In a new book, two political scientists argue convincingly that there is no stable dividing line between the “Left” and the “Right.” Instead, politically active citizens join the Left and Right (knowingly or not) as though they’re joining clubs or becoming devoted fans of sports franchises, often with their teams determining their views rather than vice versa.

Everyone agrees that American politics is becoming more polarized and that there’s something dangerous about this. But the phrase “political polarization” names at least two distinct and, in some ways, competing ideas. The first idea is extremism. The Left is moving further left, and the Right is moving further right; the reasonable center cannot hold. The second idea is sorting. You talk to and read and live with the people on your “side.” Partisan sorting happens geographically, at the level of red states and blue states but even more at the level of enclaves of urban Democrats (Austin) and stretches of rural Republicans (Texas). Partisan sorting also happens regarding consumption habits and hobbies, whereby Starbucks coffee might seem “blue” and Dunkin’ Donuts coffee might seem “red.” And maybe most crucially, partisan sorting occurs at the individual level, with people forming ideologies that “fit” what we understand to be the available groupings of beliefs held by existing groups. Pollsters find relatively few people who agree strongly with Democrats on immigration but with Republicans on abortion.