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I first found Elaine’s story in a newspaper article she had written, headlined “My glass ceiling is self-imposed”. She described herself as a female executive on the fast track to the corner office who had refused a promotion in a multinational company earning billions and felt she needed to explain why.

She detailed how her company provided every possible perk to promote women’s success, including networked home offices so they could telecommute, flexi-hours, an in-house dry cleaner and gym, an income supplement for a nanny and on-site care for sick children. It was rated one of the top 100 companies for women to work for in the United States and Europe.

Her promotion would have put her third from the top in a company of 12,000 employees with offices in more than 60 countries and on the shortlist to become the company’s chief executive within a few years. Yet she had stalled her own advancement.

I thought that Elaine just might be able to fill in the blanks about why it was becoming increasingly clear that highly capable women were pulling out of the race. Research showed that about 60% of gifted women turn down promotions or take positions with lower pay so as to weave flexibility or a social purpose into their work lives.

Elaine was eager to tell me her story. She was glamorous, with athletic good looks and an easy-going confidence, and it soon became clear that she was hardly lacking in ambition. She had assiduously worked her way up the executive ladder.

She got straight to the point, telling me that, besides a job she loved, she had two small children, a husband and parents, all of whom were central to her happiness. A promotion would require moving to another city, and while it would boost her status and salary, it would destabilise her family.

Her explicit message: work is essential but so are the needs of her family. The subtext? Saying so is somehow shameful. It’s not a wise gambit to turn down a promotion, much less ascribe one’s reasons to the time-warped notion that the feelings of loved ones matter to you as much as achievement at work.

Over lunch, Elaine described the reactions of men in her circle. “When I said no, the president just looked at me and said, ‘I think you’re nuts’. My father-in-law was almost speechless – he was CEO of a company and moved his five kids all over the world.”

She also spoke about the pressures on women to take top jobs: “The company’s desperate – they want women at the senior executive level.” To get more gender balance at the top, she told me, offers too good to refuse were being made to other deserving women, as long as they were willing to move and, if they were successful, to move again a few years later. She had known just one who had said yes – someone without a family.

Could Elaine be representative of other highly placed women? There’s plenty of evidence that many more women than men refuse promotions out of consideration for family, including women at the top of their game.

In 2006, when investment analyst Carolyn Buck Luce and economist Sylvia Ann Hewlett tried to get to the bottom of the “hidden brain drain” of female talent by surveying 2,443 women with graduate or professional degrees, they discovered that one in three American women with MBAs chose not to work full-time – compared with one in 20 male MBAs – and that 38% of high-achieving women had turned down a promotion or had deliberately taken a position with lower pay.

Instead of being forcibly barred from top positions by a glass ceiling, these women were avoiding them.

When the researchers looked at women’s motivations to work, they discovered that having a powerful position was the lowest ranked career goal of highly qualified women in every sector. For 85% of the women, other values came first: the ability to work with people they respect, to “be themselves” at work and to have flexible schedules.

Logging millions of air miles, being available 24/7 and facing unpredictable demands and tight deadlines are the mainstays of top-tier jobs. Of the small minority of women in such posts, twice as many women as men described the negative fallout on their families – connecting their kids’ behaviour, school performance, television and eating habits to their own job pressures in what Hewlett calls “a veritable portrait of guilt”. Why is this?

At school in the 1960s, I was an instant convert to Germaine Greer’s The Female Eunuch. If only women rejected their conditioned roles by avoiding “menial” jobs like teaching or nursing, it could be a different Continued on page 2 world. The assumption was that men had it made; only when women took on men’s roles would they truly be equal.

Most women in the West are now in the workplace and young women are doing exceptionally well at school and university in comparison with their male peers. But gifted, talented women with the most choices and freedoms don’t seem to be choosing the same paths, in the same numbers, as the men around them. Even with barriers stripped away, they don’t behave like male clones.

As a developmental psychologist, I began to wonder about the science. We have come to expect that there should be no real differences between the sexes. But the science that’s emerging upends the notion that male and female are interchangeable, symmetrical or the same. The psychology, neuro-science and economics of people’s choices and behaviour have exploded with amazing findings in the past 10 years alone.

In particular, an opiate-like hormone, oxytocin, which one anthropologist calls “the elixir of contentment” (it surges during breastfeeding, childbirth, sex, cuddling and nurturing), has emerged as a key to understanding Elaine’s decision to impose her own glass ceiling. LIKE Elaine, most of the women I have met in studying this phenomenon did not feel forcibly excluded from the most lucrative positions or feel that talented women are routinely barred from the top ranks.

One of my neighbours, Donna, spent 16 years as a computer science professor but always felt mismatched with the abstract work. Eventually she got herself a new “meaningful” job as a teacher.

