In 2017, three psychologists led by Zachary Hohman of Texas Tech University recruited 152 people to take a fake personality test. The participants hailed from all political parties, races, and education levels, but they were all Americans. The test arbitrarily told each participant their personality type was more similar to the typical American or more similar to the typical French person.

The Americans who were told they resembled the French felt much more uncertainty about themselves, their futures, and their place in the world. They were then more likely to answer that they “agree” with statements like “I feel that I am not really the person that I appear to be” and “Even if I wanted to, I don’t think I would tell someone what I’m really like.”

To a typical, red-blooded American, something about being told they resemble a Frenchman must be very unsettling indeed.194 “When people are made to feel that they lack fit within their group, they question who they are,” Hohman and his coauthors wrote.

Other studies have shown that when people—say, honors students or sorority members—are told that they’re very different from the other members of their group, based on personality tests, they will more heartily embrace the stereotypes associated with that group, such as “hardworking” or even “superficial.” Told that they do not fit in, people will do anything to try to prove that yes, they really do.195

Feeling excluded can be so painful, in fact, that people will turn to terrible alternatives to avoid it. Researchers are increasingly finding that the roots of various kinds of terrorism and radicalization lie in the rather banal sensation of feeling cut off from your social circle.

In the wake of the 2015 Paris terror attacks, the economist Thomas Piketty wrote an op-ed in Le Monde in which he argued that “only an equitable model for social development will overcome hatred”—suggesting,196 in essence, that economic equality is the key to extinguishing the global terrorist threat. Efraim Benmelech, an economist at Northwestern University, read the article and decided to investigate whether economic inequality really does drive terrorism. Benmelech looked at the number of fighters flowing in to join ISIS from countries other than Syria and Iraq. What he and his coauthor, Esteban Klor, found197 was very peculiar: rather than coming from economic backwaters, ISIS fighters were pouring in from supposed democratic-socialist utopias. The countries that yielded the most ISIS fighters, relative to their total Muslim populations, were Finland, Ireland, Belgium, and Sweden.