For recently minted Ph.D. scientists, having children and the choice of field to study play crucial roles in the very real gender pay gap, a new study finds. A year after receiving their doctorates, the women in the study earned nearly a third less than the men. When the researchers controlled for the field of study, the difference shrank to 11%, and when they controlled for gender, marriage, and parenthood, it vanished, with childless women matching men in income. “This suggests the presence of children contributes meaningfully to the gender wage gap,” the authors write in the May issue of American Economic Review, though they also note that “[t]hese results should be interpreted with caution” because of various complicating factors.

Using a novel combination of census data and earnings records, the researchers—Catherine Buffington and Benjamin Cerf Harris of the U.S. Census Bureau in Suitland, Maryland; Christina Jones of the American Institutes for Research in Washington, D.C.; and Bruce A. Weinberg of Ohio State University, Columbus—examined 1237 Ph.D. students (867 males and 370 females) who worked on federally funded grants at four U.S. universities over a period of 10 years and followed them as they entered the post-degree labor market. The authors found some differences between the genders in graduate school—for example, women were more likely than men to work with women faculty members—but “no clear disadvantages in the aspects of training environments that we can measure,” they write.

A “dramatic” income gap appears, however, when the new Ph.D. recipients move past graduation. Some of the difference reflects the fact that men tend to enter better-paid fields than women. “Males were more than twice as likely to complete dissertations in Engineering (45% versus 21%) and were 1.5 times as likely to study Computer Science, Math, or Physics (28% versus 19%),” the authors write. Meanwhile, 59% of the women studied biology, health, or chemistry—as compared to only 27% of the men. Beyond that, men are more likely than women to go work in industry, which pays better than government or academe, where women are likelier to work. But simply going into industry did not equalize pay. “Women and men earn the most in industry, but the gap is also larger,” the authors write. The only variables that fully accounted for the gap were the combination of gender, partnership status, and parenthood.

The crucial role that children play in female scientists’ earnings, as identified in this study, reinforces a substantial body of research on which we have previously reported. In an article we quoted in 2012, for example, Cornell University psychologists Stephen Ceci and Wendy Williams wrote that the “single most important factor in explaining women’s underrepresentation [in academic science is] a desire for children and family life.” And in the 2013 book Do Babies Matter? Gender and Family in the Ivory Tower, which we wrote about at the time, the authors answer the question resoundingly in the affirmative. “[H]aving young children dramatically reduces the likelihood” that women will get a tenure-track job offer, win a grant, or receive tenure, they write.