Sexually, the human male has changed dramatically from his monkey and ape relatives. While they usually have only one sexual strategy, he has two. The first is to fall in love and form a pair-bond with a particular female.

In evolutionary terms the advantage of human pair-formation is that, in a small tribe, it shares out the females between the male hunters. Unlike other male primates, human males had to cooperate on the hunt. Alone, the individual hunter was not fast enough or strong enough to defeat his prey. The leading hunter needed the active cooperation of his male companions. If the leading male kept all the females in the tribe for himself, he could hardly expect to be given active assistance by the other males on the hunt.

There was a second, important advantage in the pair-formation system of tribal mating. It created a family unit in which the children knew their father as well as their mother. And it added paternal care to the maternal care found in other species. At a stroke, it doubled the parental protection that children received. Powerful paternal feelings are unleashed the moment a human father holds his new baby in his arms, and in the years ahead he will devote a great deal of time and attention to the rearing of his offspring.

The reason evolution has changed the human male in this way and converted him into a good father is that our species has such a heavy parental burden that one adult cannot cope alone: the human mother has a serial litter to care for. Her babies are helpless at birth, demanding endless attention in the early months, and they are still completely dependent on her a year or two later, when the next baby arrives. And so on, until the mother is caring for a whole brood. Having the support of a protective, loving father during this process provides a major boost to the survival chances of the young ones. From the male’s point of view, the stronger his paternal feelings the more chance he has of seeing his genetic progeny thrive.

I said earlier that the human male has not one, but two sexual strategies. The first, as we have seen, is to devote a huge amount of time and effort to his family unit, ensuring that his children have the very best chance of survival.

The second is the more primitive one of scattering his seed wherever and whenever he gets the chance. If he finds himself in the company of an adult female who is not his family partner, he may feel the urge to engage in a brief bout of sexual activity with her, even if he will never encounter her again. If she produces a child as a result of this brief encounter, he will take no part in its rearing and may not even know of its existence. Lacking his paternal care, the baby will have less chance of survival than one inside a carefully protected family unit, but it will not be without any chance. Furthermore, in instances where the female concerned is already paired with another male, her permanent male partner may believe that the child is his and give it his full protection. In this way it will have an excellent chance of survival.

The inevitable question arises of why a paired human female should risk mating with a strange male when she has a permanent partner available to make her pregnant. If she is discovered it could cause serious damage to the stability of her family unit, yet it does still happen. The reason appears to be that the human female is programmed to assess human males in two different ways. In one assessment, she rates them on their supportive qualities. She senses how well they will look after her and her offspring, and how socially successful and reliable they are. In the other assessment, she rates them according to their physical fitness. Do their bodies look as though they will pass on good genes to her offspring? In an ideal relationship, the female’s permanent partner will be both reliably supportive and physically impressive, and she will have no genetic reason to stray. But if she has chosen a partner primarily for qualities of protection and caring, then she may be tempted, from time to time, to engage in a little sexual activity outside the family unit.

Today efficient DNA testing has at last made it possible to ascertain, with some accuracy, how many children are the result of the human male’s pair-bonding strategy and how many are the result of his more ancient seed-scattering strategy.

Even within a society that is, in comparison with earlier times, sexually liberated, 24 out of every 25 children are the result of the pair-bonding strategy and only one is the outcome of the seed-scattering strategy. It is clear from this that, despite his well-documented urge for philandering, the human male is essentially a pair-forming being.

How, then, can one explain the male’s endlessly roving eye? He may not father too many extramural children, but this does not mean that he is as faithful to his mate as a strict pair-bond should imply. The answer comes from an evolutionary trend that has led to the human species becoming increasingly childlike during the past million years or so. The value of this trend, called neoteny, has been that it has resulted in human beings retaining their childlike sense of playfulness and curiosity well into adulthood. Thishas enabled them to become increasingly innovative, leading to all the ingenious inventions that have given us our complex modern technologies. But at the same time it has led to this heightened level of curiosity spilling over into other aspects of life.

When an already paired male sees an attractive stranger of the opposite sex, his curiosity makes him wonder what it would be like to enjoy her sexually. In the vast majority of cases he is able to keep his curiosity operating at the level of sexual fantasy, but he also occasionally goes farther. Usually, once he has satisfied his curiosity, that is the end of the matter, but in many instances there is a disruption of the original pair-bond and often the formation of a new one. This inevitably reduces the quality of his paternal caring for the children of his original relationship.

Major family disruptions of this kind were more difficult in the small tribal communities where male reproductive patterns evolved. But modern society is more complex and the opportunities for pair-bond disruption are so much greater that the ancient system is coming under increasing strain in modern times. The divorce rate has increased dramatically and, although some figures quoted are wild exaggerations, it seems that, in 21st-century America, of those who marry, 34 per cent will experience divorce. The figure for the UK is 36 per cent.

So it seems that about a third of modern pair-bonds collapse, a fact used by anxiety-makers as a sign that society is in decay. Another way of looking at the situation, however, is that, even in the midst of modern decadence and liberal attitudes, two thirds of modern couples manage to make their pair-bonds work. Given the highly unnatural structure of urban society, to which the human tribal animal has had to adapt, this is a testimony to the tenacity of the pair-bonding mating strategy.

One argument heard is that, if pair-bonding is such a basic feature of the human species, why is it not total? If it was so valuable to tribal groups, why did evolution not make it permanent? There are stories about birds that pair for life with such an intensity that when one of them dies, the surviving partner never takes a new mate. Why did evolution not develop this extreme mechanism for humans and avoid all the misery of marriage collapse?

The answer, it seems, is that in primeval times, when the mating strategy was developing, the males were facing serious dangers in their search for prey and the females were facing difficult births due to the vertical posture of our species. Either member of a mated pair could easily die young, and this would leave the surviving partner reproductively stranded if the pairing mechanism was too rigid.

If, on the other hand, after a period of distress and mourning, the surviving young adults could find it possible to form new pair-bonds, then the reproductive rate of the very small tribes could benefit. So an almost perfect pair-bond would be better, from a survival point of view, than a perfect one. In evolutionary terms, then, the human male is programmed to form a long-term relationship with a female partner, but with the reproductively valuable possibility that, should she die, he can, after a while, form a new pair. It is this slight weakening of the bond of attachment, so useful in early days, that has been magnified into a serious problem in modern times.

The main reason for this has been the way in which the hunting pattern of the male has changed. Instead of setting out on a dangerous pursuit of prey, he now sets off for the city to engage in a different kind of hunt. Once there, he will find himself in the company of young women, who were absent from the primeval hunting fields. There were few temptations out on the savannah, but in the big city they are all around him. And this is where his less-than-per-fect tendency to stay exclusively bonded to his mate lets him down.

© Desmond Morris 2008 Extracted from The Naked Man: A Study of the Male Body, by Desmond Morris, published by Jonathan Cape on January 3. RRP £18.99, available from Times Books First for £17.09, free p&p: 0870 1608080. timesonline.co.uk/booksfirst