She backed her case with global data. Women worldwide devote an average of 4.5 hours each day to unpaid work — cooking, cleaning, changing the baby’s diaper. Men contribute less than half that much time, according to the OECD.

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The domestic division of labor remains staggeringly unbalanced in the United States, where female breadwinners now support 40 percent of homes. Women here typically spend two hours and 12 minutes daily on housework, while men spend one hour and 21 minutes.

A 2015 survey by Working Mom, furthermore, found that female breadwinners who lived with male partners still reported handling the bulk of grocery shopping, meal preparation, bill-paying and cleaning.

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“This isn’t a global plot by men to oppress women,” Melinda wrote. “It’s more subtle than that. The division of work depends on cultural norms, and we call them norms because they seem normal — so normal that many of us don’t notice the assumptions we’re making. But your generation can notice them — and keep pointing them out until the world pays attention."

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(Bill, it should be noted, drives their kids to school every other day.)

Melinda is right about cultural norms in America and beyond. Even after moms could vote and become chief executives and run for president, dads maintained their status as primary provider status in popular culture.

Consider Fusion’s new breakdown of the Oscar-winning actresses through the years: Most have played wives.

The wife penalty

Of course, that’s a highly simplified explanation for why one sex shoulders more unpaid work. And it’s not to say all women feel trapped in motherhood and the associated duties.

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But, as Gates points out, for women who want to excel in their careers, the disproportionate load can create an unfair penalty. Men, it turns out, might be holding their partners back when they don't share non-work tasks.

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Mothers in the United States who work full time, year round, make an average of $40,000, compared to $56,999 paid to fathers, according to the National Women’s Law Center. That’s 70 cents to the dollar.

Economists say this disparity probably comes from a blend of factors: Women are sometimes “mommy tracked,” or passed over for high-profile projects or promotions, when employers assume motherhood (but not fatherhood) zaps productivity. A 2015 study from the University of Georgia found that, for this reason, expectant mothers are often afraid to tell their bosses they’re pregnant.

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Women do take more time off to tend to family, but that's not always a choice. Sometimes, as the data shows, husbands just don’t pull their domestic weight. They also tend to prioritize their careers over their wives’, while wives tend to equally prioritize both partners’ careers, according to a 2014 study of 25,000 Harvard Business School graduates.

The lesbian wage premium

Evidence for the wife (or husband) penalty shows up in what researchers call the “lesbian wage premium.” Across years of research, women who have never lived with a male partner make more money than women who have.

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Last year, Marieka Klawitter, professor of public policy at the University of Washington, examined 29 studies across the Western Hemisphere on wages and sexual orientation and found a 9 percent earnings premium for lesbians over heterosexual women. (Gay men, meanwhile, faced an 11 percent penalty, compared to straight men.)

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She controlled for parenthood and concluded lesbians simply had more education and work experience than the general female population.

But another study from the University of Nevada, which used national data from the year 2000, adds a stunning asterisk to Klawitter's findings: Lesbians who had previously lived with male partners made 9.5 percent less than those who’d never cohabitated with a husband figure.

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Were men actually the drags on women’s earnings?

After controlling for characteristics like education and experience, the authors found a 6.6 percent premium for cohabiting lesbians over cohabiting heterosexual women, with the premium falling to 5.2 percent if the lesbians had ever been married to a man.

(Still, two-women households, on average, see lower combined income than man-woman arrangements.)

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What's driving this disparity isn't clear. The gender dynamics that Gates refers to could be at play: A woman married to a man could be struggling with the unequal distribution of domestic chores, causing her career to suffer. Or, as the researchers guessed, a single, straight woman may invest less in education and job training if she anticipates a future caregiver role.

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“A woman in a heterosexual relationship may opt out of work because the man’s salary is higher, or paying for good child care would cost more than she earns, or breastfeeding would be more difficult at work,” Klawitter said. “We all create narratives about why we’re doing things but those narratives are driven by conditions out there in the world.”

For example, she said, some women take their husband’s name, assuming it’s more important to him or his family. Social norms make her much less likely to say the same about herself or her family, even if she cherishes her last name or built her identity with it.

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“This all snowballs into: Other women stay home -- should I?" Klawitter said. "Gender categories influence our behavior. It’s hard to tease out the discrimination from these gender roles we all create.”

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