Do you think the initial sex disparity on OB/LW was mostly due to those communities being sexist from the start, or a selection effect caused by the subject matter, or something else?

Arguably, Overcoming Bias is (or was) an "economics blog" and men think more like economists.

From the Journal of Law and Economics:

The positive economic beliefs of economists and the general public systematically differ. What factors make non-economists think more like economists? Using the Survey of Americans and Economists on the Economy, this paper shows people think more like economists: if they are well-educated; if they are male; if their real income rose over the last five years; if they expect their real income to rise over the next five years; or if they have high degrees of job security. However, neither high income nor ideological conservatism have this effect. My findings for education, gender, and income have close parallels in political science: on tests of objective political knowledge, the better-educated and males score higher, controlling for numerous other variables, and the independent effect of income is minor.