The study — a working paper published by the Census Bureau in November — is one of several recent papers that show that children account for much of the remaining gender pay gap. That gap has narrowed significantly over the past four decades, as women have gotten more education and entered male-dominated professions, but a divide remains.

Women who have babies late typically have different career paths from those who have them early. Those who first give birth in their late 30s tend to be more educated with higher-earning jobs, while those who have babies in their early 20s have less education and lower earnings.

Low earners have a smaller pay gap in general, and people who have babies in their late 30s could have a smaller pay gap because they are less likely to have more than one child. But the fact that both groups of women recover their earnings, relative to their husbands, suggests there’s also something about having children outside the prime career-building years that hurts women’s pay less, no matter the occupation.

One explanation is that the modern economy requires time in the office and long, rigid hours across a variety of jobs — yet pay gaps are smallest when workers have some control over when and where work gets done. In high-earning jobs, hours have grown longer and people are expected to be available almost around the clock. In low-earning jobs, hours have become much less predictable, so it can be hard for working parents to arrange child care.

The issue, in general, comes down to time. Children require a lot of it, especially in the years before they start school, and mothers spend disproportionately more time than fathers on child care and related responsibilities. This seems to be particularly problematic for women building their careers, when they might have to work hardest and prove themselves most, and less so for women who have already established some seniority or who have not yet started careers.