Though it has a long way to go, the science that underlies the fight against extremism has made a lot of progress in recent years. Psychologists and social media analysts have found that people become radicalized by other members of a group. People seek out the like-minded, then enter online forums, then become more extreme by reading and communicating with others. As the study of history will also tell you, individuals will do things as part of a mob that they would not do alone.

This is the psychological mechanism that the Islamic State and other Islamist terrorist organizations have used to recruit young men online: They offer them membership and a sense of virtual belonging — then they goad them into action. This is also the mechanism that far-right groups have used to organize marches, persuade people to wave racist banners — and worse. Thomas Mair — the man who murdered Jo Cox, a British member of Parliament, in 2016 — spent his time reading neo-Nazi websites. He was a “loner” who, thanks to the Internet, very much felt himself to be part of a mob.