Nationalism is one case of a wider human pattern: people form groups, identify with them and favor them. Psychology and political theory offer tools for understanding this pattern, and they apply to religion, ethnicity, class, region and even sports teams as well as to nations.

Social identity theory

The psychologist Henri Tajfel, a Polish-born Jewish survivor of the Second World War who worked in Britain, wanted to understand how prejudice and group conflict arise. In experiments in the early 1970s, now called the minimal group studies, schoolboys were divided into groups on trivial grounds, such as whether they preferred paintings by Klee or Kandinsky. They never met their fellow group members and had nothing to gain personally. Yet when asked to share out rewards, people favored in-groups even when groups were based on trivia, and sometimes chose to maximize the gap between groups rather than the total their own group received.

With John Turner, Tajfel developed social identity theory (1979). Its central claim is that part of our self-concept comes from our group memberships. Because people want to feel good about themselves, they want their groups to compare well with other groups. This search for positive distinctiveness helps explain group pride and, under some conditions, hostility.

In-group bias and its limits

In-group bias is a tendency to favor members of one's own group. It is common and often mild: trusting a neighbor more than a stranger, cheering for a home team. Favoring one's own group does not automatically mean hating others. Hostility grows when groups compete for scarce resources, feel threatened, or are told by leaders that the other side is dangerous.

Research also shows how bias can be reduced. The psychologist Gordon Allport, in The Nature of Prejudice (1954), proposed the contact hypothesis: contact between groups tends to reduce prejudice when it involves equal status, common goals, cooperation and institutional support. Many later studies have broadly supported it, while finding that the conditions matter.

Multiple identities

Every person belongs to many groups at once: a nationality, perhaps a religion, a profession, a hometown, a generation, a language, a family, a set of political convictions. Which identity matters most changes with context. The economist and philosopher Amartya Sen, in Identity and Violence (2006), argued, in paraphrase, that reducing people to a single identity can feed violence. When someone is seen only as a member of one religion or nation, and every other affiliation is ignored, it becomes easier to treat that person as an enemy. Recognizing our many identities, and our freedom to choose which to emphasize, is part of the remedy.

Identity politics, debated fairly

The term identity politics is often traced to the Combahee River Collective's 1977 statement, written by a group of Black feminists in Boston who argued that their own experience of overlapping oppression should be the starting point of their politics. Defenders say that groups facing shared discrimination must organize together to win equal rights, and that being recognized and respected as who one is matters to human dignity. On this view, nearly all politics involves identity; the question is whose identities are noticed.

Critics, who come from several political directions, worry that it may fragment shared citizenship into rival groups, reduce individuals to group labels, and encourage competition among grievances rather than common goals. Some point out that majority groups can practice identity politics too, and that ethnic nationalism is one form of it. A fair summary is that groups and identities are inescapable, and the live debate is how to honor them while keeping a shared civic life in which everyone is treated as an individual and an equal.

Worked example: the Robbers Cave study

In the summer of 1954, the psychologist Muzafer Sherif ran a famous field study at a boys' summer camp in Robbers Cave State Park, Oklahoma. It is often taught alongside social identity theory.

About two dozen eleven-year-old boys, all similar in background, were split into two groups that at first did not know of each other. Each group bonded, chose a name (the Eagles and the Rattlers) and developed its own habits. Then the researchers introduced competitions with prizes for the winners, such as tug-of-war and baseball. Hostility grew quickly: name-calling, flag burning and raids on each other's cabins.

Simply bringing the groups together for pleasant activities, like watching films, did not end the hostility. What helped was superordinate goals: problems that both groups needed to solve together, such as restoring the camp's water supply and pulling a stuck supply truck. After working together on several such tasks, the boys became noticeably friendlier across group lines.

The study fits Allport's contact hypothesis: contact reduced prejudice when it involved common goals and cooperation, not just being in the same place. Notice also that competition for scarce prizes, not any deep difference, produced the conflict.

A caveat matters. Historians who studied Sherif's records found that an earlier version of the experiment in 1953 had not gone as planned: the boys largely refused to turn on each other. Researchers also shaped events in ways later writers have questioned. The study remains illuminating, but like Milgram's work, it should be read as a dramatic illustration rather than a final proof.