WHY DO WOMEN have smaller brains than men? Male brains weigh around 1.25kg; female brains weigh on average 100g less.

One possible answer comes from an unexpected study of foxes. In 1959 Dmitri Belyaev, a Russian geneticist, launched a long-term experiment to tame foxes. Starting with a population of caged wild animals, he selected from each generation the puppies who were friendliest (or, initially, least hostile) to humans, breeding only from them.

After 35 generations he produced animals that had been transformed from the usual snarling fearfulness of wild foxes into animals that were similar to domestic dogs. The tamed foxes wagged their tails, whined for affection, were submissive, barked like dogs and their ears flopped. As Darwin said: “Not a single domestic animal can be named which has not in some country drooping ears.” Belyaev seems to have concertinaed all this into 50 years when it took 10,000 years to domesticate wolves as dogs.

And the tame foxes’ brains were smaller. Domestic animals generally do have small brains. On average, domestic dog, cat, sheep and pig brains weigh 25 per cent less than those of wild animals. The mechanism remains a mystery. A recent study by Elena Jazin and her colleagues from Uppsala University in Sweden, published in Current Biology, reported that of 30,000 brain genes about 40 showed differences between tame and wild foxes. All we do know is that blood levels of stress hormones are lower in tame than wild animals and that brain levels of anti-stress hormones are higher.

But would it be dangerous to suppose that women’s brains are smaller than men’s because, over the millennia, we men have been selecting friendly women with whom to breed? And that therefore we have domesticated them? And that they, in turn, have selected assertive men? Interestingly, human skulls of both sexes have been shrinking over the past few tens of thousands of years, suggesting that as human beings have been increasingly domesticated, so they have shrunk their brains — without losing the gender difference.

One force that drives up brain size is social interaction. It was Robert Trivers, the biologist, who showed in his 1985 book, Social Evolution, that there is a direct correlation between the size of a species’ brain, the size of its social groups and the degree of social interaction between the animals of the group. Men and women inhabit different social spheres, and though women may enjoy deeper social interactions than men, it is probably men who, as tribal leaders, have experienced wider social interactions over evolutionary time.

In any event, small brain size seems to optimise emotional intelligence. In a paper published last month in Current Biology, Brian Hare and his colleagues at Harvard University showed that domesticated foxes were better than wild foxes at reading human social cues. For example, domesticated foxes instinctively understand a person’s intention when he or she points at an object and they investigate it. Social intelligence, therefore, seems to be increased when fearfulness and stress are lowered by selective breeding for tameness. Certainly our border terrier at home, Rusty, is uncanny in his ability to understand our intentions (when he wants to).

And then there is body size. Irrespective of other factors, there is a direct correlation between the size of an animal’s body and its brain. We human beings have haremic tendencies, so women are smaller than men. So their brains are too.

Does any of this matter? A recent paper in the British Journal of Psychology by Paul Irwing, of the University of Manchester, and Richard Lynn, of the University of Ulster, claims that more men have very high IQs than women do. But IQ tests remain so controversial and so subject to cultural factors that we will need another half century before fully understanding them. Not only that, we also simply do not know enough about our brains to draw safe conclusions from any of these observations.

But what I do know is that such speculations are dangerous in the academic world. Lawrence Summers, the President of Harvard, lost his job when he conjectured that women might not scale the same intellectual heights as men, so for me to continue this theme might be perilous.

The author is the Vice-Chancellor of the University of Buckingham.

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