John Tierney had a piece in yesterday's New York Times on why conservatives tend not to go to graduate school in the social sciences or in the humanities. Tierney found that, contrary to what he called the smug hypothesis ("that conservatives are just too close-minded and dimwitted" for grad school), there is an element of economic rationality to the phenomenon.
Tierney spoke to Prof. Peter Wood, who noted that "It's become clear to [conservatives] that they're unlikely to succeed at the same level as someone going into these fields with more socially approved political convictions and attitudes." This leads conservatives to avoid the Ph.D. route or to suppress their convictions, an effort that is typically hard to sustain for very long.
I saw this phenomenon first hand in my own Ph.D. program at the University of Texas at Austin. A few years into my time there, a friend and classmate called me up at my apartment and asked in a nervous whisper if it was true that I had written for National Review. Bracing my self for a liberal onslaught, I acknowledged that yes, I had indeed written for and was indeed a political conservative. His response: "I'm a conservative too, but I don't let anyone know." I was glad to have the co-conspirator, and would often privately talk to him about conservative politics, but I was careful never to reveal his secret.
My friend is now teaching at a large state university. As far as I know, he has not confessed his political leanings to any of his colleagues, and a Google search on him does not reveal his conservative predilections in any way. This is the odd "success story" for conservatives in academia. It is possible for a conservative to get a graduate degree, and even at times a tenured teaching position. But the sacrifices one has to make along the way may not be worth the effort.