In my dwindling band of friends who are still combining work and motherhood, there is a common fear. It is fear of promotion. Few say it, few even acknowledge it to themselves. These women are in their thirties, educated, in good jobs. But the next move up the career ladder - or at least the conventional career ladder - seems to produce in them a secret dread.

I realised this recently when one friend, who has been wanting more responsibility for ages, ducked a planned meeting with her boss. “I think I'm OK where I am,” she said. “Why risk climbing up another notch?” Only a few days later yet another friend turned down a job offer that most of the men she consulted said she'd be crazy to reject. This has become a familiar pattern. We mothers hold a steady course, fearing that any deviation will send our households veering out of control. While most of the men we know have their feet clamped hard on the career accelerator, their eyes in almost permanent rotation between the conquests ahead and the rear-view mirror.

“If you were to predict the future on the basis of school achievement,” says Susan Pinker, in her new book The Sexual Paradox, “the world would be a matriarchy.” Women are powering ahead of men in education. As graduates, many are earning more than their male peers. But by their mid-thirties they stick in the middle ranks or drop out altogether, while men who may have much more erratic educational histories are excelling. This trend is most pronounced among the most gifted women, many of whom have bosses or husbands who urge them to aim high. And it is not just a motherhood issue: educated women without children are also not choosing the same paths, in the same numbers, as educated men. As Pinker puts it: “Even with all the barriers stripped away, they don't behave like male clones.”

Why? Pinker believes that the answers are mainly biological. It is not lack of ability or opportunity that prevents so many women from reaching boardrooms and the upper echelons of science, she says, (although she does not claim that discrimination has been abolished). It is because women are wired in the womb to want different things. Baby boys are more exposed to testosterone, which drives them to be daring and aggressive. Baby girls are doused in oestrogen, which helps them to empathise. This makes women by nature resistant to investing all their energies, single-mindedly, in one thing. It makes them less extreme. Women tend to seek “inherent meaning” in their jobs, whereas men tend to seek domination.

Parents like me, who have failed to tempt their children away from gender-stereotyped toys, may nod at this. Some people will see it as an outrageous attack on equality - as I would have done in my feminist twenties. But it is really an argument for a better understanding of why some women dislike roles that are defined by male ambitions. Pinker asks why we think of the male as the standard model and the female as a version with a few optional features. All the high-powered women she interviews are happier for having left their top jobs. In different ways they explain that society impelled them towards the male model, but that it didn't quite fit.

The book is a powerful portrayal of men, too. Pinker realised that in 20 years of clinical practice most of the troubled children she had seen were boys. She discovered that some of the most fragile boys, with obsessive interests or an extreme appetite for risk, had become surprisingly successful in later life. Some of the men who have driven the world forward have (like my distant relative Henry Cavendish, in whose scientific discoveries I have always taken a nonsensical pride) been loners almost incapable of communicating - not attributes to which most women aspire.

This book in fact gives powerful support to Larry Summers' remarks that produced rage on the Harvard campus two years ago. He was the first President of Harvard to suffer a no-confidence vote, for having the temerity to suggest that there are fewer female geniuses than men and fewer women prepared to devote crazy hours to a single topic.

The book raises intriguing questions. If Pinker is right, then women who have the luxury of making career choices may actually increase, not decrease, the sexual division of labour. That is certainly what happened in kibbutzes that were studied over four generations, where all choices were freely available to men and women but where, in each generation, men chose to do progressively less childcare and women less construction work.

What does that mean for our current notions of equality? If women choose not to be corporate CEOs, does it matter? How can we find ways to better value what they do decide to do? If women really are more wired for empathy, this also raises questions about what policies are really “family-friendly”. Pinker cites potentially devastating evidence, from one Ivy League university, that male professors use parental leave to do research, while female professors use it to care for children.

He returns with a book, and she with a backlog. So greater equality in family policy could paradoxically discriminate against women.

To me, this book comes as a relief. I have never felt that diatribes about discrimination chimed with my personal experience, although it does with some of my friends. I have never bought the idea that women aren't competitive: we are. But I see so many able women who are fed up with the idea that the only real progress has to be perpetual upward motion. There's a time for that, but it should be in our own time.

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