To Donna this was more important than status and money. But was this true for women in general? The more women I spoke to the more I came across evidence that it was. They had been taught that they could do anything a man could do but had ended up feeling completely out of place.

As a girl, Anita wanted to be a nurse but was “pushed” into engineering because she was good at maths. “It’s great that people want more women in these disciplines but the women have to want it too,” she said. She, too, now teaches.

And there was Sonia, who was unhappy for 20 years before summoning the nerve to get out of her career in academic science. Yes, she also became a teacher.

There were many others; and what all these women expressed was how high-achievers fall in step with expectations of them to take “male” roles when they are young – and regret it when they are older. So what was going on?

The science of sex differences is a grab bag of surprises. In the early 1980s I was not alone in thinking that men and women had nearly identical brains but had been socialised to take on different roles.

If my husband, a doting father, could leave our newborn daughter after two weeks at home and go to work for 10 hours a day without a backward glance, the script dictated that this was because he had learnt that his role was to be the provider.

And if I felt physical distress about tearing myself away from the baby to go back to work, I had internalised my role as the care giver.

Many of us thought that if only women could tame their outdated sentimentality, if only men offered their babies more bottles, then our parental roles could be reversed. I was amused but admiring when a male friend strapped on a device at a dinner party that allowed him to simulate breastfeeding.

At the time we assumed that men and women were equals – not just in rights and opportunities, as they should be, but also in underlying psychology and behaviour. Any differences, including physical differences, could be fixed via technology, policy or force of will.

This is the “plain vanilla” gender assumption: that female is just a variation of male. It is far from reality.

More than 20 years after my daughter was born, brain imaging and neuroendocrinology have unveiled many of the biological networks underlying mothers’ specific longing for their infants and their drive to nurture them.

There are distinctive design elements in female brains that evolved to promote the survival of infants. An avalanche of hormones at childbirth and during nursing trigger behaviour and emotions that don’t vanish simply because the new mothers have to go to work.

Breastfeeding releases hormones and neurotransmitters that induce euphoria in mothers. Prolactin turns on breastfeeding in females and circulates any time feeding, nurturing or protecting is on the agenda. And oxytocin, “the elixir of contentment”, is evolution’s way of making proximity to infants and feeding them so attractive.

Regular intimate contact becomes a physiological imperative. After infusing her brain with the analgesic and pleasure-inducing effects of oxytocin every few hours when she nurses her baby, a mother is suddenly cut off from her supply when not breastfeeding. That’s why nursing mothers newly returned to full-time work can’t wait to get home to feed the baby again. HORMONES are the catalysts that set dynamic sex differences in motion. Based on studies in animals, scientists expect that certain regions of the brain are not just transformed by hormones early on but are also endowed with receptors that enable the hormones to continue to play a role throughout life.

Rats help us to get a deeper understanding. The behavioural geneticist Michael Meaney has found that a mother rat’s style of nurturing can switch on genetic functions in the pups that skew their emotions and ability to deal with stress.

“Under normal circumstances, high-licking mothers are less anxious and their female offspring are less anxious,” he explained.

Anxious mothers lick and groom their pups less. But when Meaney matched high-licking mothers with foster pups, he found that a stress regulating capacity in the newborns’ genes was switched on. Providing all is well in the environment around them, these more relaxed pups then pass on the attentive mothering they received to their own pups, whose brain circuits are altered in the same way – via their mother’s nurturing.

This clever study shows how maternal behaviour, hormones, empathy, stress and genes interact. But they’re rats and we’re humans, you might say. Rats don’t know much about the Mummy Wars – whether to consider children’s needs first or take a much-vaunted promotion.

While it’s hard to infer empathy in rats, there are hormonal and neural pathways that are common to all mammals. A mechanism that allows a human mother and infant to transmit their emotional states to each other would have survival benefits and Meaney’s work, among others, shows that just such a system exists.

We see the evidence in the pleasure pathways lit up by holding and feeding babies – a phenomenon I noticed whenever a newborn was in my own clinic. I could hear from the chorus of high-pitched cooing that the female family doctors and office staff had gathered yet again around a new mother, admiring her baby and waiting for their “fix” – the opportunity to hold it close.

All these women had already chosen work that would bring them into contact with children, but babies under four months continued to exert a magnetic pull that never wore off. In animals, the nurturing relationship is so inherently rewarding to mothers that when given the choice, new mother rats choose newborn pups over cocaine.

The same pathways may allow Barbary macaques, the monkeys found in Gibraltar, to get stress-reduction benefits from grooming other monkeys. It has long been thought that, just as at the hair salon, the monkey being groomed gets the stress reduction. But Kathryn Shutt of Roehampton University has found that the longer a female macaque spends grooming others, the less stressed she is. For female macaques, anyway, it is better to give than to receive.

