Interesting study summarized here; some excerpts:

Mainly because only about one third of students who either enroll in the undergraduate major or stay after introductory classes are women.  I.e., at least in North America, the main pipeline leak occurs at the first stage of a potential academic career….

Why, then, do women either not enroll in the first place or drop out after the introduction?  I.e., why is philosophy not an attractive major to women?

…The two most important factors turned out to be self-perceptions of brilliance and of belonging…

[T]he typical philosophy major perceives themselves as systematizing, as brilliant, does not value having a family, nor making money—all compared to the typical psychology major.  Fewer women than men show these self-perceptions and preferences.

The study is available from the authors, and there is lots more detail at the link, above.