Computer scientists are very highly represented here; a show of hands on IRC found more than half had some CS background. This site is particularly appealing to the CS mindset, so that's not so surprising, but it means that Less Wrong inherits the same massive gender imbalance that computer science has. Of course, this only pushes the question one step away, to the reasons why CS has a gender imbalance; but that's a question that's already been studied, with many hypotheses put forth.