Shelley Taylor, a psychologist at UCLA, was the first to theorise that nurturing and stress reactions are tightly bound together in humans. Having interviewed women with cancer about how they dealt with their stresses, she was struck by how many chalked up their resilience to their social connections. Could something biochemical be triggered by reaching out to others?

Oxytocin, the underlying driver in tending children, is also the hormone of befriending. Besides being triggered by childbirth, breastfeeding, nurturing and orgasm, it is also released at critical moments in women’s relationships and menstrual cycles, damping down other stress responses. It helps to keep mothers going, providing sedative and analgesic effects, calming and immediately rewarding the women who instinctively reach out to others when they are in trouble.

Oxytocin is not just a feel-good, nurturing drug. It helps people to read emotions in other’s faces and increases their trust, according to two studies at the University of Zurich. These showed that oxytocin in nasal spray even has a positive effect on men’s usual behavioural limitations: it boosts their trust in social situations and their ability to read facial expressions.

Both studies bolster the idea that this hormone secreted in greater quantities in females – when they have babies, when they nurture them, when they cuddle or have sex with their partners, or when they reach out to others – facilitates females’ capacity for empathy and their trust in others.

Here is evidence, then, that biochemical drivers underlie some of the most obvious behavioural differences we see between the sexes.

Testosterone, secreted in greater quantities in males, may alter some neural connections related to reading others’ emotional states. And oxytocin seems to do the reverse. It seems to help women guess what’s going on inside the heads of other people, enabling them to trust them enough to seek them out, especially when they’re stressed, and to feel pleasure and relief when they do.

Studies have also shown that women on average perceive, experience and remember emotional events more intensely than men do and that these experiences are encoded in more areas of their brains than in men’s. From this, it makes sense that their emotional attachments will figure more strongly in their career decisions.

In the context of male-dominated “extreme” jobs, being aware of others’ needs can be a liability if promotion is the yardstick of success.

Ingrid, a former senior executive in the automotive industry, routinely flew from North America on the redeye for a day of meetings in Europe, having to leave before her children went to bed and getting home after her husband or baby-sitter had put them to bed the next night. The adrenaline was addictive and 16-hour days were common. Accepting the 24/7 executive yoke, she rose through the ranks.

“There weren’t many women in the kind of work I was doing and I was going to make damn sure I didn’t look like one,” she said. “We were the first generation of women allowed to work the crazy life of men and we had to show them we could do it.”

As one of the first women in a male industry, she felt an ineffable internal pressure that drove her to her limits, pushing her to put in 60 to 80-hour weeks when her two children were small, overshadowing her other needs: “I wanted so much to be somebody who counted.”

She had a husband and a nanny to take over at home and there was never any question about her competence or her commitment to her work: “Let’s say you had a capacity for 100%. I thought even at 120% it would be a weakness. I didn’t think I could ever say no.”

But her extreme job put her in conflict with her emotions. “I was a wreck. I never saw my kids. I wasn’t even home to put them to bed. I never made them a meal myself,” she said.

Her starting assumption had been that women were versions of men and if men could do it and were expected to, so could she: “That first batch of women – we wanted so much we didn’t realise that people wouldn’t have thought twice if we wanted to go home occasionally. I didn’t know how miserable I was.” THE science showing that many women feel empathy more acutely than many men doesn’t mean they must or should make trade-offs over their work. It simply explains why some women might want to, as the British sociologist Catherine Hakim understands.

For years she has pricked the ire of the European feminist establishment by asserting that persisting gender gaps in pay are the result of women’s deep-seated preferences.

Her worst sin, according to her critics, was asserting that social policy could never allow the majority of women to have it all, since a measurable slice of the population – 10% to 30% – never wanted it all, anyway, and another 60% adapt their ambitions to their family’s needs.

“If you are seriously interested in a career you don’t have time for children and if you are seriously interested in bringing up more than one child, you don’t have the time, effort and imagination for getting to the top of a career,” she told me.

Half of all women in the top professional and managerial grades are childless, Hakim reports, which is similar to women in academic science and engineering. Reliable contraception has allowed them to choose how they want to direct their energies and to plan their ascent.

In Hakim’s case, over the past eight years she has written six books and “there’s no way I could have done that if I had had children. The fact is that children are a 20-year project and a career is a 20 to 40-year project and there is an incompatibility there”.

And she added mildly: “If someone tested me, I’m sure I’d have the highest level of testosterone.”

© Susan Pinker 2008

Extracted from The Sexual Paradox by Susan Pinker, to be published by Atlantic Books on March 1 at £12.99. Copies can be ordered for £11.69, including postage, from The Sunday Times BooksFirst on 0870 165 8585

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