Sociologists Neil Gross and Ethan Fosse are trying to figure out why professors in general, and sociologists in particular, as so liberal. Patricia Cohen has a pretty good story about it in the New York Times. They argue that the academy has become "politically typed" on the analogy of some jobs becoming "gender typed." It is not that conservatives get all the way through Ph.D. programs only to get turned away from professoring jobs by political discrimination. Rather, conservatives don't even start down the path of professor training. They are more likely to head to business and the professions in the first place.
I am wrestling with this argument. I think it is mostly true. I see a broad political mix of undergraduate students. Of those who head on to graduate school to be professors, quite a few are very liberal; almost none (actually, none that I can think of) are strong conservatives. The liberals expect the academy to be an easier and friendlier place for them to make their way than any other occupation would be. I attract and encourage centrists, some of whom go on to academic careers. Some of them are, indeed, pushed left by academia, though just as often they are pushed more to the center in reaction. This center movement is especially true for religious and family-oriented centrists.
In my own case, as I have moved more to the center, I have encountered some resistance from liberal academics who regard liberalism as a requirement of being a professor. Just recently I proposed a family sociology textbook that would be an empirical and centrist compilation of the basic facts of most people's family lives. A potential publisher said, reasonably, that such a book would be so controversial that it would be too risky to publish - not enough professors would assign it to make it financially viable.