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“It’s feelings based,” said Lilliana Mason, a political scientist at Johns Hopkins University and author of “Uncivil Agreement: How Politics Became Our Identity.” “It’s polarization that’s based on our feelings for each other, not based on extremely divergent policy preferences.”

The tendency to form tightly knit groups has roots in evolution, according to experts in political psychology. Humans evolved in a challenging world of limited resources in which survival required cooperation — and identifying the rivals, the competitors for those resources.

“The evolution of cooperation required out-group hatred, which is really sad,” said Nicholas Christakis, a Yale sociologist and author of “Blueprint: The Evolutionary Origins of a Good Society.”

This is just as true on today’s political stage. There are two major parties, and their contests are viewed as zero-sum outcomes. Win or lose. The presidency is the ultimate example: There are no consolation prizes for the loser.

No researcher argues that human nature is the sole, or even the primary, cause of today’s polarization. But savvy political operatives can exploit, leverage and encourage it. And those operatives are learning from their triumphs in divide-and-conquer politics.