The United States has gotten much more sorted, geographically and socially. People are less likely to date and marry across political lines. Since 1973, the rate of politically mixed marriages in America has declined by 50 percent. Neighbors are more likely to agree on politics than they were 15 years ago. This sorting leads to prejudice, as racial or religious segregation does. “Separation triggers a series of interlocking processes that inflame group conflict,” the social psychologist Thomas Pettigrew has written. “Negative stereotypes are magnified; distrust cumulates; and awkwardness typifies the limited intergroup interaction that does take place.”