Tajfel’s studies demonstrated two things: first, that people are social; second, that people are antisocial. People are social because they like the members of their own group. People are antisocial because they don’t like the members of other groups. Exactly why this is so has been the subject of continual debate. I think it might be a solution to a complex problem of optimization. Such problems arise, for example, when two or more factors are important, but none cannot be maximized without diminishing the others. A problem of this sort emerges, for example, because of the antipathy between cooperation and competition, both of which are socially and psychologically desirable. Cooperation is for safety, security and companionship. Competition is for personal growth and status. However, if a given group is too small, it has no power or prestige, and cannot fend off other groups. In consequence, being one of its members is not that useful. If the group is too large, however, the probability of climbing near or to the top declines. So, it becomes too hard to get ahead. Perhaps people identify with groups at the flip of a coin because they deeply want to organize themselves, protect themselves, and still have some reasonable probability of climbing the dominance hierarchy. Then they favour their own group, because favouring it helps it thrive—and climbing something that is failing is not a useful strategy.

Jordan B. Peterson, 12 Rules for Life An Antido…, loc. 961