The Economist( link requires subscription) says that women do worse than men because they are less competitive:

“A second study, of physical tasks, showed similar results. When nine- and ten-year-old children?ran a race alone, boys and girls clocked similar speeds. When children raced in pairs, girls’ speed hardly altered. But boys ran faster when paired with a boy, and faster still when racing against a girl. Mr Gneezy points out that, if men try harder when competing, they will disproportionately win the top jobs, even when to do the job well does not require an ability to compete. Job selection is itself highly competitive”

and also because they are uncomfortable with the idea of negotiation!

that women may do worse than men even when they win a job, because they take a different approach to negotiation. Ms Babcock, who recounts her studies in a forthcoming book?, noticed that male graduates with a master’s degree from her university earned starting salaries almost $4,000, or 7.6%, higher than female students. But when she asked who had simply accepted the initial pay offer and who had asked for more, only 7% of women, compared with 57% of men, turned out to have negotiated. On average, those who negotiated raised the initial offer by $4,053-almost exactly the difference between men’s and women’s starting pay.

Huh? What about all those women I see negotiating with bhajiwalis?