ecology. None the less, there are some universal features that stand out. First, women most commonly seek monogamous marriage – even in societies that allow polygamy. Rare exceptions notwithstanding, they want to choose carefully and then, so long as he remains worthy, monopolize a man for life, gain his assistance in rearing the children and perhaps even die with him. Second, women do not seek sexual variety per se. There are exceptions, of course, but fictional and real women regularly deny that nymphomania holds any attractions for them and there is no reason we should disbelieve them. The temptress interested in a one-night stand with a man whose name she does not know is a fantasy fed by male pornography. Lesbians, free of constraints imposed by male nature, do not suddenly indulge in sexual promiscuity; on the contrary, they are remarkably monogamous. None of this is surprising: female animals gain little from sexual opportunism, for their reproductive ability is limited not by how many males they mate with but by how long it takes to bear offspring. In this respect men and women are very different.

But third, women are sometimes unfaithful. Not all adultery is caused by men. Though she may rarely or never be interested in casual sex with a male prostitute or stranger, a woman, in life as in soap opera, is perfectly capable of accepting or provoking an offer of an affair with one man whom she knows, even if she is ‘happily’ married at the time. This is a paradox. It can be resolved in one of three ways. We can blame adultery on men, asserting that the persuasive powers of seducers will always win some hearts, even the most reluctant. Call this the ‘Dangerous Liaisons’ explanation. Or we can blame it on modern society and say that the frustrations and complexities of modern life, of unhappy marriages and so on, have upset the natural pattern and introduced an alien habit into human females. Call this the ‘Dallas’ explanation. Or we can suggest that there is some valid biological reason for seeking sex outside marriage without abandoning the marriage – some instinct in women not to deny themselves the option of a sexual plan B when A does not work out so well. Call this the ‘Emma Bovary’ strategy.

I am going to argue in this chapter that adultery may have had a big part to play in shaping human society, because there have often been advantages to both sexes from within a monogamous marriage, in seeking alternative sexual partners. This conclusion is based on studies of human society, both modern and tribal, and on comparisons with apes and birds. By describing adultery as a force that shaped our mating system, I am not ‘justifying’ it. Nothing is more ‘natural’ than that people should have evolved the tendency to object to being cuckolded or cheated on, so if my analysis were to be interpreted as justifying adultery, it would be even more obviously interpreted as justifying the social and legal mechanisms for discouraging adultery. What I am claiming is that adultery and its disapproval are both ‘natural’.

In the 1970s, Roger Short, a British biologist who later moved to Australia, noticed something peculiar about ape anatomy. Chimpanzees have gigantic testicles; gorillas have minuscule ones. Although gorillas are four times the weight of chimpanzees, chimpanzees’ testicles weigh four times as much as gorillas