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For decades after the 1920 ratification of the Nineteenth Amendment, which extended the franchise to women, women voted less frequently than men—and, mostly, for the same candidates. Mid-century politicians and pollsters explained this phenomenon by claiming that most women didn’t care about politics and would always duplicate their husband’s vote, if they showed up to the polls at all. As George Gallup said in 1940 of women: “How will they vote on election day? Just exactly as they were told the night before.”

But in the past 40 years, Gallup’s theory has been obliterated. In each of the last two presidential contests, women have outvoted men by about 10 million ballots. That’s equal to the total number of votes cast in the state of Texas in 2016. From an electoral perspective, the United States of Women is larger than the United States of Men by a full Lone Star State. Women aren’t copying men’s ballots, either: Even as men have migrated toward the Republican Party, women have become a dependable force for Democrats.

Why did women leapfrog American men in voting in just a few decades—and why did they lurch left?

The first mystery is easier to solve. And it starts with the Baby Boomers.

“Women’s relative gains in turnout over time appear to be explained by generational change,” says Elizabeth U. Cascio, a professor at Dartmouth College and a co-author of a new paper on the 100-year history of the American female voter. “My grandmother, born in the 1930s, used to stay in the car while my grandfather went to vote. My mom votes as often as my dad.”

Women born in the late 19th or early 20th century were less likely than men to go to the polls, either because voting was an unfamiliar ritual, or because they felt discouraged by their families from participating in politics (or both). The Boomers were far less sheepish. They grew up expecting a college education and a work-life outside the home. Voting, to them, seemed natural. They may also have been galvanized by the fight over the Equal Rights Amendment, which brought home the importance of politics in women’s lives.

As for why women overall become consistent Democrats, according to Cascio, the brief answer is: Women’s politics changed a little, and party politics changed a lot. The longer answer starts with Ronald Reagan.

With the nomination of Reagan for president in 1980, the Republican Party moved sharply to the right on a handful of issues relevant to women. The party dropped its support of the Equal Rights Amendment, embraced an anti-abortion position, and courted conservative Christians who lamented the effect of working women on “traditional” families. Although Reagan handily won election, he lost women by eight points. “It wasn’t until Reagan that Republicans clearly showed women that there are sides,” Kathleen Dolan, a political-science professor at the University of Wisconsin at Milwaukee, told The Washington Post.

The unprecedented split between male and female voters in 1980 caused a media frenzy. The National Organization for Women headlined its newspaper’s following issue “Women Vote Differently Than Men: Feminist Bloc Emerges in 1980 Elections,” and the first mention of a “gender gap” in mainstream media appeared in The Washington Post in 1981.

In the next few decades, the parties became more polarized. Conservative elites sorted into the Republican Party, while liberal elites sorted into the Democratic Party; and voters followed. This alignment had specific ramifications for the gender gap. Since the beginning of modern polling in the U.S., men had consistently held more conservative positions than women on a range of issues, including welfare spending, homosexuality, and use of force in foreign policy. As the parties became more ideological, the gender gap kept growing—from eight percentage points in 1980, to 12 points in 2000, to 13 points in 2016. Notably, Democrats lost all of those elections, as men moved even more sharply into the Republican Party. Since 1980, a majority of men have never once supported the Democratic candidate for president. In 2016, a paltry 41 percent of men (and just 32 percent of white men) voted for Hillary Clinton.

Perhaps because the Democratic Party has become reliant on winning female votes, its policies are attuned to women’s priorities. Women are more likely to live below the poverty threshold and rely on food stamps and other welfare services—part of a global phenomenon known as “the feminization of poverty.” This fact may make them more receptive to Democrats’ relatively consistent promises to expand the welfare state. As the Washington Post columnist Catherine Rampell writes, women are also more likely to work or be employed in government and government-regulated sectors, such as education and health care. It stands to reason that these employment trends make women less likely to vote for a Republican Party that has, for four decades, consistently promised to slash taxes and shrink government.

The gender gap may be a reflection of other schisms in the electorate, like education and geography. Women account for a majority of college graduates in the U.S. and a majority of residents in most metro areas. Both college education and density correlate with Democratic voting.

Today, Donald Trump’s approval rating among women is below 40 percent, while the front-runner for the Democratic nomination, Bernie Sanders, draws his strongest support not just from young voters but specifically young women. In 40 years, the gender gap has gone from a nonentity that didn’t even have a name to one of the most important fault lines in American politics.

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