Eye-popping chart from Kieran Healy:
I always got the sense that this mostly reflected a kind of pure arbitrary path dependency in a very small field. There are few women in philosophy, and the resulting boys club atmosphere leads to an unusually high quantity of sexist bullshi. Given that it’s not a particularly remunerative or socially influential field, there’s little specific pressure to overcome these barriers to women stay away and the cycle continues. As Ned Resnikoff notes, this seems to end up leading to a fair number of invalid philosophical arguments gaining acceptance.
Back when I was a philosophy major, I think half the professors in the department were women, and there were also (and, I think, not coincidentally) a higher-than-average number of women graduate students.