Tierney, John. "The Left-Leaning Tower." NEW YORK TIMES July 22, 2011.

Why are conservatives such a minority at so many graduate schools? Conservatives like to blame liberal bias. Liberals like other explanations.
One — the most tactful hypothesis — is that conservatives just aren’t interested in academic careers. Another — the most smug hypothesis — is that conservatives are just too close-minded and dimwitted.       

Now, fortunately, we have something beyond hypotheses, courtesy of scholars who have been taking a close look at their colleagues. Some even conducted a small sting operation that they believe is the first field experiment on political bias in academia. The perpetrators include Neil Gross, a professor of sociology at the University of British Columbia, whose previous work showed that Democrats outnumber Republicans by about 4 to 1 among professors, by at least 6 to 1 at elite universities, and by still higher ratios in departments of the humanities and social sciences. . . .

http://www.nytimes.com/2011/07/24/education/edl-24notebook-t.html?_r=3&ref=edlife&pagewanted=all