On the topic of Philosophy’s uneven sex ratios: Gina Schouten has a really interesting paper about stereotype threat as a possible explanation of those ratios (PDF). Her paper is, as she says, an armchair reflection on the hypothesis, but I think it would be useful to anyone wanting to study the causes of the sex ratios empirically.
The reason she has to do an armchair reflection is that Philosophy is a small discipline, and one the composition of which does not have huge social consequences, so the incentives for empirical researchers to give it the kind of attention they give the STEM subjects, the subjects for which the stereotype threat hypothesis was formed, and has been tested, and for which treatments have been devised, are small (Kieran seems to do it as a strange sort of hobby – but I don’t think his discipline promises great rewards for this part of his work).
Her reflections, though, are interesting and useful. She points out that the main leak in the pipeline is between the first philosophy course and the major. It would be really handy if it turned out that stereotype threat explained the exit of students at this point in the pipeline, because psychologists have devised interventions to counter stereotype threat that are extremely cheap and easy to implement, and seem to be highly effective (see footnote ). We could adapt some of those interventions relatively easily to Philosophy courses. (Then we could continue to be completely insensitive and rude in the way we teach, without suffering the consequence of depriving our discipline of talent!)
Problem is that we don’t have a lot of evidence, and some of the features of stereotype threat seem to be absent. For example, the fact that girls get lower average grades in any given STEM course is prima facie evidence that they are underperforming (one indicator of stereotype threat). I don’t have data on how well girls and boys do in intro level courses, but anecdote suggests that girls do not get worse grades than boys (Ok, ok, I’m writing this, and realize I should just get someone to check for my dept, and I’ll report back if it’s legal to). Of course, “underperformance” means something like “lower performance than the student should perform given his or her prior achievement”, and given that girls at most institutions have significantly higher prior achievement on most measures, they could be getting higher grades than boys and still be underperforming.
Another problem with the idea that stereotype threat explains why girls leave after the first one or two courses is that they just lack the stereotype. After all, philosophy is a found major, and because they have no experience of it, our students lack the relevant stereotypes: girls don’t think that philosophy is the kind of thing that girls do badly, or that others think that, because they don’t know what it is. In so far as they do have beliefs about what philosophy is , those beliefs are usually quite wrong, and we disabuse them pretty quickly.
However, as she points out, their first encounter with the subject might easily introduce a stereotype to them:
The first bit of data we provide is the gender composition of the disciplinary experts in the classroom: the instructors and teaching assistants. Many undergraduates will be exposed to only male experts during their first experience with philosophy. Especially in large classes with multiple teaching assistants, gender homogeneity may constitute a striking bit of data indeed. Students might explicitly infer from the prevalence of male experts that men tend to excel in philosophy, whereas women do not. Or they might reach this conclusion indirectly, noticing that the gender makeup of philosophy resembles that of math and science, and then applying gendered stereotypes from other disciplines to philosophy.
The second bit of data from which students may infer the existence of gendered stereotypes about philosophy is the course syllabus. Even the most well-meaning among us are likely to distribute syllabi on which men’s contributions far exceed those of women. Even if students do not notice this immediately, they are likely during the first few weeks of the course to notice that all the articles they are reading were written by men, if only because we refer to the authors using male pronouns.
The course content during the first few weeks of class often constitutes a third data point for students. Many of us begin our courses with a brief introduction to logic. From this, students may quickly come to believe that philosophy generally is similar to math, or that an important foundational component of philosophy is similar to math. They may infer that being good (bad) at math bodes well (ill) for one’s prospects in philosophy. Students may then apply gendered math stereotypes to philosophy, or simply experience anxiety at the prospect of confirming math stereotypes in the philosophy classroom, much as they have been shown to experience anxiety at the prospect of confirming math stereotypes in physics classrooms.
There’s a lot else in the paper, but I want to focus on this part of it. Among the interventions Schouten suggests are altering syllabuses to include more women authors, and doing our best to ensure that students are exposed to live female philosophers, both by assigning women teachers to classes where they are likely to teach a good number of female students and by using guest lecturers. 
A discussion with a (feminist) colleague who teaches ethics at another institution indicated how difficult the first suggestion – changing the sex ratios of the assigned authors – is on some approaches to philosophy. Said colleague teaches her introductory ethics course using the historical approach (the same I used the first time I taught it): but on the historical approach, you are going to teach the historical figures: probably a couple out of Plato, Hobbes, Rousseau, Nietzsche, Marx and Rawls and definitely all of Aristotle, Hume, Kant and Mill. No women. You can force women in – both times I taught it that way I used Mary Wollstonecraft, and my colleague ended her course with Philippa Foot. But neither of these figures has the stature of any of the men I’ve mentioned, and my guess is that the students can pretty much see through the token gesture (and if you teach it chronologically, the women come along pretty late anyway). It is, obviously, much easier to change the ratios if you take a problem-based approach – teach about realism and relativism, the significance of consequences, integrity, virtue ethics, partiality, sexual morality, forgiveness, the significance of death, etc. Keep it mainly focused on contemporary authors and you could easily design a sensible syllabus, using only very high quality philosophical literature with just one man on it (Williams is who I would find indispensible – I’m not suggesting that it would be a good idea to have only one man on the syllabus, just pointing out how easy it would be to alter the sex ratios). The same is probably true for an intro to philosophy course (I say probably because I’m not sure I could do it, but I’m not sure I could design a good syllabus for an Intro to Philosophy course, period.
Couldn’t a department experiment with this? My guess is that the historical approach is dominant in many departments as things stand, and that it should be possible to persuade a few colleagues to adopt a problem-based approach with significant numbers of female authors on the syllabus, and see whether rate at which girls take a second Philosophy course changes (using colleagues who have not changed their approach as a control). Of course, an experiment like this would require finding out what people are actually teaching, which goes against the grain, but might be valuable for other reasons.
 Sorcery maybe?
 I’ll disclose that Schouten herself has been used (by me) in exactly the latter way – for about 3 years she was my go-to substitute for numerous reasons, including that my large classes are largely female and I want those students to see a female philosopher in action (esp. one who is evidently extremely talented both as a teacher and as a philosopher). Also, that both she and I see the limitations of and problems with this suggestion.