The NYT thinks they have the answer: typecasting
. Chicken or egg?
From the news article: "Nearly half of the political lopsidedness in academia can be traced to four characteristics that liberals in general, and professors in particular, share: advanced degrees; a nonconservative religious theology (which includes liberal Protestants and Jews, and the nonreligious); an expressed tolerance for controversial ideas; and a disparity between education and income."
Among other things, the article fails to account for why, if their theory holds, some conservatives pursue an advanced degree. Condeleeza Rice comes immediately to mind. However I thought you all might find the news article interesting.