In New York last week I met — as I always try to do in that city — the behavioral psychologist Jonathan Haidt, whose new book, The Coddling of the American Mind, offers a brilliant explanation of why students, almost overnight, have become so unwilling to countenance opposed opinions. Haidt explains that all human beings are, on a genetic level, tribal. The really remarkable thing is that we have managed to repress that tribalism in order to build the shared institutions of a pluralist open society. Those institutions need to be sustained by an application of reason and tolerance that does not come naturally. Stop inculcating these values and people quickly revert to their hunter-gatherer instincts.

To put it another way, the rise of tribal identities of every kind — the determination to categorize people politically by color, gender and sexual preference — has called into being an answering identitarianism from white people and men.

It shouldn’t need saying that, in a multi-ethnic, immigrant-descended population, these tendencies are dangerous. The centripetal force of the postwar era, when a high proportion of Americans were native-born, watched the same three TV channels and shared a common patriotism, has been replaced by a centrifugal force that defines and separates people by micro-culture. Without shared national identity, opponents become enemies. A strong sense of common national loyalty distinguishes the United States from, say, Syria or Somalia. Think hard before you throw it away.