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 Even though that may be an outdated stereotype, it influences younger people’s ideas about what they want to be when they grow up.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.

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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 mismatch between schooling and salary complements a theory that the Harvard professor Louis Menand raises in his new book “The Marketplace of Ideas.” He argues that the way higher education was structured by progressive reformers in the late 19th century is partly responsible for the political uniformity of today. In the view of the early reformers, the only way to ensure that quality, rather than profit, would be rewarded was to protect the profession from outside competition. The tradeoff for lower salaries was control; professors decide who gets to enter their profession and who doesn’t.

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, 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.”

Other contributors to the book, Matthew Woessner and April Kelly-Woessner, who are husband and wife, also found that conservatives are less interested in pursuing advanced degrees than liberals.