Women are rising through the corporate ranks but they will never achieve parity with men, argues Kingsley Browne

Women have swept through the workplace in the past 30 years. In the United States now, 46 per cent of the workforce is female. Women can be found in abundance in the lower echelons of management, yet on many measures, they still lag behind. Women make up only 5-7 per cent of senior executives in large corporations and less than 3 per cent of firefighters. Full-time women workers earn only 75 cents for every dollar earned full time by men.

Many women believe these statistics point to a monumental injustice. This belief is often based on the assumption that men and women are interchangeable and have the same work preferences, leaving discrimination as the most plausible explanation for any disparities between the sexes. It is an assumption that is fundamentally flawed.

conventional wisdom in the past few decades has been that human behaviour is chiefly a product of social conditioning and that it is our "sexist society" that causes men and women to act differently - that causes men, for instance, to be aggressive and competitive and women caring and nurturing. But modern science casts doubt on this. Anthropologists have demonstrated that, rather than being idiosyncratic social artefacts, many sex differences are universal. Moreover, personality traits, such as shyness and sensation-seeking, once thought to be purely products of a person's upbringing, are now known to be hugely affected by genes. Average differences in temperament between the sexes turn out to be more than "social constructs".

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Some scientists now believe that these temperamental differences result from "sexual selection", a special form of natural selection. Broadly speaking, natural selection is the process whereby those best adapted to their environment survive and have children, thus passing on their genes to the next generation. Sexual selection favours those features that give one animal a reproductive advantage over others of the same sex. Male deer, for example, have elaborate antlers that they use vigorously in competing with each other for female mates. The male can inseminate many females in a breeding season, whereas females cannot similarly increase their reproductive success by mating with many males. There would be little reproductive advantage to female deer from fighting among themselves for access to males, while the payoff for the victorious male deer can be immense.

Men, like male deer, can increase their reproductive success by increasing their number of sexual partners. Suspected terrorist Osama Bin Ladin, for example, is the 17th of his father's 52 children, a brood no woman could hope (or would want) to duplicate.

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What is it that has traditionally characterised reproductively successful men? Or, to put it another way, what qualities do women seek in a mate? Today's women tend to prefer dominant men of high status with lots of money, and there is little reason to believe that their ancestors differed. For much of human history male status came from being a skilled hunter and warrior and from the ability to influence others through muscle or wit. This history has left men more disposed than women to strive for status and engage in the risky, competitive and sometimes aggressive behaviour often required to ascend hierarchies. Women with a taste for winners would likely be in a position to pass their preference on to more children than would women with a penchant for failures.

Women would not ordinarily enhance their reproductive success through direct competition and risk-taking. Indeed, it would promise little reproductive payoff and could be a very dicey strategy because a bad outcome would imperil a woman's future reproduction and the well-being of existing children. Instead, women, to a far greater extent than men, have enhanced reproductive success by looking after their children as they grow up, resulting in stronger bonds between mother and child than between father and child.

Given that the sexes are genetically identical except for the Y chromosome possessed by males, how do parents transmit different temperamental traits to their sons and daughters? The answer involves sex hormones. Hormones manufactured by the male foetus cause its brain as well as its body to develop in a male direction, and differential hormonal exposure throughout postnatal life, especially at and after puberty, enhances the difference. These hormones produce a mind oriented more towards risk and competition, just as they produce an anatomy that is larger and more muscular. Women exposed to these hormones tend to be more masculine in behaviour, often being tomboys in youth and competitive women in adulthood.

Sex differences in labour-market behaviour are predictable consequences of these psychological sex differences. Today, for probably the first time in history, men and women work side by side doing the same tasks and competing for positions in the same hierarchies. Although women do quite well by many measures, what accounts for their sparse representation among senior executives? As studies of successful executives have shown, temperament has a big impact on achievement. Those who reach the top tend to be aggressive, competitive and willing to take risks - traits men possess disproportionately.

A study of women's career achievement found that the more "masculine" the woman (assertive, competitive), the greater her career achievement; achievement was negatively correlated with "femininity" (nurturing, warmth).

The "gender gap" between male and female salaries has similar causes. Most economists do not believe it is caused by wage discrimination but rather by such obvious reasons as men working longer hours and in more dangerous jobs in worse conditions. Even within occupations, the sexes have somewhat different career orientations. Female physicians and lawyers are much more likely than their male counterparts to be employed for a salary and to work regular (and shorter) hours. Men are more likely to be in private practice, work longer hours and bear the economic risk of business failure.

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While sex differences in workplace behaviour exist even for single workers, they are greater for married ones and even more so for parents. Marriage and parenthood affect the behaviour of both men and women, causing women to decrease their workplace commitment and men to increase theirs. This is consistent with a psychology inclining men towards seeking status and money and women towards caring for offspring. Although many have advocated an increase in state-subsidised daycare to ameliorate the glass ceiling, it is not lack of money but an unwillingness to delegate the care of their children to others that causes women to eschew single-minded devotion to career.

Every choice entails trade-offs. Time devoted to family is time that cannot be devoted to career. Women in increasing numbers now face a choice long faced by men. For many the choice is particularly poignant because women (like other mammalian females) are often reluctant to separate from their young. They feel more guilt than men for having done so. This does not mean they have no choice; it means that they are predisposed to make the choice differently from men.

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For reasons understandable in evolutionary terms, men's self-esteem is more related to workplace success than that of women, which is tied more to success in social relationships. Thus, the psychic rewards of working and not working are often different for men and women. Once one sees that women invest less in the workplace to obtain greater psychic payoff domestically, their lesser workplace reward may seem less of an injustice.

But there is substantial overlap between the sexes; the differences described are merely average differences. Just as many women are taller than many men, many women are more competitive and more risk-oriented than many men. Nonetheless, among those who are unusually tall or unusually competitive and risk-oriented, males will be substantially over-represented.

Moreover, nothing said here justifies treating individual men and women differently. Rather, it explains why, even if they are given the same opportunities, they are likely to respond differently. Neither does my argument deny the continued discrimination against women, although government pressure to raise female representation at work ensures there is also discrimination against men.

This explanation does not show that the current percentage of women in executive suites is at the "right" or "biologically determined" level. Those there today are there because of a career trajectory set decades ago when there was less opportunity and when women's educational and career choices reflected that fact. We can expect a continued rise in the number of women ascending the corporate ranks, but we should not expect them to reach parity with men.

Even with today's strong social consensus against sex discrimination and with girls being urged to be anything they want to be, we should not be surprised if marked sex differences in the labour market persist.

Kingsley R. Browne is professor of law at Wayne State University Law School, Detroit, Michigan. He is author of Divided Labour: An Evolutionary View of Women at Work, Weidenfeld and Nicolson, Pounds 4.99.

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