Here's an interesting paper about why women are under-represented in philosophy. Wesley Buckwalter and Stephen Stich propose that part (but only part) of the explanation is that women's philosophical intuitions differ from men's. When students are presented with philosophical thought experiments (Thomson's famous violinist, Jackson's case of Mary, Foot's trolley cases, Gettier cases, etc.), their study reveals that men and women don't have the same reactions. The authors think women's intuitions may make them feel "different" and thus less inclined to join the profession.
There's a problem though... In several of their cases it's women, not men, who have "the right intuitions"-- the ones that "most people have," as philosophers like to say. How does that square with the exclusion hypothesis? The authors speculate that philosophy is more often taught by men, who are likely to have the intuitions seen more often in men, so female students are likely to find themselves at odds with their professors. The resulting sense of alienation will outweigh whatever validation comes from one's intuitions meshing with a well-known author's.
I'm not so sure about that. One of the thought experiments is Frank Jackson's case of Mary, the color-blind neuroscientist. She learns all the physical facts about the color red. Will she thereby be in a position to know what it is like to see red? The data shows that females are more skeptical than males. Are we really to believe that male philosophy professors are typical males, instead of having Jackson's intuitions--that Mary cannot know what it is like? That would surprise me a great deal. Still, in the majority of the cases Buckwalter and Stich look at, it's men who have "the right intuitions"--the ones that match the author's. That's got to be off-putting for women. What to do? Read the paper for their answer....
The paper got me thinking about other aspects of philosophy that may be off-putting for women (research needed--I'm just speculating). For example, there are the thought experiments themselves. A lot of them are really weird. Both male and female students can be impatient with these things, and it could be that women feel especially that way. The good news is that the thought experiments are easily transposed into the key of real.
Take, for example, Thomson's case of the famous violinist. You are kidnapped by the Society of Music Lovers and hooked up to him, and have to lie next to him for 9 months giving him life support. Granted, he has a right to life; does that mean you have to stay there and keep him alive? The analogy is supposed to help us think about what would follow if a fetus were a person with a right to life. We are supposed to conclude: not much. Women would still have a right to choose abortion.
The violinist story is a lot of fun, but it's not impossible to replace it with a real life story. I don't have the perfect story to suggest, but there's an interesting variation on the violinist theme in Joe Simpson's book Touching the Void. Joe is mountain climbing in the Andes with a friend. The two are connected to each other by a rope, and Joe falls into a deep crevasse. Joe is (of course) a person with a right to life. Does that mean that the friend who is connected to him must stay there trying to save his life? The friend thought otherwise--he cut the cord. Was that permissible or impermissible? For some students, that would be a more engaging question than the one about the violinist.
Another potentially aversive aspect of philosophy classes is the way the most opinionated people are rewarded. If you are "still thinking" and not ready to declare a preference for one view over the others, let alone to defend it tooth and nail, you will get very few pats on the head. The expectation that one will quickly have firm views might be more off-putting to women than to men. And it could be a bigger problem than aggressive combat--a feature of philosophy often said to be particularly bothersome to women. It's fun to fight (isn't it?) when you're 100% sure what you think. Not fun at all when you're still thinking, still trying to understand. In philosophy culture as it is, one must have a view now, not tomorrow, not next week.
[Update: Why would women especially resent the "must have an opinion" culture of philosophy? I can think of lots of possible explanations, but I don't think it's because they're simply slower then men. Heavens!]
So--there are lots of potential factors that could be studied, and if they really do affect men and women differently, some are fairly easily changed.