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The interesting thing that happens when a Republican marries a Democrat

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The gulf in American politics between the left and the right can seem insurmountable. There are red states and blue states, liberal media sources and conservative media sources, even jobs and names that differ by political party. Research has suggested that liberals and conservatives even wash with different shampoos and soaps and thus have different smells.

But actually, millions of Americans cross that gulf every day — by being married to someone of a different political party.

In a new paper, two researchers examined American couples to see how common "mixed" political marriages really are. Eitan Hersh, a professor of political science at Yale University, and Yair Ghitza, a chief scientist at political data firm Catalist, studied a database of more than 18 million couples drawn from voter registration records.

They found that most married couples — 70 percent of them — were made up of people of the same political affiliation (either Democratic, Republican or independent/other). Married couples overall were somewhat more likely to be Republicans than Democrats.

But, as the graphic below shows, “mixed” political marriages certainly do happen. Three percent of married couples are female Republicans married to male Democrats (upper right rectangle), while 6 percent of married couples are male Republicans with female Democrats (lower left rectangle). So, overall, nearly 1 in 10 married couples contain both a Republican and a Democrat. Nineteen percent of married couples are a Republican or a Democrat with an independent.

In total, nearly 30 percent of all married couples are of mixed party affiliation.

Meanwhile, 30 percent of married couples were exclusively Republican, 25 percent of married couples were all Democrat, and 15 percent were all independent.

Those findings might be surprising for people who are used to reading about America's divided politics, Hersh said.

“There’s actually a lot of people all mixed up together in terms of their politics," Hersh said.

Hersh and Ghitza also compared political affiliation to other factors that often guide people’s choice of a marital partner — like age, race and geographical vicinity. Of course, all of these factors also have strong ties to a person’s political affiliation.

First, they found that marrying across political parties is far more common in the United States than marrying across racial categories. (This is partly due to the way political and racial categories are split up — America is majority white but not majority Democratic or Republican). In the United States, 71 percent of couples are in same-party marriages, compared with 93 percent in same-race marriages.

Their research also supported previous findings that Democrats live in more partisan neighborhoods than Republicans do — partly due to concentrated African American communities, where most people are Democrats. In neighborhoods that were more than 90 percent Democratic, they found that 68 percent of households were made up of two Democrats. But in neighborhoods that were more than 90 percent Republican, only 55 percent of households were made up of two Republicans.

Also in line with prior research, they found that men are more likely to be Republican. The study shows that there are twice as many Democratic-Republican households in which the husband is a Republican than in which the wife is a Republican.

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They also found that older couples were much more likely to be of the same political party than younger couples are. Among married couples under 30, fewer than half were Democrat-Democrat or Republican-Republican pairs, the researchers say. But among couples over 80, 70 percent were. This is partly because more young people are independents.

You can see those trends in the chart below, which shows how the household party registration of married couples changes by age. The bands in the middle get narrower as you go from the left to the right, showing that, as people get older, fewer are independents or in "mixed" political marriages.

Instead, Democrat-Democrat and Republican-Republican couples become more common as people age. The researchers say this could be due to the effects of cohabitation — after years of living together, one spouse may be persuaded to join the other’s team.

Hersh says that effect was much larger than he expected. “For two people who live in the same state, of the same age and race, if they’re living with someone of their own party they’re voting at much higher rates than if they’re living with someone of a different party."

But these trends vary depending on which specific parties you’re talking about.

For both Republicans and Democrats, for example, being married to an independent tends to depress the person’s vote significantly more than being married to a person on the opposite side of the political spectrum. “Being married to an independent, it brings down the turnout of the partisan, and it never brings up the turnout of the independent,” he says.

The graphic below shows the percentage drop in the likelihood of voting if one spouse has a different party registration from the other. In the 2012 primary, for example, a Democrat married to an independent or member of a third party was 13 percent less likely to vote than a Democrat married to another Democrat.

As the graphic shows, being married to someone of a different party appeared to affect the vote of Republicans much more than Democrats, for unknown reasons.

“Depending on whether you’re a Democrat, or a Republican or an independent, the effects vary a lot,” Hersh said. “And they’re particularly strong for Republicans. A Republican married to a Democrat was 10 percentage points less likely to vote than a Republican married to a Republican. But a Democrat married to a Republican was only 3 percentage points less likely to vote than a Democrat married to a Democrat.”

The paper is designed to explain what precisely is happening, not why it’s happening, so the researchers don’t know exactly why a spouse’s political party changes a person’s likelihood of voting.

But they speculate that the reason could be twofold: that people who are less politically active are more likely to marry outside of their own party and that living together might influence a person’s politics and willingness to vote. Some people of different parties, for example, might have the feeling that their votes are “canceling each other out” and decide to stay at home on the sofa instead.

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