Amber (Prettier than Napoleon) asks the question, and it's a very interesting one.

Is the cause possible differences in innate intelligence at the tail ends of the bell curve (what I'd heard called the idiot-genius syndrome, which leads men to be overrepresented both among the very low-IQ and the very high-IQ)? Sex discrimination in law school classes (whether on the exam or before) or in hiring? Social pressures that push some women away from law school? Differences in innate ambition? Social pressures that lead men to be more ambitious than women (for instance, because less ambitious men face more condemnation from parents, peers, or prospective girlfriends than do less ambitious women, or because more ambitious women face more such condemnation than more ambitious men)? The tendency of women to marry at a somewhat younger age than men, coupled with a tendency of married people to on average be less likely than single people to move? (Moving is often needed to get the prestigious appellate clerkship that can help lead to a Supreme Court clerkship.) The greater tendency of women than men to have spouses or lovers who aren't easily movable, which may again make it less likely that women would move to get the prestigious appellate clerkship? A combination of some or all of the above?

I'd love to hear speculation, but even more I'd love to hear actual data.

UPDATE: By the way, some data, from my year clerking (1993-94): Of the 38 clerks (including 4 who were clerking for retired Justices), 11 were women. As I recall, 5 of the 11 women were married, none had children, and at least 4 of the married women had left their husbands in a different city.

Of the 27 men, only 5 were married, 5 had children (including one who was divorced and whom I didn't include in the 5), and 4 of the married men's wives were with them in D.C.

This is just one year, and any serious study would have to look at much more than one year. But it led me to wonder whether the women who had the law school credentials to get the prestigious but often out-of-town appellate clerkships that are stepping-stones to the Supreme Court

1. might be more likely to be married than comparable men (presumably because women marry slightly younger than men),
2. might have more difficulty getting their husbands to move with them than men would have getting their wives to move with them (perhaps because the women's spouses are more likely to have hard-to-move jobs than the men's spouses), and
3. might have more difficulty clerking, especially in a highly demanding clerkship, if they have children than comparable men would.

If the answers to some of these questions are yes, then this might lead some of these women to drop out of the clerkship race, likely by not looking for a prestigious out-of-town appellate clerkship.