Above all, tribe members protect one another from perceived attacks by outsiders. Last April, when the MSNBC host Joy Reid was found to have posted homophobic content on a now-defunct blog (and claimed, dubiously, to have been hacked), many liberals rallied to her side anyway, pointing out that the posts were more than 10 years old and urging others to accept her profuse apologies. Had such posts been attributed to a Fox News personality, however, it’s almost certain those same liberals would have offered no opportunity for forgiveness. The gift of absolution is given within a tribe, and rarely outside it.

Political tribes can organize along stark lines: the working class versus the 1 percent, baby boomers versus millennials, city dwellers versus rural people. But they can also be more nebulous, forming around subtleties of education, lifestyle or cultural taste. Some years ago, when Howard Dean was the front-runner for the Democratic nomination for the presidency, the conservative PAC Club for Growth ran a TV ad in Iowa featuring an elderly white couple being asked about Dean’s tax proposal. “What do I think?” the husband says. “I think Howard Dean should take his tax-hiking, government-expanding, latte-drinking, sushi-eating, Volvo-driving, New York Times-reading —” Then his wife interrupts: “body-piercing, Hollywood-loving, left-wing freak show back to Vermont, where it belongs.”

The question was at least putatively about Dean’s plan to repeal George W. Bush’s tax cuts, but instead of eliciting a coherent opinion on how much tax should reasonably be withheld, from whom and for what services, it provoked a rant against a particular group of people, who were characterized almost entirely through their lifestyle and consumer choices. There was no need to talk policy, because the policy was reframed as an embrace of one tribe and a rejection of the other.

In principle, the United States is a country where various tribes are supposed to work in coalition to form what the founders called “a more perfect union.” Americans also pride themselves on having a “melting pot” model of immigration, in which each new group is thrown into the mix, contributing to the overall sustenance of the nation. But the reality is that, for most of this country’s history, one tribe has held power, deciding who was allowed to settle the land and who could be dispossessed, who was free and who was enslaved, who had the right to vote and who did not. The hegemony of white landowners prompted few, if any, complaints about tribalism in the national conversation. It was only when other factions began to demand justice and recognition — the “seat at the table” that Trump, but not Obama, was applauded for encouraging people to seek — that the debate about which tribe holds power became explicit rather than implicit.