Much of that happens early in people’s careers, during women’s childbearing years. The American Economic Review paper, which examined people born around 1970, found that almost all of the pay gap for college graduates came from ages 26 to 33.

The researchers used demographic data from the 2000 census and work history from 1995 to 2008 from the Census Bureau’s Longitudinal Employer-Household Dynamics program, which covers private-sector companies. These two data sets have rarely been combined, which allowed the researchers to connect people’s work histories with demographic data like age, education, marriage and childbirth.

The pay gap is larger for college graduates because their earnings are higher, and men dominate the highest-paying jobs. These jobs also place more value on long, inflexible hours.

People without college degrees start out with a slightly larger pay gap, but it is smaller throughout their careers. Part of the reason is that less educated men have fewer high-paying job options than they used to. “The pay gap is not because non-college-educated women do so well, but because non-college-educated men are not doing well,” Ms. Kerr said.