The author of the particularly dismal email was referring to a 2015 World Economic Forum report, which predicted that global gender parity, or the economic and social equality of the sexes, would not arrive for 177 years. This time last year, the Institute for Women’s Policy made a similar assertion, arguing that American men would significantly out-earn American women until 2058.

These estimates, of course, rely on the oft-debated gender wage gap. The Census Bureau calculates that the median woman in the United States makes 79 cents for every buck paid to the median man. The gap widens by race, with black women earning 60 cents and Hispanic earning 55 cents to every white man's dollar.

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Critics who accuse politicians of whipping out these numbers without nuance or context say the figures are misleading. Sure, a gap persists — with little movement since the 1990s — but significant parts of it can be attributed to choice. And federal law has prohibited pay discrimination since 1963.

Since women comprise about half the labor force and now outpace men in college enrollment, economists say it’s important to study why one gender, on average, still makes much less money than the other. Let’s break it down into three parts. Full disclosure: They're highly simplified, but less simplified than proclaiming women make 79 cents for every men's dollar.

How a woman works

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In a January study, Francine Blau and Lawrence Kahn, Cornell University economists who studies pay disparities, crunched national data that included labor market experience, slicing the gap into sections that better explain the American woman’s paycheck. Career decisions, they found, account for roughly half the gap.

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Evidence emerges in the early years. When young women start jobs, they typically earn nearly as much as young men. In one 2013 sample, among workers who were ages 25 to 34, women on average made 93 percent as much as men, according to the Pew Research Center.

The disparity grows, however, as women age. One popular explanation: motherhood. Women, more often than men, tend to leave the workforce for extended periods to have and raise children. They’re also more likely to leave the office early to pick up the kids or take a day off when a family member is sick.

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Both genders, though, valuable flexible schedules and time with family, according to recent surveys of working parents.

That’s where “choice” gets complicated: The division of domestic labor remains unbalanced in the United States, where female breadwinners support 40 percent of homes. Women typically devote two hours and 12 minutes daily to housework, while men spend one hour and 21 minutes.

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Some economists argue that women’s outsized burden at home stalls their careers. Mothers in the United States who work full time, year round, meanwhile, make an average of $40,000, compared to $56,999 paid to fathers, according to the National Women’s Law Center.

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June O’Neill, former director of the Congressional Budget Office, has noted that many women simply prefer to take less demanding, more flexible roles in order to better accommodate their families. Their choices, O’Neill argues, doesn’t require government intervention. “Any wage gap is rooted more in social trends and tendencies than malicious discrimination by employers,” she wrote.

Where she works

In Blau and Kahn’s explanation of the wage gap, career decision combines two factors: industry choice (17.6 percent of the gap, they write) and occupation choice (32.9 percent).

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Women tend to concentrate in low-paying jobs, according to the Bureau of Labor Statistics. Male-dominated fields, like finance and technology, meanwhile tend to pay more. The trend is global. An analysis of 142 countries by the International Labour Organization, released Tuesday, shows that women worldwide tend to be overrepresented in the lowest-paid occupations. Particularly in developed nations, women outnumber men in service jobs “associated with part-time employment and low pay,” the authors noted.

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A 2014 report from the National Women’s Law Center found women are two-thirds of America’s 20 million low-wage workers. Men in the United States, the researchers found, require less education to achieve higher pay: Female workers with a high school degree make up nearly a quarter of the low-wage workforce, while male workers with the same education level comprise 12 percent.

How she's treated at work

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After accounting for job, industry, education and experience, Blau and Kahn determined that 38 percent of the wage gap comes from factors “unexplained.”

They attribute this mystery portion to something tough to prove: discrimination. "Discrimination is not necessarily overt or conscious," Blau said. "It could be subtle. People have stereotypes about males and females, and they can creep in during hiring or promotion decisions."

The Brookings Institution illustrated Tuesday how the unexplained element has changed over the years. As society evolves to support more women at work, the unexplained slice has shrunk but remains significant.