Split strangers into groups by a coin flip, and they'll still favor their own

Henri Tajfel randomly sorted participants into groups using trivial, meaningless criteria, even a coin toss. Despite knowing the grouping was arbitrary and having no history with their 'team', people still gave more rewards to fellow group members than to outsiders. In-group favoritism, it turns out, needs almost nothing to switch on.

— Henri Tajfel, Experiments in Intergroup Discrimination — Scientific American, 1970