"Diversity is overrated."

That argument against immigration — once confined to the alt-right gutter — has climbed its way into respectable right-wing circles in the Trump era. The idea is apparently that people have a natural desire to be around their own, so there is nothing wrong with limiting "mass" immigration, especially from non-European countries that are too dissimilar from America.

And who does the right invoke when making its case? Not the Nazi philosopher Carl Schmitt, who famously argued that maintaining healthy polities requires treating cultural strangers like enemies. No, they are increasingly dusting off the work of liberal Harvard political scientist Robert Putnam, whose research purports to show the pitfalls of diversity.

The trouble is that Putnam oversold his own research — and conservatives are overselling Putnam.

Putnam, whose 1990 landmark Bowling Alone bemoaning the growing atomization of Americans became a household name, published a paper 10 years ago showing that ethnically diverse communities in America suffered a loss of what he called "social capital" or solidarity. He studied 30,000 Americans across 41 communities — ranging from declining inner cities like Detroit, rural areas like South Dakota, and bustling metropolises like San Francisco. He found that regardless of income level or crime rate, the more diverse the community, the less it trusted not just other ethnic groups but also, remarkably, its own. People don't riot in the streets, he found, they vacate them, retreating, turtle like, into their homes to watch TV rather than participate in community activities or neighborhood projects.

To some extent, this makes sense. It is incontrovertible that people are more comfortable with those who share their way of life and cultural outlook. One doesn't have to harbor animus toward other groups to prefer one's own. Yet it becomes harder to cut through the multi-ethnic thicket in super diverse communities and find one's cultural kin, leaving us isolated — unable to reach out to our own and unable to connect with others.

But just because one can find a plausible explanation for the finding doesn't mean it's the whole truth — or even the main truth. It is not easy to reduce complex cultural phenomena to measurable metrics. And even though Putnam's study is among the more thorough of its kind, his way of measuring trust — basically by asking people to rate on a three-point scale whether they would "say that most people can be trusted" — is arguably quite crude. Furthermore, George Mason University's Bryan Caplan notes, Putnam conveniently forgot to highlight that part of his research that showed that many other factors, particularly homeownership, correlate far more strongly with social trust than homogeneity. So why did Putnam bury them and highlight a less important factor instead? Essentially because it's more in line with his thesis in Bowling Alone. It's a classic case of "confirmation bias."

Furthermore, as Putnam forthrightly acknowledges — but his right-wing appropriators ignore — the loss of trust due to increasing diversity is a short-term phenomenon. Over the long run, people reconstitute new identities and bonds based on other shared characteristics. Yesterday's "them" become tomorrow's "us." For example, Putnam notes, in the 1920s, Americans were acutely conscious of divisions among European sub-groups — the Irish, Italians, Germans, Eastern Europeans — and in the 1950s of various Protestant denominations — Methodists, Lutherans, Baptists. None of these distinctions matter anymore.

Right-wing diversity critics argue that race is different. Unlike religion and nationality, it is an immutable fact of life and given the inherently tribal nature of humans, ignoring it to build a racially eclectic society means inviting conflict (which Putnam's study did not find, incidentally). In other words, the defining project of American liberalism to transcend the tribal ties of "blood and soil" through a commitment to a universalistic creed of liberty and equality is a farce in the eyes of these self-styled American patriots.

But a society dedicated to slicing and dicing people to ensure demographic homogeneity will wind up far more fractious. It is actually quite hilarious to witness the bitter disputes among alt-righters about who exactly is genetically pure enough to qualify as a member of their tribe. While in liberal societies big differences become irrelevant over time, in an ethnically homogeneous society small differences inevitably grow to oversized importance. Indeed, hierarchies over minor impurities or cultural differences will become magnified, breeding not harmony but strife. In 17th century Europe, people were almost entirely white. But it wasn't exactly a picture of comity. They simply replaced the internecine warfare among various European tribes in previous centuries with sectarian warfare among Protestants and Catholics. It was liberalism, with its commitment to protecting the freedom of all without regard to caste, class, or ethnic and religious differences, that finally brought a halt to Europe's centuries-long state of constant war.

The pursuit of ethnic and cultural homogeneity won't make America less conflict prone, but it will make it collectively dumber. Putnam's Harvard colleague, anthropologist Joe Henrich, notes that cultural evolution works much like biological evolution. It needs an environment rich in variation and complexity where different cultures can challenge, compete, and combine with each other to generate new ideas while jettisoning old habits and counterproductive traditions. Indeed, small homogeneous societies inhabited by ethnic clones become less dynamic and even regress. America led the world in cutting-edge innovations in the 20th century, especially the IT revolution, precisely because its relatively free immigration and trade policies allowed for a much more rapid and free flow of information among diverse groups.

Ethno-state purists like to point to Japan as an example of a society that has successfully combined a radical program of ethnic homogeneity with modern standards of living. But Japan is paying a heavy price for its cultural purity and restrictive immigration policies. It's in the midst of a demographic collapse as its population rate plummets far below replacement levels. And if social trust and isolation is a problem in large diverse societies, it can be an even bigger problem in small homogeneous ones. Because the Japanese don't have a critical mass of people to find the right mate or friend, they are hunkering down like Putnam's turtles, unable to form strong bonds. Indeed, Japanese people are slightly less trusting than Americans — and a whole lot less so than "high trust" Australia, a poster child for diversity.

To be sure, the American left overplayed its hand by insisting on a forced program of diversity on college campuses and elsewhere. But the right's program of forced homogeneity based on a tendentious reading of social science will be far worse.