Evolutionary psychology has been stretched far beyond what it is really capable of explaining. Take the changes in attitudes to sex during the twenties.

Evolutionary psychologist John Townsend interviewed 20 male and 20 female medical students in their twenties about their sex lives. Regarding one-night stands, one of the women in her late twenties said: 'I would do them two years ago, but I wouldn't do it now. There was nothing at stake then. Now every man is a potential mate.'

She had been to seven weddings the previous summer and she was feeling under pressure. 'Guys will go for the young, pretty girls. When we qualify we'll be 30 or 35 and they'll want women who are 20 and 25.' Like most of her female peers, she was looking for relationships with baby-making potential.

The men were travelling in the opposite direction. Whereas 90 per cent of the women replied 'yes' to the statement 'I would prefer to have a serious relationship but have not met the right person yet,' only 10 per cent of the men did.

They were refusing opportunities to sleep with their classmates because of the grief it entailed - getting a bad name for not calling back after a night of passion, the demand for commitment.

Townsend maintained that these differences reflected genetically inherited tendencies: women start looking for a steady hunter-gatherer to support them as child-rearing approaches; men, increasingly confident and of high status, want to distribute their selfish genes.

The evolutionists claim that there are genes making women think it's a bad idea to do casual sex in their late twenties and other genes driving male promiscuity. Sure, evolution is the most likely explanation for why the looks of women have maximum pulling power when they are young - it would make the opposite of reproductive sense for post-menopausal attributes to be maximally sexy.

But there is no need at all to suppose there are exclusively female gene combinations directly driving women's changing psychological needs. Rather, consciousness of the biological clock means women simply become aware that they need to get their skates on if they want children and that their looks are declining. While genes determine that decline, its psychological meaning varies according to their upbringing and the society they are in.

The same is true of the men. Genes may predispose them to prefer Anna Kournikova to a granny and explain their relative powerlessness to pull when in their mid-teens. But environment explains how they react after that - as they get older they find there is a far larger universe of attractive women who are 'up for it' and this changes the way they think.

There is no need to posit some exclusively male shagging gene to explain why. Just as their female peers did in their teens and early twenties, men are more liable to want to sleep around when spoilt for choice.

· Next Week: Why is desire the basis for picking parenting partners?