In the late 1960s and 1970s, the Democratic and Republican Parties were more ideologically, religiously, and racially diverse than they are today. Back then, for instance, some Republicans supported abortion, and some Democrats were virulent, fearmongering racists. As Vox cofounder Ezra Klein argues in his new book, Why We're Polarized, that dynamic has radically changed. Over the last several decades, American political parties have sorted themselves into two teams based on identities. For the most part, the blue team is a godless, multiracial coalition of city dwellers, and the red team is a Christian army composed largely of white men from the suburbs and rural areas. Given this reality, Klein argues, a democratic system based on compromise has become nearly impossible to maintain. When groups feel like their identity—their very being—is under attack, they're less likely to try to work out differences, and much more likely to try to wipe out their rivals. If you listen to Klein's podcast, The Ezra Klein Show, you know this book is essentially a product of the conversations he's had with political scientists, researchers, and Big Thinkers over the last several months. What I like about Klein's approach to this topic is that he doesn't merely point to the "divide" in American politics as a roundabout way to bemoan some perceived decline of "civility." He looks at the larger structures powerful people have built to break us up, and then shows us how they work. If we really want to create a less polarized society, it's going to take good, clear analyses such as Klein's to figure out how to improve those structures, or else dismantle them entirely.