Our hyperpartisan era has occasioned a sea change in understandings of American political behavior—or perhaps more accurately, a returning tide.

Following decades of research divining substantively “rational” behavior in the heuristics employed by American voters, recent work has reemphasized the affective, group-oriented, psychological drivers of party affiliation and voting first detailed in classics of mid-twentieth-century behavioralism. Lilliana Mason’s impressive new book serves as a definitive statement of this view as it relates to polarization, masterfully synthesizing existing work with a battery of original studies while applying a novel framework borrowed from social identity theory in psychology. All politics is identity politics, Mason argues, and the key to our current woes is that social sorting has turned our two major parties into all-consuming “mega-identities” for the majority of the electorate.

In a well-structured argument conveyed in pristine prose, Mason first documents the universally experienced, psychologically ingrained, and inherently conflictive qualities of group-based identification. What distinguishes contemporary party politics is that powerful group identities that used to crosscut the party divide have now sorted themselves alongside it. Divisions by race, religion, and ideology