Leila is three years old, at nursery, practising her first words. Among them are “dolly” and “shopping”. She plays quietly with other girls and takes turns to share their toys. Not so Joseph, who, at the same age, is less verbal. He often plays by himself and when he does join other boys in the group they tend to wreck what Leila and her friends have done.

The boys push the girls around and refuse to take turns. By the end of the morning session the girls have had enough and retreated to play far away from the boys. It’s a standoff we have all witnessed so often. But according to Dr Louann Brizendine, the author of a new book about gender differences, it’s just the beginning of a mutual, lifelong incomprehension that never quite disappears.

For Brizendine, a 54-year-old neuropsychia-trist living in California, the battle of the sexes is specifically a battle of hormones. These seep into or “marinate” the brain, resulting in different circuitry or processing. So when men get angry and women cry, when men demand sex and women crave a massage, it’s the result of different “wiring” that has evolved in our brains over millions of years.

Testosterone in the male foetus, for instance, will shrink the brain’s communication centre, reduce the hearing cortex and make the part of the brain that processes sex twice as large as that of the female. Thoughts about sex enter a woman’s brain perhaps once a day, but in a man every time he sees an attractive woman.

Such details fascinate her. But how significant are they? “Biology isn’t destiny,” she says. “Not if you know what it’s doing to you. Of course there is always an interplay of nature and nurture in our lives. But knowledge of that differing chemistry brings power.” Biology may not be destiny, but it certainly can bring bewilderment.

The Female Brain, published this week in Britain by Bantam Press, has topped the bestseller lists in the United States. In the tradition of popular science Brizendine explains why it is that men and women can’t seem to connect smoothly. Her views appear to back up many of the stereotypes that dominate our culture. Women talk; men keep silent. Women are perceptive and intuitive; men are blunt instruments.

“I think there is always a kernel of truth in these observations,” she says. “When it comes to reading people and understanding their emotional nuances, women have a superhighway that interprets signals and body language. Look inside men’s brains and they have a country Broad by comparison.” And when it comes to sex, the opposite holds sway.

“The area for sexual pursuit in the brain is found in the hypothalamus, the instinctual core,” she says. “It is 2-2Å times larger in the male relative to the female. So in this department he’s got the Maserati, you’ve got the VW Beetle.” Should a man and a woman argue, she adds, he is 20 times as likely as she is to get aggressive. “A woman’s prime role is to recoil from conflict. She will chew on the emotion and get angry later because it’s her job to maintain relationships.”

Brizendine guarantees, however, that her brain will record every single detail of the fight. “Research shows that females can recall emotional events such as first dates and holidays and arguments more vividly than men. So a fight that happened 10 years ago can be indelible. She can remember what room she was in and what she was wearing. But he won’t recollect it at all.”

Scientists believe this elephantine memory results from females having a relatively larger hippocampus, the place where emotions are tagged and retained, than males.

But Brizendine will have nothing to do with sexual one-upmanship: she is not in the business of engendering competition or trading insults. “Men and women can perform the same tasks. One kind of brain isn’t better than another. Both sexes can study the same subjects and have the same careers. They just arrive at the same place in different ways.” So, for the record, hold off saying to your husband when he forgets your anniversary: “It’s because your hippocampus is tiny.” Abuse won’t get you anywhere.

Unlike many other relationship studies, such as the bestselling Men Are from Mars, Women Are from Venus, her book has pages of footnotes citing scientific studies. Brizendine, who studied neurobiology at Berkeley, medicine at Yale and psychiatry at Harvard, has an impeccable pedigree to help her defend some of her most controversial claims.

One of them, for instance, which suggests that women use 20,000 words a day compared with 7,000 for men, has been hotly disputed. As ever, context and interpretation of data are key. “That’s merely in a social setting,” she counters. “In a boardroom women often say very little.”

By far the most controversial claim refers to what she calls the “mommy brain”. It describes the syndrome whereby highflying career women who once ran business empires give birth and then find themselves unable to function as they once did. “It’s true that the female brain shrinks by about 8% during pregnancy. That’s the bad news, but the good news is that it recovers about six to 12 months afterwards to create large maternal circuits.”

Married to a neuroscientist, she is the mother of a son, now 17. So she has felt at a personal level what she calls “baby lust”, the deep desire for a woman to have a child. “It can hit her after she has held someone else’s newborn. And on a brain scan this ‘mommy love’ can look a lot like romantic love.”

These maternal circuits, she suggests, also give rise to what she calls “maternal aggression” when women crave food, shelter and protection. Woe betide husbands if they do not get those things. “The mother becomes hyper-vigilant about every aspect of the home,” she says. “She becomes obsessed about safety and babyproofing the house.” If we haven’t been there ourselves, we know someone who has.

So shocked was she by some of her revelations, the result of more than three years’ evaluation of more than 1,000 studies, that she considered ditching the project. “I was a first-wave feminist in the 1970s,” she says. “I was determined to grow up gender-neutral. And when it came to motherhood my generation was determined to raise gender-neutral sons. One day our daughters-in-law would thank us for it. But when I gave my son a Bar-bie doll, he started to use it as a spear.

“When I was a student the climate was unisex. You couldn’t begin to talk about differences. But the idea of a unisex brain is a myth. And I think that all those bio-differ-ences were stuffed in a closet of political correctness. So I asked myself, ‘Do I really want to put all this stuff out there so brazenly?’ Then I decided that I had to stand on the terra firma of research.”

Much of the science she cites is well known. But the advent of brain-imaging technology such as PET (positron emission tom-ography) and fMRI (functional magnetic resonance imaging), which allows us to see inside the brain in real time while we cry, laugh, express love and retrieve memories, has made analysis of brain function increasingly sophisticated.

To appease feminists who think this book could be damaging to the female cause, she points out that men and women share 99% of their DNA. “The male and female brain are more alike than different. But the differences are the things we remember. We get hung up on that 1%.” Nor are any of us passive victims at the mercy of armies of oestro-gen and progesterone, testosterone and androgen marching through our bodies. “We all experience anger and aggression, fear and passion. But we can also bring intellect and reason to bear.”

It was after a spell in London studying the work of Freud that she first developed an interest in hormones. “I was being taught that women were frigid because of emotional problems relating to sex,” she says. As a physician she wondered what a blast of testosterone (the hormone that drives sexual appetite in both genders) would do for such women.

“I began prescribing it and, wow, their libido exploded.” Her Freudian textbooks went back to the library. Since 1994 she has run the Women’s Mood and Hormone Clinic at the University of California, San Fran-cisco, the only one of its kind in America. Last week a testosterone patch to treat low libido in women became available in Britain.

Typical of the patients attending her clinic are the couple for whom decades of mutual incomprehension have taken their toll. The following scenario, she says, has long informed her views about gender. “I often turn to the woman and ask her, ‘How do you know he loves you?’ She invariably replies, ‘Because he talks to me about my feelings.’ Then I turn to the man and ask him the same question. He always answers, ‘Because she wants to have sex with me’.” Explaining their primal instincts, the legacy of evolution, helps them to understand their emotions.

And what of her own marriage? What steps has she taken to promote harmony? When she takes her worries to her husband she finds it profoundly irritating that he says, “Now this is what you should do”, before coming up with a solution.

“I think men are wired to want to solve problems. It reflects their brain architecture. But he needs to know that all I want him to do is understand my feelings and empathise with me. I don’t want a lecture.” So she has come up with the following compromise. Stuck to his computer are the words she wants him to preface all his remarks with: “Honey, I understand what you are feeling but . . .” Only then is he allowed to continue. Brizendine, not surprisingly, is now at work on her next study — the male brain.

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