Some polarization is healthy—even necessary—for democracy. And indeed, the historical experience of democracies in Western Europe shows us that norms can be sustained even where parties are separated by considerable ideological differences. But when societies grow so deeply divided that parties become wedded to incompatible worldviews, and especially when their members are so socially segregated that they rarely interact, stable partisan rivalries eventually give way to perceptions of mutual threat. As mutual toleration disappears, politicians grow tempted to abandon forbearance and try to win
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