Women are shunning academic careers in math-intensive fields because the lifestyle is incompatible with motherhood, researchers at Cornell University found in a study to be published next month in American Scientist Magazine.

Universities have long been criticized for hiring and evaluation policies that discriminate against women, but the findings of this new study point to the female biological clock as a main reason why so few women end up as professors in fields such as math, engineering, physics and computer science.

A woman who wants a family looks at the rigorous path to a tenured position and considers how old she will be before she can start a family and how little time she will have to raise her children. Many of those women opt for a more flexible career.

"Universities have been largely inflexible about anything other than the standard time table, which is you kill yourself for years and only then would you consider getting pregnant," said Wendy Williams, a human development professor at Cornell who co-authored the study with her husband, Stephen Ceci.

Williams and Ceci analyzed data about the academic careers of men and women with and without children. Before women became mothers, they had careers equivalent to or more successful than their male peers. But once children entered the equation, the dynamic changed.

Women in other academic fields such as the humanities and social sciences face similar hurdles and often leave academia as well. But because there are so many women in those Ph.D. programs, enough ultimately stay to amount to a critical mass of female professors.

In math-heavy fields, however, women make up a tiny minority of the graduate students. So when the rare few who make it through a Ph.D. program leave because universities are insensitive to their needs as mothers, the net result is virtually no women represented on faculty rosters, the study said.