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Female gibbons, also, live alone. Male gibbons are capable of defending the home ranges of up to five females, and they could easily practise the same kind of polygamy as orang-utans: one male patrolling the territories of five females and mating with them all. What is more, male gibbons are of little use as fathers. They do not feed the young, they do not protect them from eagles, they do not even teach them much. So why do they stick with one female faithfully? The one enormous danger to a young gibbon that its father can guard against is murder by another male gibbon. Robin Dunbar of University College, London, believes that male gibbons are monogamous to prevent infanticide.5

A female gorilla is as faithful to her husband as any gibbon; she goes where he goes and does what he does. And he is faithful to her, in a manner of speaking. He stays with her for many years and watches her raise his children. But there is one big difference from gibbons. He has several females in his harem and is, as it were, equally faithful to each. Richard Wrangham of Harvard University believes the gorilla social system is largely designed around the prevention of infanticide, but that for females there is safety in numbers (for fruit-eating gibbons there is not enough food in a territory to feed more than one female). So a male keeps his harem safe from the attentions of rival males and pays his children the immense favour of preventing their murder.6

The chimpanzee has further refined the anti-infanticide strategy by inventing a rather different social system. Because they eat scattered but abundant food like fruit, and spend more time on the ground and in the open, chimpanzees live in larger groups (a big group has more pairs of eyes than a small group) that regularly fragment into smaller groups before coming back together. These ‘fission–fusion’ groups are too large and too flexible for a single male to dominate. The way to the top of the political tree for a male chimpanzee is by building alliances with other males, and chimpanzee troops contain many males. So a female is now accompanied by many dangerous stepfathers. Her solution is to share her sexual favours more widely with the effect that all the stepfathers might be the father. As a result, there is only one circumstance in which a male chimpanzee can be certain that an infant he meets is not his: when he has never seen the female before. And, as Jane Goodall found, male chimpanzees attack strange females that are carrying infants and kill the infants. They do not attack childless females.7

Hrdy’s problem is solved. Female promiscuity in monkeys and apes can be explained by the need to share paternity among many males to prevent infanticide. But does it apply to mankind?

The short answer is no. It is a fact that stepchildren are sixty-five times more likely to die than children living with their true parents8 and it is inescapable that young children often have a terror of new stepfathers that is hard to overcome. But neither of these facts is of much relevance, for both apply to older children, not to suckling infants. Their deaths do not free the mother to breed again.

Moreover, the fact that we are apes can be misleading. Our sex lives are very different from those of our cousins. If we were like orang-utans, women would live alone and apart from each other. Men, too, would live alone, but would each visit several women (or none) for occasional sex. If two men even met there would be an almighty, violent battle. If we were gibbons, our life would be unrecognizable. Every couple would live miles apart and fight to the death any intrusion into their home range – which they would never leave. Despite the occasional anti-social neighbour, that is not how we live. Even people who retreat to their sacred suburban homes do not pretend to remain there forever, let alone keep out all strangers. We spend much of our lives on common territory, at work, or shopping, or at play. We are gregarious and social. We