Female MBA graduates not only earn significantly less than their male colleagues, but the gap grows over time, according to a paper from the National Bureau of Economic Research. The authors tracked the earnings of 1,600 MBAs who graduated from the University of Chicago’s Graduate School of Business between 1990 and 2006; they attribute the differences to choice of subjects (women took fewer finance courses), that women’s careers were subject to more interruptions, and that the women worked fewer hours than the men.

The main underlying reason for the second and third of these explanations is that once women MBAs had children, many cut back on hours, or dropped out—13% of the women in the study were not working at all ten years after graduating, while the corresponding figure for men was a mere 1%. The researchers stress that active discrimination is unlikely to have played much of a role in the pay gap—but the continued influence of gender roles in determining career paths is clear.