In the United States, “among students whose first major is philosophy, 36% are women” while “among students whose second major is Philosophy, 43% are women.”Those are findings from Eric Schwitzgebel (UC Riverside), who notes:
The difference between 36% and 43% might not seem large, but I’ve spent over a decade looking at percentages of women in philosophy, and anything over 40% is rare. For decades, until a recent uptick, the percentage of women majoring in philosophy stayed consistently in a band between 30% and 34%. So that 43% pops out.
He also finds that: “schools in which a second major [in general, not just philosophy] is common also tend to have philosophy majors that are more evenly divided between men and women.”
More details here.