The single most important intellectual trend of our time is the popular rediscovery of human tribalism. We thought we had it licked. For roughly 200,000 years, humans ran around in small, clannish groups, hunting and mating together while variously raiding or befriending other groups. But in the past couple of centuries, we wised up and replaced tribal social organization with depersonalized, rules-based institutions: markets to organize our economies, elections to organize our politics and science to organize our search for knowledge. To satisfy our hankering for group affinity, we transferred our tribal loyalties from clan and caste to abstractions like the Constitution and the free-enterprise system. The results were spectacular, a step change in human potential. We had figured it out. Or so we thought.

Only we couldn’t fool Mother Nature. Amy Chua’s compact, insightful, disquieting, yet ultimately hopeful book is both a sign of the rediscovery of the primacy of tribalism and a lucid guide to its implications.

Science has shown that tribalism is hard-wired. Experiments and evidence dating back generations, in psychology, sociology and anthropology, have established firmly that human opinions and emotions, loyalties and affiliations, religions and customs, and even perceptions are shaped by our need to belong to a group — and by our proclivity to hate rival groups. Experimental subjects will spontaneously form in-group loyalties and out-group antipathies when assigned to teams randomly. Subjects will deny the evidence of their own eyes to agree with those around them, even if the discrepancy is blatant. There need be no trigger for tribalism, no cause or conflict. If we do not already have a tribe and a reason to be loyal to it, we will create a tribe and invent a rationale. Some versions of this behavior, such as loyalties to professional sports franchises, are relatively benign. Others, such as blind political partisanship, can be quite malignant.

In "Political Tribes," Chua, a law professor at Yale, takes stock. Her short book relies on a handful of case studies and examples to draw broad conclusions, so scholars will want to be cautious with it; but her accessible and provocative treatment sets up just the right public conversation. She takes her argument in two directions, one foreign, the other domestic.

British imperialists made a science of understanding the tribal structures of societies they colonized. In places such as Iraq and Afghanistan, where they privileged Sunnis and Pashtuns to exploit tribal divisions, they played favorites adeptly. Americans, being more ideological and less cynical, have never been as good at dividing to conquer, but in earlier eras we were known to do it quite well — by pitting Indian tribes against one another, for example, and by exploiting class divisions in the campaign to pacify the Philippines.

More recently, however, anti-communism and enthusiasm for democracy became all-purpose prisms and often distorted Americans’ view. In Vietnam, Chua argues, the United States misunderstood its adversaries as communist fanatics kowtowing to foreign sponsors, when in reality, the North Vietnamese were motivated more by nationalism and ethnic grievance. Later, in Afghanistan and Iraq, Americans imagined that democracy and peace would bloom once everyone could vote. Instead, vicious tribalism erupted. “In many parts of the world, far from neutralizing tribal hatred, democracy catalyzes it,” Chua writes.

Terrorism, too, is often driven by tribal impulses. Young people join radical groups in search of an identity they can call their own; the groups desensitize them to outsiders’ humanity and send them off in suicide vests. “The key to contemporary Islamic terrorism lies in the proliferation not merely of fundamentalist Muslim teachings but of the belief that Muslims, as a group, are being attacked, humiliated, and persecuted by an evil Western enemy,” Chua writes. The worst thing the United States can do, in this view, is to indulge in us-vs.-them rhetoric that helps militant Islamists win converts.

Chua’s observations on international affairs, although useful and timely, will not surprise anyone who has been paying attention. Nor will she win an originality prize for recommending that American interventionists be mindful of local ethnography and realistic about nation-building. More interesting, and more challenging, is her take on tribalism here at home.

Like Mark Lilla, in his recent book “The Once and Future Liberal: After Identity Politics,” Chua decries American progressives’ shift away from messages that appeal to shared values and toward themes that dwell on ever-narrowing group identities. The great mid-century civil rights leaders saw themselves as delivering on the promise of the Declaration of Independence; Martin Luther King Jr.’s dream was the American Dream. Franklin Kameny, America’s greatest gay rights advocate, likened his fight for equality to the combat he waged against totalitarianism in World War II. During the war, as he wrote in a 1961 brief to the Supreme Court (seeking an end to anti-gay discrimination in federal employment), “petitioner did not hesitate to fight the Germans, with bullets, in order to help preserve his rights and freedoms and liberties, and those of others.” In his lawsuit, he continued, he was fighting for “those same rights, freedoms, and liberties, for himself and others.” He saw himself as advancing justice for all Americans, not as distinguishing homosexuals from other groups.

