This gap has received significant attention. But many observers have ignored the fact that women also were more likely to vote in Democratic primaries than were men, which boosted Clinton’s margins at the polls. Recent electoral history suggests the same thing could happen in November. If so, women would help Clinton beat Donald Trump.

In Democratic primaries, women turn out at higher rates than men

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In every Democratic primary in 2016, women turned out at higher rates than men. This turnout gap averaged 16 percentage points, ranging from six points in West Virginia to 28 points in Mississippi.

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The pattern is not unique to 2016. In the last contested Democratic primary, in 2008, the gender gap in turnout averaged 15 percentage points. Of course, Clinton was on the ballot then, too. But even in years in which a major female candidate has not been running, women voted at higher rates than men did. Women outpaced men by 14 percentage points in 2000 and by 10 points in 2004.

Why are there more female voters than male voters in the Democratic primaries?

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Let’s look at three possible answers.

1. Women are more likely than men to be Democrats.

This is what we call the “partisan gender gap.” Beginning in the 1960s, men became more conservative and moved into the Republican Party, while women continued to support the Democratic Party.

According to the American National Elections Studies survey, the partisan gender gap in 2012 stood at five percentage points: 49 percent of women and only 44 percent of men identified as Democrats (that’s including Democratic-leaning independents). That translated into a five-point gender gap in support for President Obama in the November election.

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That gender gap in partisanship is smaller than the gender gap in turnout for Democratic primaries — 15 percentage points. Yet there’s variation between states, so in some, a partisan gender gap could account for the turnout gap in the Democratic primary.

Let’s compare the partisan gender gap in each state (measured by the 2004 and 2008 general election exit polls) to the turnout gap in those states’ 2016 Democratic primaries. The graph below shows the results. While in some states, such as Illinois and Wisconsin, differences in the partisan gender gap can explain the gap in primary participation, this is not always the case. Mississippi and Maryland show the opposite pattern, where the gap in primary participation exceeds the gap in party identification. In general, the turnout gap is larger than the partisan gender gap by, on average, six percentage points.

So that’s not the whole answer. What else?

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2. African American women turned out for Clinton in the primaries, big time.

Clinton actively courted African American women’s votes through advertisements and campaign stops, and they turned out for her.

As a result, some of the largest gender gaps in the 2016 Democratic primaries’ turnout occurred in Southern states: 28 points in Mississippi, 24 points in Georgia, 22 points in South Carolina and 20 points in Alabama. In Mississippi, black women were 47 percent of the entire Democratic primary electorate; 90 percent of them voted for Clinton.

3. Women are more likely to vote than men are.

A third explanation goes beyond Democratic primaries — and has implications beyond the nomination. Since the 1980s, women have been more likely than men to vote in the general election. In short, they vote more consistently than men do.

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For instance, using Current Population Survey data, the Center for American Women and Politics at Rutgers University reports that the turnout rate among women in 2012 was about four percentage points higher than that of men. In raw votes, that means that women cast 9.8 million more ballots than did men. Results were similar in the 2004 and 2008 presidential races.

Women vote at higher rates than men do. What does that mean for November?

Early polls show Donald Trump with low favorability among women. For example, a recent ABC News/Washington Post survey found that 77 percent of female respondents had an unfavorable impression of Trump, compared with 62 percent of men. Meanwhile, Clinton has struggled to win men’s support. Current differences in men’s and women’s candidate preferences may foreshadow a large gender gap in the fall election.

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Of course, we’re far from the general election, and opinions will still change. Some Republican women who have reservations about Trump may “come home” because they identify so strongly with their party. How well each candidate identifies and mobilizes their voters will affect various groups’ turnout rates. And mobilizing voters in a year in which both candidates have relatively high unfavorability ratings may be more complicated than normal.

But for now, it appears possible that the gender gap in turnout could help Clinton. Trump would need men to turn out at unusually high rates to overcome his deficit with women. That’s a difficult hurdle given the turnout trends of the past decades.