Recently, the psychologist David Buss's team at the University of Texas at Austin reported that men, when looking for one-night stands, check out women's bodies. Or as they put it, "men, but not women, have a condition-dependent adaptive proclivity to prioritize facial cues in long-term mating contexts, but shift their priorities toward bodily cues in short-term mating contexts."
Like many results in evolutionary psychology, this may seem blindingly obvious, but that does not stop it from being controversial. Earlier this month a neuroscientist in Britain, Gina Rippon, lambasted what she called the "neurohype" about sex differences: "There may be some very small differences between the genders, but the similarities are far, far greater."
She has a point. Compared with, say, chimpanzees, men and women are not very different. Most of the interesting things about people—language, laughter, love, laptops—come just as naturally to both sexes.
Yet we would be a very peculiar animal species if we did not have sex differences in behavior as well as anatomy. In virtually every mammal and nearly all birds, males are more aggressive, females more nurturing. It is a distinction that goes right back to active sperm competing for stationary eggs in the primeval ocean. It was only reinforced when the invention of the placenta and the mammary gland gave male mammals a gigantic prize to compete for: nine months and several years of somebody else's bodily efforts. Wombs are worth fighting over—and granting to favored applicants only.
So it's no zoological accident that in all societies, however peaceful or violent, men are about 50 times more likely to kill other men than women are to kill women, and they do so most in young adulthood, when most actively competing for mates. Likewise, it is no neurophysiological accident that women coo over newborn babies more enthusiastically than men do. Women who showed interest in babies left more genes behind than those who were indifferent; men who turned violent left more genes.
These are the kinds of sex differences that we share with all other mammals. What intrigues me, though, is the possibility that human beings have other sex differences peculiar to themselves and derived from uniquely human habits of more recent origin.
Take the cliché of the golf-playing husband and the shoe-shopping wife. Not even an evolutionary psychologist would claim to find monkey equivalents to this. Yet the obsession with the trajectory of ballistic objects is as baffling to most women as the obsession with searching and re-searching every store for the perfect shoe bargain is to men. (I know there are exceptions, but admit it: Marriage surprises most people by revealing the truth of such clichés.)
In all hunter-gatherer societies there is a sharp difference between the foraging strategies of the two sexes. Men generally travel far in search of mobile prey that they need to bring down with well-aimed projectiles. Women generally go out in groups and search for good sources of roots, ripe berries or nuts, which they use their acute powers of observation to spot and collect.
Without knowing it, golf-course designers are setting up a sort of idealized abstraction of the hunting ground, while shoe retailers are setting up a sort of ersatz echo of the gathering field.
This sexual division of labor over foraging is not only far more marked in people than in most other animals (it was, arguably, the first "gain from trade" we stumbled upon, benefitting both sides), but it may be a relatively recent feature of our evolutionary history, invented in Africa just 150,000 to 300,000 years ago. Some archaeologists have concluded that Neanderthals did not practice it: that female Neanderthals were co-operative hunters with men, not gatherers.
That still gave the sexual division of labor plenty of time to leave its instinctive marks on the human psyche through genetic changes, raising the intriguing thought that some of our sex differences might be caused by our culture, yet also ingrained in our genes.
—Matt Ridley's many books include, most recently, "The Rational Optimist" and "Francis Crick." His weekly column explores the science of human nature and its implications.