First, as mass immigration increases diversity, it reduces social cohesion and civic trust. This is not a universal law, as the economics writer Noah Smith has pointed out; there are counter-examples and ways to resist the trend. However, it is a finding that strongly comports with the real-world experience of Europe and America, where as cultural diversity has increased so has social distrust, elite-populist conflict, and the racial, religious and generational polarization of political parties.

Moreover, the trust problem is not a simple matter of racist natives mistrusting foreigners, since social trust is often weakest among minorities — which is one reason why the most diverse generation in American history, the millennials, is also the least trusting. So you can see the political effects of distrust even if you ignore the Trump Republicans entirely: It’s one reason why campus politics are so toxic, why Democrats struggle to keep their diverse coalition politically engaged, and why the Bernie-Hillary contest produced so many cries of racism and sexism.

Then linked to these ethno-cultural tensions are the tensions of class, where mass immigration favors stratification and elite self-segregation. In the United States, as in France and England, regions and cities with the largest immigrant populations are often the wealthiest and most dynamic. But this doesn’t mean that poorer regions are dying from their own xenophobia, as is sometimes suggested. The hinterlands are also filled with people who might want to move to wealthier regions (or who used to live there) but can’t because an immigrants-and-professionals ecosystem effectively prices out the middle class.

It is a testament to immigrants’ grit and determination that they can thrive working long hours for low wages while living in crowded housing with long commutes. But the social order of, say, the Bay Area or greater Paris is not one that can serve for an entire country — and it ill-serves not only lower-middle-class natives but also the descendants of the immigrants themselves, whose ability to advance beyond their parents is limited by a continued arrival of new workers who compete with them for jobs and wages and housing.