Nearly half of all college math majors are women, and females now score as well as males on standardized math tests. Yet only about 30 percent of Ph.D.s in mathematics—and fewer in computer science, physics and engineering—are awarded to women every year, and men far outnumber women in science- and math-related tenure-track positions at U.S. universities. Why? For decades researchers have blamed sex discrimination and bias, but research suggests that there may now be a less sinister culprit: motherhood.

There is no arguing that women in science have had to fight sex discrimination for decades. But Wendy Williams and Stephen Ceci, a husband-and-wife team of psychologists at Cornell University, recently reviewed the literature on whether female scientists still have more trouble landing jobs, publishing papers or winning grants when compared with men. They found no evidence of lingering bias. “The problem is that women don’t apply for the jobs, not that they’re discriminated against once they apply,” explains Williams, who initially published the research in the Proceedings of the National Academy of Sciences USA last year and wrote a follow-up article in the March/April issue of American Scientist.

According to a report by the National Academy of Sciences, which Williams and Ceci cite, 27 percent of Ph.D.s in math are awarded to women, but females make up only 20 percent of the tenure-track applicant pool for positions in mathematics. In chemistry, the loss is greater: 32 percent of Ph.D.s are awarded to women, but only 18 percent of tenure-track chemistry job applicants are female.

What holds women back, Williams says, is the realization that they cannot juggle the many demands of an academic career and also have a family. The busiest years of a researcher’s life are in her 20s and 30s, which corresponds with the time her biological clock is ticking most loudly. Men can put off having kids longer and can also more easily juggle career and family because women still “do the lion’s share” of child care, Ceci adds. Recent research by Adam Maltese, a science education researcher at the University of Indiana, shows that men are 5 to 10 percent more likely than women to have kids while in graduate school.*

Not everyone believes this is the whole story, however. “Motherhood and family do have an impact on women’s career trajectories in the sciences, but I think that this is too simplistic,” says Shirley Malcom, head of education and human resources at the American Association for the Advancement of Science. Plenty of successful female scientists have families, she notes.

But Malcom, Williams and Ceci agree that universities should give women the option of working part-time or flexible hours when they want to start families and “stopping the tenure clock” so that women can take more time with their careers. Many universities have started offering family leave to graduate students, extending stipends and health benefits while suspending academic deadlines for those expecting babies. Women should never be forced to pick between career and family, Malcom says, and institutions need to “create a climate that allows them to not have to make these really tough, terrible choices.”

This article was published in print as "The Motherhood Gap."

*Erratum (6/4/12): Maltese's affiliation is given incorrectly. He is at Indiana University Bloomington.