Our politics, we lament, is disfigured by a deep and ugly partisan divide. The left accuses the right of starving the government of funds needed to perform its essential functions and of wishing to impose on the country outworn and repressive moral and political beliefs. The right accuses the left of betraying the American experiment in individual freedom and limited government by maintaining a massive and ever-expanding welfare state. The left believes that the right is selfish and cruel. The right believes that behind the left’s vaunted compassion is an impulse to ruthlessly control people’s conduct and prescribe correct beliefs. And partisans of both sides find justification for their uncompromising ways in the conviction that the other side’s partisanship has never been more shameless or extreme.

Various explanations have been proffered to account for what political scientists call...