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CRONIN: No. It's not to do with definitions. For an evolutionary biologist, the defining characteristic of females and males is their sex cells: eggs or sperm. But that's just the fundamental difference from which all other sex differences proliferate. So we can leave definitions behind, move on from eggs and sperm, and ask which characteristics are sex-typical in our species — just as we ask which characteristics are species-typical or age-typical or typical of stages of development, in our species or any other.

Now, unlike whether you're an egg-bearer or a sperm-bearer, this kind of characterisation doesn't cleave our species neatly into two. And people often seize on this as anti-Darwinian ammunition. I'm sure you've heard the argument: "But the differences within the sexes are greater than the differences between them". Well, I said it's an argument. But it's usually stated just like that — as premisses without a conclusion. I think the implication is meant to be that there's so much overlap in the distributions that the Darwinian interest in differences is misleading.

But is that right? It needs thinking through — but when I try, the argument tends to fall apart. For a start, how important the difference is depends on why you're interested in it, what your aim is. If your aim is to get rich, don't try selling pornography to women or romantic novels to men; don't try selling 'Kill! Kill!' computer games to girls or 'people' games to boys. And, anyway, you can't simply generalise about how large the overlap is; it depends on the characteristic. There'll be almost no overlap if you pitch boys against girls in throwing missiles — the boys will win every time; and almost no overlap in fluency of speech — nine out of ten men will do worse than women. Then there's the fact that, even if the mean differences are small, there can be huge differences at the extremes. Men are on average only a few inches taller than women; but all the very tallest people are male. So men might end up ahead just for that statistical reason alone.

There's also a curious fact — it's one that's been uncovered by evolutionary biology — about the shapes of the distribution curves for most male-female differences. Darwin remarked on it and it holds robustly across other species, too. It's that males are far more variable than females — they are over-represented both at the top of the heap and at the bottom of the barrel. For some characteristics, people might not care. But what about this implication? Fewer women are likely to be dunces but also fewer will be geniuses. When I mentioned this in a seminar in the States, I was sharply corrected by a group of feminists: "There's no such thing as genius". I later discovered that this had become a fairly standard 'feminist studies' line. I couldn't help wondering whether 'genius' had been airbrushed out because there weren't many women in the picture. Darwinian theory also suggests that it's important to look at differences in disposition and interests, as well as abilities. Will the top piano student become the international star? Being competitive, status-conscious, dedicated, single-minded, persevering — it can make all the difference to success. And these are qualities that a lot of men are far more likely to possess, often in alarming abundance.

So what you have to do for any characteristic is ask how important it is that there's much overlap, what difference it makes to the policy you're dealing with and so on. The 'differences within and between' argument somehow has a politically correct air. But it's actually useless — or downright misleading — as a guide to making decisions.