Tribal Psychology
Tribal psychology is the evolved human tendency to divide the social world into in-groups and out-groups, extending cooperation and trust to the in-group while treating the out-group with suspicion. The wiring is thought to have evolved in small Pleistocene bands, but the trigger fires on hometown, religion, political party, sports team, profession, or brand just as readily as it once fired on kinship. Henri Tajfel's minimal-group experiments showed how little is needed to switch it on, randomly assigning strangers to arbitrary groups was enough to produce in-group favouritism.
What it is
The claim underneath tribal psychology is that human beings evolved in an environment of small, bounded social groups, bands of perhaps fifty to a hundred and fifty people, in which cooperating with the in-group and being wary of outsiders had real survival value. Resources, protection, and information flowed reliably within the band and far less reliably, or not at all, from outside it. The proposed result is a piece of social cognition that automatically sorts people into "us" and "them" and applies different rules to each category, generosity, trust, and benefit of the doubt toward the in-group, and wariness or hostility toward the out-group.
What makes the concept useful for describing the modern world is not the Pleistocene backstory on its own, but the observation that the categorisation machinery does not appear to need a real tribe, a real kinship group, or even a group with any actual history, to switch on. It fires on markers: a shared hometown, a shared religion, a shared political affiliation, a shared sports team, a shared profession, a shared taste in music or a shared brand. None of these markers existed in the ancestral environment the theory describes, which is part of the argument, the underlying sorting mechanism is old and general-purpose, while the specific cues that trigger it in a given era are whatever happens to be socially meaningful at the time.
Henri Tajfel's minimal-group experiments are the clearest demonstration of how little a cue needs to carry to switch the mechanism on. Strangers with no history, no future interaction, and no material stake in the outcome were sorted into arbitrary categories, in Tajfel's original work, ostensibly based on trivial preferences, and even under those conditions people allocated resources more generously to members of their own arbitrary group than to the other one. If categorisation this thin is sufficient to produce in-group favouritism, the argument goes, then the much richer, more emotionally loaded categories modern life actually offers, nationality, party, faith, team, should be expected to trigger the same machinery considerably more strongly.
Tribal psychology functions in this vault less as a single theory with one named author than as an umbrella term covering a family of related findings, from Tajfel's minimal-group work through decades of subsequent social identity research, that converge on the same underlying picture, in-group and out-group categorisation is fast, automatic, and easily triggered by minimal cues, whatever the content of those cues happens to be in a given time and place.