By PATRICIA COHEN


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 Even though that may be an outdated stereotype, it influences younger people’s ideas about what they want to be when they grow up.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, they cited the low numbers of male nurses. 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.

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.

“These types of occupational reputations affect people’s career aspirations,” he added in a telephone interview from his office at the University of British Columbia. Mr. Fosse, his co-author, is a Ph.D. candidate at Harvard.

The academic profession “has acquired such a strong reputation for liberalism and secularism that over the last 35 years few politically or religiously conservative students, but many liberal and secular ones, have formed the aspiration to become professors,” they write in the paper, “Why Are Professors Liberal?” That is especially true of their own field, sociology, which has become associated with “the study of race, class and gender inequality - a set of concerns especially important to liberals.”

What distinguishes Mr. Gross and Mr. Fosse’s research from so much of the hubbub that surrounds this subject is their methodology.

Whereas most arguments have primarily relied on anecdotes, this is one of the only studies to use data from the General Social Survey of opinions and social behaviors and compare professors with the rest of Americans.

Mr. Gross and Mr. Fosse linked those empirical results to the broader question of why some occupations - just like ethnic groups or religions - have a clear political hue. Using an econometric technique, they were then able to test which of the theories frequently discussed were supported by evidence and which were not.

Intentional discrimination, one of the most frequent and volatile charges made by conservatives, turned out not to play a significant role.

Typecasting, of course, is not the only cause for the liberal tilt.

The characteristics that define one’s political orientation are also at the fore of certain jobs, the sociologists reported.

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.

The tendency of people in any institution or organization to try to fit in also reinforces the political one-sidedness. In “The Politically Correct University: Problems, Scope and Reforms,” a collection of essays published by the American Enterprise Institute in Washington, D.C., a conservative research group, Daniel B. Klein, an economist at George Mason University in Virginia, and Charlotta Stern, a sociologist at Stockholm University, argue that when it comes to hiring, “the majority will tend to support candidates like them in the matter of fundamental beliefs, values and commitments.”