Chua sees the emergence of something quite different: “A shift in tone, rhetoric, and logic has moved identity politics away from inclusion — which had always been the left’s watchword — toward exclusion and division.” Facebook, she notes, lists more than 50 gender designations, “from genderqueer to intersex to pangender.” Activists compete to be offended if their particularism is not acknowledged. “Gay” becomes LGB, then LGBT, then LGBTQ, then LGBTQQIAAP and other variants — a terminological balkanization that Kameny lived to witness but never accepted. “For today’s Left,” Chua observes, “group blindness is the ultimate sin, because it masks the reality of group hierarchies and oppression in America.”

Political actions beget equal and opposite reactions. Recent polling finds that a majority of white Americans — including about two-thirds of whites without college degrees and three-fourths of white Republicans — believe there is discrimination against white people in America today. Whites, Christians and other traditionally predominant groups are developing their own narratives of beleaguered solidarity and group victimhood, and Steve Bannon and President Trump are standing by to take their calls.

Two consequences follow, both troubling. America’s unique achievement, for Chua, is its emergence as a “super-group — the only one among the major powers of the world. We have forged a national identity that transcends tribal politics — an identity that does not belong to any subgroup, that is strong and capacious enough to hold together an incredibly diverse population, making us all Americans. This status was hard-won; it is precious.”

Of course, in practice, the ideal of an Americanness that transcends race and ethnicity and all the other categories can never be perfectly attained. But we can struggle to that end, and we have fought a very long way toward it, as I can attest: Here I am, in America, married to another man. Tribalism of both right and left endangers progress toward sharing the country. Worse, it endangers the idea that we should share the country. “At different times in the past,” Chua writes, “both the American Left and the American Right have stood for group-transcending values. Neither does today.”

Moreover, tribalism is a dynamic force, not a static one. It exacerbates itself by making every group feel endangered by the others, inducing all to circle their wagons still more tightly. “Today, no group in America feels comfortably dominant,” Chua writes. “The Left believes that right-wing tribalism — bigotry, racism — is tearing the country apart. The Right believes that left-wing tribalism — identity politics, political correctness — is tearing the country apart. They are both right.” I wish I could disagree.

Remedies? Chua sees hopeful signs. Psychological research shows that tribalism can be countered and overcome by teamwork: by projects that join individuals in a common task on an equal footing. One such task, it turns out, can be to reduce tribalism. In other words, with conscious effort, humans can break the tribal spiral, and many are trying. “You’d never know it from cable news or social media,” Chua writes, “but all over the country there are signs of people trying to cross divides and break out of their political tribes.”

She lists examples, and I can add my own. My involvement with the Better Angels project, a grass-roots depolarization movement that is gaining traction in communities across the country, has convinced me that millions of Americans are hungry for conciliation and willing to work for it. Last summer, at a Better Angels workshop in Virginia, I watched as eight Trump supporters and eight Hillary Clinton supporters participated in a day of structured interactions. Under rules that encouraged listening without challenging or proselytizing, they explained their values and examined their stereotypes. No one’s political opinions changed (or were expected to), but everyone left the room feeling less animus and believing that ordinary people can fight back against polarization.

Tribalism is humans’ default mode. De-tribalizing requires effort. Americans’ atavistic impulses got the better of us because we grew complacent. Progressives failed to imagine that identity-mongering and victim-worshiping would not only take over the academy but could help elect Trump to the presidency. Now they know. Conservatives failed to imagine that rage-mongering and conspiracy-theorizing would not only take over conservative media but could help elect Trump to the presidency. Now they know. Those who hold with what Chua calls group-transcending values were caught flatfooted and are only beginning to gather their forces and find their voices. But they are assembling, and the tribalists have lost the advantage of surprise.