The overwhelmingly liberal tilt of university professors has been explained by everything from outright bias to higher I.Q. scores. Now new research suggests that critics may have been asking the wrong question. Instead of looking at why most professors are liberal, they should ask why so many liberals  and so few conservatives  want to be professors.

A pair of sociologists think they may have an answer: typecasting. Conjure up the classic image of a humanities or social sciences professor, the fields where the imbalance is greatest: tweed jacket, pipe, nerdy, longwinded, secular  and liberal. Even though that may be an outdated stereotype, it influences younger people’s ideas about what they want to be when they grow up.

Jobs can be typecast in different ways, said Neil Gross and Ethan Fosse, who undertook the study. For instance, less than 6 percent of nurses today are men. Discrimination against male candidates may be a factor, but the primary reason for the disparity is that most people consider nursing to be a woman’s career, Mr. Gross said. That means not many men aspire to become nurses in the first place  a point made in the recent Lee Daniels film “Precious: Based on the Novel ‘Push’ by Sapphire.” When John (Lenny Kravitz) asks the 16-year-old Precious (Gabourey Sidibe) and her friends whether they’ve ever seen a male nurse before, all answer no amid giddy laughter.

Nursing is what sociologists call “gender typed.” Mr. Gross said that “professors and a number of other fields are politically typed.” Journalism, art, fashion, social work and therapy are dominated by liberals; while law enforcement, farming, dentistry, medicine and the military attract more conservatives.