Understanding polarization

If you were to reduce the past 30 years of American elections down to a single word, it might well be one you’d never think of: “sorting.” The dominant trend in American politics during that time has been polarization, but that word alone doesn’t capture what’s going on. America is not just becoming more liberal and more conservative. It is sorting into two distinct identities based on partisan ideology.

Consider the 1960s, for instance. America went through dramatic social change, but Congress was still able to cooperate and pass legislation....