A new demography paper argues that there is a reason more black women have voted for Democrats than men have since 1980 - more black men are in jail.

Black voters have historically voted overwhelmingly Democratic, even after 1972, when moderate white Democrats switched to the Republican party due to Democratic lack of support for racial equity, as in the Civil Rights Act of 1966, and having an avowed segregationist, former Alabama Gov. George Wallace, as the front-runner for the party's presidential nomination. Since 1980, Republicans have made little progress, notably among Black women, and the authors suggest the voting would be more equal among the sexes except that Black men have disproportionately high rates of death, incarceration, and disenfranchisement due to criminal convictions—disparities that reduce the share of men voters who are Black and lead to a higher percentage of Black voters who are women relative to other racial groups.

“We’ve long known about both gender and racial gaps in voting, but the relationship between the two is less clear,” says Dean of Social Science at NYU Abu Dhabi Paula England, the lead author of the paper, which appears in the Proceedings of the National Academy of Sciences. “These findings show the fact that a higher percentage of women than men voters are Black explains about a quarter of the gender gap in voting Democratic.”

The sociologists used results of General Social Survey (GSS) data from the 1980 through the 2016 presidential elections, replicating this analysis with data from the American National Election Surveys. Their analysis showed that the gender difference in racial composition explains 24% of the gender gap in voting Democratic. In other words, given that approximately 90% of Black voters back Democrats—a much higher proportion relative to other groups—the fact that a higher percentage of women than men voters is Black tilts women’s votes toward Democrats.

To rule out another factor that may explain the gender gap, the researchers also studied income. Specifically, among unmarried voters, they looked to see if the fact that single women are poorer than single men was a reason that women are more likely to vote more Democratic.

In fact, the gender gap in voting Democratic was especially large among those who are unmarried. However, while unmarried women are poorer than unmarried men, lower-income voters vote only slightly more Democratic than more affluent voters.

“No matter how we measured income, controlling for it did not reduce the gender gap,” the authors write. “One key factor in the gender gap is racial composition.”