“We’ve never had a perfect left-right sort before, and now we do,” said Haidt, author of “The Righteous Mind: Why Good People Are Divided by Politics and Religion” (Pantheon, 2012). This is troubling, he said, because people tend to cluster around their moral in-group and view outsiders with only suspicion, not understanding.

“You engage all these tribal moral dynamics,” Haidt said…

The mid-20th century period when polarization was low was an anomaly in history, Haidt said, and it’s unlikely we’ll get back to that point. But being as polarized as America is now is dangerous, he said. When politics are tied inextricably to morals, everything becomes sacred, from guns to flags to race. And when sacred values are threatened, people lash out. Debates are no longer disagreement, they’re treason. And political opponents aren’t just people with a different point of view.