When men and women finish school and start working, they’re paid pretty much equally. But a gender pay gap soon appears, and it grows significantly over the next two decades.

So what changes? The answer can be found by looking at when the pay gap widens most sharply. It’s the late 20s to mid-30s, according to two new studies — in other words, when many women have children. Unmarried women without children continue to earn closer to what men do.

The big reason that having children, and even marrying in the first place, hurts women’s pay relative to men’s is that the division of labor at home is still unequal, even when both spouses work full time. That’s especially true for college-educated women in high-earning occupations: Children are particularly damaging to their careers.

But even married women without children earn less, research shows, because women are more likely to give up job opportunities to either move or stay put for their husband’s job. Married women might also take less intensive jobs in preparation for children, or employers might not give them more responsibility because they assume they’ll have babies and take time off.

“One person focuses on career, and the other one does the lion’s share of the work at home,” said Sari Kerr, an economist at Wellesley College and an author of both papers published in May. One published in the American Economic Review; the other was published as a working paper by the National Bureau of Economic Research. The other researchers were Claudia Goldin of Harvard, Claudia Olivetti of Boston College and Erling Barth of the Institute for Social Research in Oslo.

It is logical for couples to decide that the person who earns less, usually a woman, does more of the household chores and child care, Kerr said. But it’s also a reason women earn less in the first place. “That reinforces the pay gap in the labor market, and we’re trapped in this self-reinforcing cycle,” she said.

Some women work less once they have children, but many don’t, and employers pay them less, too, seemingly because they assume they will be less committed, research shows.

Even when mothers cut back at work, they are not paid proportionately less. When their pay is calculated on an hourly basis, they are still paid less than men for the hours they work, Goldin has shown in previous work. Employers, especially for jobs that require a college degree, pay people disproportionately more for working long hours and disproportionately less for working flexibly.

To achieve greater pay equality, social scientists say — other than women avoiding marriage and children — changes would have to take place in workplaces and public policy that applied to both men and women. Examples could be companies putting less priority on long hours and face time, and the government providing subsidized child care and moderate-length parental leave.

According to the data, Kerr said, college-educated women make about 90 percent as much as men at age 25 and about 55 percent as much at age 45.

The new working paper, which covered the broadest group of people over time, found that between ages 25 and 45, the gender pay gap for college graduates, which starts close to zero, widens by 55 percentage points. For those without college degrees, it widens by 28 percentage points.

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,” Kerr said.

Twenty-seven percent of the overall pay gap is from men being more likely to jump to higher-paying firms, the economists found. When married women leave jobs, they are less likely to get a big pay bump as a result. Previous research has found they are more likely to leave without another job lined up; they may move for their husband’s job or take time off with children.

But the bulk of the pay gap — 73 percent, they found — is from women not getting raises and promotions at the rate of men within companies. Seniority and experience seem to pay off much more for men than for women.

“On every possible front, women are getting the short end of the stick,” Kerr said. “Whether they’re changing jobs or trying to stick with the current employer, the returns are always smaller.”

The average college-educated man, for instance, improves his earnings by 77 percent from age 25 to 45, while similar women improve their earnings by only 31 percent. Men without college degrees increase their earnings much faster than similar women in the first decade of their careers, but by age 45, women catch up.

Even women who catch up, however, pay a long-term price. They’ve lost a significant amount of pay — in wages, raises and retirement savings — along the way.

This article originally appeared in The New York Times.