“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.