The two new studies draw their data from the Health and Retirement Study at the University of Michigan and from the Current Population Survey and the Survey of Income and Program Participation at the Census Bureau. The data included surveys that tracked the same women over time; in some cases, the researchers studied respondents’ income tax and Social Security records.

Women now in their 60s and 70s were the first generation to become professionals in large numbers, but that doesn’t explain all of the increase in older women who work, which began in the late 1980s.

For some women, the decision to keep working was unexpected and not necessarily welcome, because of late-in-life divorce, pension or real estate losses, or changes in Social Security. A recent paper by the economists Annamaria Lusardi and Olivia S. Mitchell examined women who were working into old age because they had more debt than in previous generations and lacked financial savvy. Older workers who lost their jobs during the financial crisis were more likely to be unemployed long term — especially women, in part because their résumés tended to be spottier.

But most of the time, Ms. Goldin and Mr. Katz found, women are working longer because of decisions they made much earlier in their lives — to get an education and spend years building a career.

If people work when they’re younger, economists say, they’re more likely to work when they’re older. And because women are marrying and having babies later, they spend more time pursuing careers first. That means that even if they take breaks to care of children, they are likely to return to work and to work past a typical retirement age. Children had no effect on working later in life, the analysis found.

The same thing is happening among women in their 60s in most developed countries, according to data from the Organization for Economic Cooperation and Development.