Acknowledging that men and women are not the same is not an argument for genetic determinism, writes Janet Radcliffe Richards.

Were traditionalists right about men and women after all? They always said the sexes were different, and they used their ideas about those differences as the excuse - when excuses were needed - for the relegation of women to their subordinate sphere. If science seems now to be confirming these old beliefs, it is not surprising that there is widespread alarm and insistent denial among feminists and their sympathisers.

It was the scientific approach to human nature that first challenged the traditional view. The sexes might seem very different; but, as John Stuart Mill pointed out, the extent to which these appearances reflected natural differences was impossible to tell because men and women had always been in systematically different environments. And the case against tradition seemed to gather strength as this agnosticism was replaced by the conviction of social scientists that observed differences between the sexes were cultural constructs. Feminists adapted the term "gender" to refer to such non-biological differences, and it is a sign of how essential the social construction view has become to feminism that the substitution of "gender" for "sex" seems to have become compulsory among the politically enlightened.

But this seems to be changing. Techniques of observation and control have appeared that were unavailable in Mill's time, and these seem to be confirming that the sexes really are different by nature. Similar results have come from the new approach provided by evolutionary psychology.

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Evolutionary psychology tackles the problem of disentangling natural and cultural differences between the sexes by using our understanding of evolutionary processes to generate hypotheses about natural differences. Darwin recognised that as soon as evolution produced creatures with emotions and intelligence, these qualities would be as relevant to the evolutionary fate of their possessors as would anything else about them. And when the sexes - of almost any sentient species - are considered from this viewpoint, it becomes clear that males and females should be expected to have significantly different temperaments, just because their reproductive systems are so different.

A human female, reproducing flat out, can produce only about one child a year. A human male's reproductive potential is limited only - though entirely - by his ability to impregnate women. This alone suggests that different psychological characteristics would have been needed for evolutionary success. The problem is that many familiar differences between men and women, dismissed by recent feminism as culturally induced, are turning out to be ones that natural selection might be expected to have planted deep in our natures.

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Evolutionary reasoning has suggested, for instance, that women should be attracted to impressive, high-status males from whom they seek undivided support and commitment, and should be strongly devoted to the care of their children. Men should be competitive, adventurous and anxious to possess women and control their sexuality. They should prefer youth and beauty in women, but also be eager to grasp whatever sexual opportunities present themselves.

It begins to sound as though, after all its early promise as an ally of liberation, the science of human nature is leading straight back to the traditional view. It is hardly surprising that, in many quarters, the whole project of evolutionary psychology is dismissed as politically motivated pseudo-science and is met with accusations of genetic determinism, essentialism, gross oversimplification, insensitivity to variation and overlap, categorising, stereotyping and rampant sexism.

But the new claims about sex differences, while they may sound like the old ones, are really quite different. Many old ideas are not upheld - there is nothing in evolutionary psychology, for instance, to suggest that women are less intelligent than men.

There is also a more fundamental, and more subtle, point. Even when modern claims about male and female differences of nature sound like traditional ones, they are not because there has been a radical change in the idea of nature and what it means to understand the nature of something.

The familiar, traditional claims about the natures of men and women were made against the background of a long-established view of the world as a naturally ordered whole, in which all was harmonious as long as everything stayed in its ordained place. If things went wrong, that was because of interference in the natural order of things. This idea comes in many different versions, of which the most familiar is the religious view that sees order and complexity as underlain by intelligent design. Against the background of such traditions, to understand the nature of something is to understand its place in the scheme of things, and to understand the nature of men and women is to know how they should live harmoniously together. As Mill's contemporary, critic James Fizjames Stephen, claimed, the sexes could no more have different interests than could different parts of the same body.

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But Darwin's account of evolution by natural selection, which showed in principle how complexity could arise from simplicity without any design or intention, revealed a quite different world in which there was no underlying moral order and no natural harmony. In this world - as, indeed, in the world of modern science in general - to describe the nature of something is to say nothing at all about its natural place or what is good for it. It is only to give a neutral account of what it is like and how it interacts with other things. The trouble is, however, that pre-scientific, pre-Darwinian ideas about nature are deeply ingrained in our consciousness and persist even among people who have in theory abandoned them. This leads to systematic misinterpretation - or misrepresentation - of the kinds of claim made by evolutionary psychology.

In a Darwinian world, for instance, claims about the way evolution has shaped male and female emotions carry no implications of psychological homogeneity within the sexes, or of firm boundaries between them. Since variation is the raw material of evolution by natural selection, it is to be expected. Ideas of fixed essences and clear distinctions between natural kinds belong only to earlier ideas of an ordered universe.

Similarly, claims about sex differences carry no implication of genetic determinism of any kind. To say that men and women are different by nature does not imply that their development and actions are fixed in their genes; it implies only that - to the extent that they are different - they will react in different ways to similar environments.

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To understand the nature of something is to understand the circumstances under which that nature will change. The idea that natures are unchanging also belongs to the pre-Darwinian world and has nothing to do with the claims of evolutionary psychology.

Most important of all, in a Darwinian world, discoveries about the natures of the sexes have no direct implications whatever for how they should live and relate to each other. There is not the slightest reason to expect their interests - either evolutionary or personal - to coincide; indeed the evolutionary writer Robert Wright says the sexes almost seem designed to make each other miserable. Natural selection produces harmony only to the extent that harmony promotes reproduction. Evolutionarily speaking, the sexes are rivals. Whatever may be achieved by well-matched or well-motivated individuals, there is no natural prescription for domestic harmony or social justice. To think otherwise is once again to import incompatible, traditional ideas of an ordered universe into the world of modern science.

Paradoxically, the same mistake shows even in the apparently opposed idea that Darwinism justifies constant struggle. An idea popular among some males of the species is that if their inclinations to philander or even rape are evolutionary adaptations, we should not impede evolutionary progress by trying to curtail them. But ideas of evolution as all-purpose progress also depend on the idea of a natural order, through which evolution progresses. Darwinian evolution has no such onward-and-upward path. The only hope for progress of any kind lies in our deciding what counts as progress and then trying to bring it about.

Whatever we hope to do, we cannot do it without understanding what we are up against. If science is showing us that men and women are different, that is something we need to know. The last thing we need is resistance that comes from encumbering Darwinian claims about human nature with fossils from a pre-Darwinian world.

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Janet Radcliffe Richards is reader in bioethics at University College London and author of Human Nature after Darwin (2000).