I have tried to show that the gender gap in the sciences is not the result of cognitive differences, but that begs the question about what else to which it can be attributed. It could be that it is the result of conscious or unconscious discriminatory behavior. However, it could also be the result of the personal and economic decisions of the women involved -- particularly with respect to having babies. If some women decide to leave their careers early or for long periods to have children, then this will affect their average representation among tenured faculty.

A recent NBER paper by Donna Ginther and Shulamit Kahn argues that the second explanation -- economic and personal decisions -- is the explanation for the disparity. Here is the abstract:

Many studies have shown that women are under-represented in tenured ranks in the sciences. We evaluate whether gender differences in the likelihood of obtaining a tenure track job, promotion to tenure, and promotion to full professor explain these facts using the 1973-2001 Survey of Doctorate Recipients. We find that women are less likely to take tenure track positions in science, but the gender gap is entirely explained by fertility decisions. We find that in science overall, there is no gender difference in promotion to tenure or full professor after controlling for demographic, family, employer and productivity covariates and that in many cases, there is no gender difference in promotion to tenure or full professor even without controlling for covariates. However, family characteristics have different impacts on women's and men's promotion probabilities. Single women do better at each stage than single men, although this might be due to selection. Children make it less likely that women in science will advance up the academic job ladder beyond their early post-doctorate years, while both marriage and children increase men's likelihood of advancing.

I guess the results of this paper coincide more with my personal experiences. Granted, I am not a woman trying to get tenure. Yet, it has always bothered me that if I were to believe that there is epidemic of discrimination, I have never personally witnessed any discrimination of this type. My personal experiences in the sciences have shown me that women who work hard are rewarded. I work at an institution with many female faculty members (and at the moment I think we may have more female than male graduate students).

With respect to this issue of fertility decisions, it is another matter whether it is fair to expect women to be the prime movers with respect to child rearing -- torpedoing their careers in the process. That is an issue to be worked out by the couple involved. However, it at least gives me hope that I wasn't participating in a vast conspiracy without knowing it, and that -- at least of recent date -- our institutions to encourage equal opportunity of the sexes are functioning.

Hat-tip: Jonah Lehrer and Tyler Cowen.

UPDATE: Matthew Yglesias has this to say:

I don't know whether that conclusion's right, but I certainly don't think it's especially politically incorrect. You say the gap is explained by "fertility decisions" I say it's explained by "structural sexism." Here, as in much of life, women and men are now allowed to compete on "equal" terms. The terms, however, were set up long ago -- by men -- before that was the case, operating under the implicit assumption that the competitors would be men who, if they had children, would have wives at home to take care of the children. So things work out nicely if you're a man, or if you're a single woman, but not so well if you're a woman who -- like most women -- has children at some point. You can't just pin this problem on science departments and university administrators, since obviously larger social forces are at work, but it's hardly fair. And university administration genuinely concerned with the situation could do things to mitigate the situation, though probably they couldn't solve it all on their own.