One recent study found that adding more women to the syllabus didn't make much difference, which is what I would have expected. Anecdotally, I have had the impression that sexual harassment has been a major deterrent for female students, though lower levels of tolerance for this misconduct may help change that. But a senior female philosopher elsewhere writes with an interesting and different take:
My assessment of the undergrad women in philosophy thing: undergrad women get sick of being talked over and strawmanned by their peers in and out of the classroom, and get sick of classes where the male students endlessly hold forth about their own thoughts. Relatedly, they find many thought experiments overall less compelling perhaps because, (a) women may start out in philosophy feeling less impressed with a priori speculation in general, perhaps because of confidence issues and the topics, and more importantly (b) they are less able to receive credence for their thoughts about the thought experiments when they try to engage with their peers, and (c) a lot of it seems like just more blowhard bullshitting by a bunch of dudes, on esoteric topics that they aren’t really allowed to engage in fully (see (b)). (Metaphysics is really a lot like that. Trad epistemology too.)
I will say that over two decades of teaching, it has seemed to me that the students who speak out of proportion to what they have to say are overwhelmingly male. Maintaining control of the classroom, and creating a welcoming environment for all student contributions, can probably go some distance to rectifying this--but that, of course, supposes levels of pedagogical talent and sensitivity that many philosophy faculty probably lack. But I'm curious what readers make of this diagnosis.