Extremists hold extreme views, and struggle to persuade others of their views, or even to get them to engage such views. Since most people are not extremists, you might think extremists focus mostly on persuading non-extremists. If so, they should have a common cause in getting ordinary people to think outside the usual boxes. They should want to join together to say that the usual views tend to gain from conformity pressures, and that such views are held overconfidently.
But in fact extremists don’t seem interested in joining together to support extremism. While each individual extremist tends to hold multiple extreme views, extremists groups go out of their way to distance themselves from other extremist groups. Not only do they often hate close cousins who they see as having betrayed their cause, they are also hostile to extremists groups on orthogonal topics.
This all makes sense if, as I’ve suggested, there are extremist personality types. Extremist groups have a better chance of attracting these types to their particular sort of extremism, relative to persuading ordinary folks to adopt extreme views.