Tribal psychology, the tendency to divide the world into “us” and “them,” goes back to early human history. It gets its name because in earlier times, survival required loyalty to the tribe and suspicion of outsiders. In modern society, people of all kinds mix, but people still feel uncomfortable about those who are different from them. Sometimes they construct differences where there’s no real issue. Fan groups are a mostly harmless example. Red Sox fans hate the Yankees and Yankees fans hate the Red Sox, but it’s usually in fun. Science fiction fans disparage non-fans as “mundanes,” usually as a joke but sometimes with real disdain.

Problems arise when people use tribalist inclinations to treat outsiders as inferior or evil. That course can lead to violence, systemic discrimination, and legal persecution. It hurts members of the in-group as well, giving them fewer opportunities for valuable interactions and limiting them economically. It’s the antithesis of the liberal worldview, the idea that people should be regarded on their individual merits and have certain rights regardless of who they are.

People sometimes try to make sense of tribalism by reducing it to a single cause. Racism is an especially nasty form of it, but it’s only one of the foundations. Many on the left treat it as humanity’s Original Sin and ascribe all kinds of group hostility to racial attitudes, but it’s just one of the factors. Its influence has declined from past centuries. When it was acceptable, people used it to disparage groups that looked different. Today it’s less acceptable, so people define their groups in other ways.

Nationalism rose as a force in the nineteenth century, and it’s powerful today. Hostility based on foreignness is often considered acceptable, and many people have ferocious disdain for immigrants. People are often hostile to those with a different language or accent. Religion isn’t as powerful a factor as it was when you could be executed for heresy, but there’s a widespread view that (Evangelical) Christianity should get preferential legal status, and some countries still enforce state religions.

Irrational rage is useful for promoting hostility, the more irrational and enraged the better. This seems strange but makes historical sense. If an invading tribe posed a threat, the leaders had to stir the men up to fight. It isn’t easy to get calm, clear-headed people to march against people who want to kill them. The leaders of both sides needed fury to beat the other side. Today fury stirs people up even if they aren’t in combat. An enraged crowd is also good for electing candidates and forming mass protests.

It’s the “us vs. them” thinking which is at the core. The specifics of who’s “us” and who’s “them” vary with the fashions.