(Bloomberg) — More and more older Americans are working longer. Women, new research suggests, are much of the reason why.

Working into your late 60s and even 70s is hardly as rare as it used to be; just ask Hillary Clinton, 68, or Donald Trump, 70. Today, Americans are more likely to work past 65 than at any point since before Medicare was created in the 1960s—but most are still men, whom we're used to seeing work well into old age. (Think Carl Icahn, still moving the market with his activist investing at 80, or Warren Buffett, at 86 still heading $360 billion Berkshire Hathaway.)

Now, more women are working at ages when their mothers and grandmothers were long retired. Economists and other academics are trying to figure out why, and new research suggests the trend will continue, and could accelerate.

A prime driver of working in old age is education: Both women and men with college educations are far more likely to be working in their late 60s and 70s than are less educated Americans, and the number of college graduates is on the rise.

Past work history also matters, and the surge of women into the workforce in the 1970s and 1980s means that these women, now older, have job skills, connections, and careers they can continue to pursue.

As the oldest Baby Boomers reach their 70s, they're not only working but increasingly are working full-time. Almost half of women working in their late 60s are in full-time, year-round jobs, up from about 30% 20 years ago, Harvard University economics professors Claudia Goldin and Lawrence Katz found in new research.

A major factor in whether women postpone retirement is whether they like their jobs, said Goldin and Katz, who analyzed survey data linked to Social Security earnings records. “As jobs become more enjoyable and less onerous and as various positions become part of one’s identity, women work longer,” they wrote.

Children, on the other hand, aren't much of a factor in whether women work to 65 and beyond, Goldin and Katz found. Having kids does make it tougher for women to stay in the work force full-time from ages 25 to 44—something Goldin and Katz blame partly on the fact that parental leave of longer than 12 weeks isn't mandated—but it doesn’t affect their participation later in life. While these mothers may end up earning less than if they hadn’t had kids, they do seem to be restarting their interrupted careers once their children are older.

Not all the trends are so cheery.

By the age of 65, women have typically spent less time in the workforce than men—which means less time saving for retirement and qualifying for Social Security benefits. Meanwhile, women in their mid-60s can expect to live longer than men; a current 65-year-old American man's average life expectancy is 83, while women can expect to live to almost 86 on average.

In a perfectly rational world, women might keep working a little longer to maximize their retirement benefits, but that’s not happening—at least not among married straight women, according to new research by Harvard University health policy professor Nicole Maestas. Women are typically younger than their husbands, but they tend to retire at about the same time, often in their “peak earnings years,” Maestas wrote. (Their husbands tend to be already past their peak years.) Those early-retiring women not only miss out on the chance to bank assets and maximize Social Security benefits but also may be too young to qualify for Medicare.

Other older women end up stuck working far longer than they’d like, financially unable to retire.

“Americans now work more hours per day, days per week, weeks per year, and years per lifetime than almost all rich countries on the planet,” said New School economics professor Teresa Ghilarducci in an e-mail. Working even longer, she said, is a “lazy answer” to the American “retirement income security crisis."

“Older Americans are nearing retirement with increasingly concerning levels of debt,” wrote Annamaria Lusardi of George Washington University and Olivia Mitchell of the University of Pennsylvania’s Wharton School in a forthcoming paper. Older women today, they found, are more indebted and "financially fragile" than older women in the past.

Borrowers between the ages of 50 and 80 saw their debt loads rise about 60% from 2003 to 2015, even as younger borrowers' debt loads fell somewhat. Two-thirds of people 65 to 74 have debt, and people 65 and older are the fastest-growing group of bankruptcy filers. And women with debt—particularly mortgage debt—are more likely to end up working at age 65, Lusardi and Mitchell found.

Expect more and more older women to keep working, whether they want to or not. Younger women’s participation in the labor force has actually fallen in recent years, while college graduation rates—the other important factor spurring women to work longer—continues to rise. But after analyzing the most recent data, Goldin and Katz found neither factor fully explained the rising number of women working past 65.

"Today's younger women will likely retire later than one would have predicted based on their educational attainment and lifecycle participation rates," Goldin and Katz wrote. Particularly among women 67 and younger, they wrote, “something else is keeping them in the labor force at older ages.”

At age 70, Carol Gardner, a former advertising art director, now works “more than full-time” on a business she started 18 years ago, after a divorce left her depressed and in debt. Her business, Zelda Wisdom, started out selling greeting cards featuring Gardner’s bulldog, Zelda, and expanded into other merchandising, as well as an animated film series.

“I think back to my mother and grandmother,” Gardner said in a phone interview from her home in Portland, Oregon. “When they said they were 70, I thought, ‘My god, their lives are over.’”
Gardner hardly feels the same now—so why retire?

“I’m having so much fun," she said. "I get physical exercise and I get mental exercise from it."
Among women like her, she said, her attitude is hardly unique. “There are more opportunities for people to keep doing what they love to do," she said.

Then she had to cut the interview short. She had a meeting with her business's accountants.

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