A few thoughts:

(1) I agree with Nanani, and think it would be awful to actively try to "recruit" females, or even really do anything to entice them to come/stay. Though I appreciate the spirit of the post nonetheless because I think it's a very interesting and important issue, and I think it's okay to acknowledge it and question it. If anything, efforts to even out the male/female imbalance would have to be made on a much greater scale to start to see change.

(2) Do people really think that it's an issue of females frequenting Less Wrong and then LEAVING? I doubt it. I suspect that a much lower proportion of females even happen upon the blog in the first place. This would eliminate a number of the explanations.

(3) This is an issue that deeply intrigues me. I have some fairly simple theories. Unfortunately, I am not well-versed enough in evol. psych., gender studies, history, sociology, etc. to feel like I have enough background to really get at the heart of the matter. So most of my ideas are purely anecdotal.

I believe that females on a whole are less interested in intellectual pursuits. Particularly intellectual pursuits that are HARD and take a higher amount of mental horsepower to grasp. Period. The question is: Why?

From my own experience, I've found myself to be less INNATELY CURIOUS than many of my male counterparts. Once I get onto a topic, I can puzzle over it for hours at a high level, but if the topic is not in front of me, my brain can be content to space out and think trivial things. Once I realized this was the case, I started to actively work to be more curious and to think more. When I'm sitting around spacing out, I will actually tell myself that I should start thinking about a problem. My brain does not do this automatically.

Now, I don't know if this is purely a messed up issue that I have to deal with, or if it extends across the female gender. From observing other females, it doesn't seem unreasonable that others would face the same lack of intellectual curiosity.

My big question is where does this come from? It's either biological or social. I used to think it was biological (this helped me reconcile the fact that I had to work overtime and be more aware so that I could become more interested in things in the first place). Now I think it's entirely possible that the explanation is social and that females, through media/peer groups/etc. simply are not encouraged to be as curious about intellectual issues and by the time they are older, they've simply stopped thinking. (This is all pertaining to females as a group, not particular individuals).

(4) I don't think the atmosphere (meanness) of this site is the problem. Enough females are thick-skinned. I think it's simply the subject matter. Though I agree that it's possible that the ratio of females is slightly higher than what is apparent because of their relative silence. I personally have a much higher fear of rejection to comments, etc. This extends to in-person interactions, and upon the slightest rejection, I will quickly shut up.