United We Fall: The more homogenous the parties become, the uglier the divide between them.

• Author: Drutman, Lee
• Position: On political books

United Agreement: How Politics Became Our Identity

by Lilliana Mason

University of Chicago Press, 192 pp.

In June 2012, a hopeful Barack Obama predicted that once he won reelection, the hyper-partisan "fever" that had been roiling Washington might finally "break." Congressional Republicans had spent Obama's first term committed to blocking whatever he proposed, sight unseen, in order to make him a "one-term president," as Mitch McConnell tartly put it. But, Obama reasoned, once there was no second term to prevent, "we can start getting some cooperation again."

Six years later, Obama is gone from the White House, and Republicans control all three branches of government. "The fever may break" now looks more like a sentence missing a direct object: American democracy.

For a solid decade, wonks and pundits have been charting America's growing political polarization. By now, a reasonable consensus seems to be that, first, partisan polarization is at record-high levels; second, it exists not only among elites, but also among the mass public; third, it is increasingly driven by negative partisanship--dislike for the other party--rather than substantive issue-based differences; and, fourth, it is the end product of a half century of geographic and demographic sorting. Fifty years ago, both parties contained broad and overlapping coalitions. But today the Republicans are overwhelmingly the party of white, primarily rural and exurban Christians who call themselves "conservatives," and the Democrats are the party of everyone else.

The case for optimism ("The fever may break") is that, ultimately, there are decent people on both sides who care more about their country than they do about their party winning. And that at some point, a new equilibrium will emerge, because it has to. Or that somehow, we'll muddle through, because we always have.

The case for pessimism ("The fever may break us") is that once all political conflict becomes organized around a single us-versus-them dimension, something dark and destructive happens to our collective psyche. We see each other not as fellow Americans, but rather as enemies who would use their power to destroy us and betray the true promise of the country. And once that happens, it's hard to get back to peace without some kind of breakdown or violence.

I was pessimistic before reading Lilliana Mason's new book, Uncivil Agreement. I am even more pessimistic now. Mason, a political scientist at the University of Maryland, has written an extremely important analysis of the social psychology of what happens when the political world gets divided into two warring tribes, with no overlap. The central contribution of the book is to show that partisan sorting isn't just a consequence of identity--it also creates identity. This is critical, because, as Mason observes, "identities themselves have psychological effects of their own."

Mason draws on a tradition known as social-identity theory, which explains how we construct our sense of self based on group memberships. Humans need to both fit in and feel special. Affiliating with a larger identity group--a religion, a race, a profession, a class, a hometown--gives us both an us and a...