Studies of animals show that, at least in some species, females maximise their chances of reproducing their selfish genes by cunningly cheating on their mate. With barely a pause for thought, evolutionary psychologists like to leap straight from this fact to humans.
At least in some parts of Britain (eg Glasgow), as many as a quarter of children are unrelated to the man their mother encouraged them to believe was their biological father. Cue endless TV progs and newspaper articles about the guile of the female of the species.
In fact, there are very strong grounds for doubting that many women are unfaithful to their partners once they have started having babies together. For one thing, women are half as likely as men to do it. The survey of nearly 20,000 Britons' sexual behaviour which Baroness Thatcher tried to ban in the early 90s found that only 15 per cent of the men and eight per cent of the women reported having had an affair in the previous five years.
Among the married, the figures were even smaller: eight per cent of husbands, three per cent of wives. While the lifetime figures would be far higher, it would still make infidelity pretty rare in the general population - maybe a third of men and a fifth of women at the most.
These facts flatly contradict the evolutionists. If women really did behave like sticklebacks or orang-utans, surely the rates would be much higher. Nearly every woman probably still goes on fancying attractive men, but only a small subgroup do anything about it.
It is far likelier that the minority of people who actually stray are expressing their personality type.
While I would not want to suggest that all straying is a sign of personality disorder or fecklessness, in many cases it definitely is. A sample of weak-identitied women known as Borderline Personalities studied at a marriage-guidance clinic were compared with non-Borderlines. The Borderlines were filled with self-deceit. They had greater sexual dissatisfaction and depression about their sex lives, yet thought of themselves as highly desirable. They reported more problems in their relationships, greater sexual boredom, difficulties in achieving orgasm and proneness to affairs, but they identified their partner as the one with the sexual problem. The Borderlines were also more likely to report lesbian desires and half had suffered sexual or physical abuse as children.
Other unfaithful subtypes have a Ludic personality, in which love is seen as a game. Winning equals manipulating the person into having sex, after which there is no further interest.
Of course, all this applies equally to Borderline and Ludic men. The difference in the infidelity rates may come down to the simple impact of having babies: it leaves most women shagged out and provides an emotional companion who soaks up any spare capacity for other relationships. If men spent as much time holding the baby as their partners, infidelity rates might be the same for both sexes.
· Next week: unfaithful men.