Last week, some striking data emerged into the political blogosphere, showing that inter-party has become increasingly frowned-upon on both sides of the aisle, but more so on the Republican side. The data come from a paper by Stanford communications scholar Shanto Iyengar and his colleagues, but were given striking visual form by Kevin Drum:

Well, how about a psychological explanation?

First of all, the parties are clearly much more polarized than before. So partisans on both sides simply feel more negatively about one another than they did in, say, 1970.

But why are we so polarized--and more important for our purposes here, why would this polarization extend to non-political issues, like whether your darling daughter marries a Democrat?

Simple: As I explain in The Republican Brain, the parties are more polarized because they are better sorted psychologically than they used to be. In other words, Republicans are increasingly similar to one another psychologically, and so are Democrats—even as the two groups are also increasingly dissimilar from one another. And therefore, the differences between Democrats and Republicans are not just ideological—they are deeply rooted in , values, and psychological needs. They aren't really thought out or rational—rather, they are felt at the very core of our beings.

This is the broad consequence of what is called Richard Nixon’s “Southern Strategy,” which gradually picked off Southern and Sun Belt states for the GOP, and moved many longtime loyal Democrats into the Republican column. That included many conservative Christians who became “Reagan Democrats” and then, gradually, the “Religious Right.” They were mobilized politically around culture war issues, like women’s rights, prayer in schools, race, and so on. This cultural and values schism is, broadly speaking, the legacy of the Sixties.

As political scientists Marc Hetherington and Jonathan Weiler have shown, this has led to the parties being increasingly split over something called psychological authoritarianism. Republicans are increasingly authoritarian—craving certainty, inclined to view the world in black and white terms, you’re either with me or you’re against me—and Democrats are increasingly the opposite.

And of course this filters into everything, because if you’re authoritarian…well, one thing you are not very likely to be is open to new experiences. In other words, you’re likely to score relatively lower on the personality trait of Openness, one of the Big Five traits. And we know that one key factor about is that, well, Open people tend to date and marry other Open people, and vice versa. As the personality psychologists Robert McCrae and Sutin put it:

Whether single, , or married, people have a good idea of what they want in their ideal partner—someone just like themselves, particularly on Openness. When contemplating the ideal mate, single individuals prefer partners who strongly resemble them on Openness, with and coming in a distant second and third, respectively…

And of course this makes sense—you want to marry someone with similar values, someone who writes poetry like you do, who appreciates the Beatles like you do, who likes to hang out in coffee shops like you do. Indeed, McCrae and Sutin rather nonchalantly add that “political assortive mating” is a sort of bygone conclusion:

Recently, McCrae and colleagues (in press) analyzed trait similarity using both self reports and spouse ratings of personality in married couples across four cultures, controlling for these potential biases. Consistent with previous research, similarity correlations for the broad domains were generally modest and Openness had the largest correlation (mean r for Openness across the three cultures = .22). Facet-level analyses revealed that couples were drawn together on some aspects of Openness more than others. Across the different cultures, Openness to Values consistently showed the most evidence for trait similarity: liberals seek out other liberals, whereas conservatives seek out other conservatives.

And, we might add, liberal couples raise more little liberals, and conservative couples raise more little conservatives. And the cycle continues.

This is true across cultures, according to McCrae and Sutin. But it is increasingly stark in the United States, due to our increasingly level of political—and psychological—polarization.

But that still doesn’t answer the question: Why do Republicans disapprove of inter-party marriages somewhat more than Democrats do?

Well, the difference is probably related to Openness again—Open people are more willing to try different and new things, Closed people are not—but also, I think, to interesting data about political tribalism from Jonathan Haidt and his colleagues. This research shows that conservatives, more than liberals, perceive a strong distinction between the in-group and the out-group, those who are part of the team or unit and those who are not. As you can see in this figure, this is one of several “moral foundations” where conservatives are more strongly responsive than either liberals or libertarians:

As we’ve become increasingly polarized in America, conservatives have also increasingly defined liberals as the “out-group.” And of course, you don’t want your precious children marrying members of the out-group, who don’t share your values, and raising kids who don't share your values!

What I think is really revealing about the new interparty marriage data—and about this sort of analysis in general—is that it forces us to see that is not just about ideas. In truth, the ideas are often a kind of afterthought (empahsis on thought). The core of politics, especially in today’s U.S., is visceral and emotional.

We will never have a less divided America until we recognize this, and acknowledge what is really tearing us apart.