Fifty years ago, if you knew whether someone was a Republican or a Democrat, you didn’t necessarily know a lot about that person’s moral values; party affiliation told you even less about someone’s preferences in restaurants or movies. There was so much diversity within each party—plenty of conservative Democrats and liberal Republicans—that stereotyping was harder, and cross-party alliances were much easier.

But since the 1980s, the two parties have become ever more perfectly sorted. The Democratic party is now the liberal party, dominant in places with a population density greater than about 800 people per square mile, and the Republican Party is the conservative party, dominant in lower density areas. Nowadays you can make predictions about people’s values and votes from just a few seemingly unrelated things, such as whether they find novel cuisines appealing or how messy their desks are.

Having well-sorted parties could be a good thing. In 1950, the American Political Science Association called for just such a comparatively sharp polarization, so that Americans could be presented with clearer policy choices from two very different perspectives. Unfortunately, as the parties developed more divergent values and lifestyles, they also developed divergent facts. Republicans and Democrats believe different things—about history, the Constitution, science, and above all economics.

Read full article as published in TIME

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Jonathan Haidt is the Thomas Cooley Professor of Ethical Leadership.