Even within identical demographics, spouses are more like one another than they are like other members of their group. For example, Catholics seek out Catholics who share their politics, and Jews seek out other Jews with similar politics, the study found. Even strong Republicans tended to end up with strong Republicans, and moderate Democrats with moderate Democrats. The implication, Hibbing said, is that politically similar marriages arise from people’s choices, not just their backgrounds.

Parents pass on their political views to their children, both through socialization and perhaps via genetics, Hibbing said, so the tendency to couple with politically similar people might, like left-wing and right-wing media sources, widen the gap between liberals and conservatives. (Political scientists agree that professional politicians are increasingly polarized these days, but not everyone accepts that the general public is more divided.)