Before anyone starts wondering if they're reading some weird hybrid of New Scientist and Loaded, let me reassure you I'm simply reflecting on evolutionary psychology - the new black of science fashion. Its findings have been popularised in books, in television series such as Why Men Don't Iron and by star proselytisers such as the American Steven Pinker, author of How The Mind Works, and Helena Cronin of the LSE. Final proof that it has gone Big Time is the publication this week of Introducing Evolutionary Psychology (Icon Books, £8.99). Co-written by Dylan Evans, a research student at the LSE, it's a beginners' guide, complete with cartoons. So what's it all about?

Evolutionary psychology is a combination of cognitive psychology and the evolutionary biology pioneered by Charles Darwin. It claims to explain many aspects of human behaviour, but the area that gets most attention is male-female relations. In other words, sex. It is pivotal to evolutionary psychology that humans' sexual behaviour is motivated by the drive to reproduce. Both sexes, it is argued, have an interest in mating with a range of partners because doing so increases their chance of passing on their genes via healthy children. But men want to do it more often than women because in the distant past - so distant that the pill and the CSA hadn't been invented - the consequences for them were far less awkward than for women.

Evolutionary theory contends that to find out why, we need to travel back a few million years. Hence a cavewoman who laid a flighty, scrawny, short-sighted caveman might end up feeling fat and lousy for nine months, then have to give birth to some feeble specimen who wouldn't spot an approaching mammoth until it was too late. The caveman, meanwhile, could just pursue someone else. So, the theory goes, men's and women's minds evolved in different ways.

From this, thousands of women might wearily conclude that not a lot has changed. But even my crude summation makes it easy to see how the increasing credibility of evolutionary psychology may have serious implications for those of us concerned with gender justice. For example, if men and women are innately given to different sexual behaviour, where does this leave the feminist contention that such conduct results not from differing biological impulses but from patriarchal, social pressures on women not to practise sexual choice? And what about its thesis on sexual attraction? Men, it seems, have always been attracted to women whose waist measurement is seven-tenths that of their hips because, deep in our psyches, we believe such women are more fertile. As for women, they go for men with money because that means they'll be better protected.

On the face of it, such claims provide ammunition for conservative campaigns around such vital gender issues as sexual freedom, family life and the "gender gap" in work and education. So how can evolutionary psychology be a force for progress? Evans says there could be a new "Darwinian feminism" which "faces up" to the fact that women do some things differently for evolutionary reasons and so their best interests may not always be served by struggling for equality with men. "It may be that the desire for more stable families and greater equality in work are not compatible," he says. "It might be better to have some sort of trade-off so the world of work is organised to suit the different needs of women and men."

To some, such words will sound similar to those used by the conservative campaigners. Yet they also sound at least superficially consistent with those strands of feminism that argue men and women are indeed different in fundamental ways which should not be diminished.

Germaine Greer is among these "essentialists". Her last book, The Whole Woman, is constructed largely upon her rejection of the notion of equality in favour of the goal of liberation. "Liberation struggles," she writes, "are not about assimilation but about asserting difference, endowing that difference with dignity and prestige, and insisting on it as a condition of self-definition and self-determination."

Is it impossible to square Greer's position with the kinds of social initiatives Evans suggests? If not, what happens next? Could the new Darwinians and stalwarts of the "sex war" find they are marching hand in hand? Could the next Big Idea for this time of sexual turmoil define progress as a struggle not to acknowledge gender likeness but to instigate a more equitable balance of difference?

I would have misgivings: whose political interests would be served? Wouldn't it turn out to be just another way of justifying sex discrimination? As for evolutionary psychology itself, I can't see that it tells us much about gender relations in the modern age, even if its science is flawless. But one thing is for certain: it's not going away.