From The Better Angels of Our Nature:

The dark side of our communal feelings is a desire for our own group to dominate another group, no matter how we feel about its members as individuals. In a set of famous experiments, the psychologist Henri Tajfel told participants that they belonged to one of two groups defined by some trivial difference, such as whether they preferred the paintings of Paul Klee or Wassily Kandinsky. He then gave them an opportunity to distribute money between a member of their group and a member of the other group; the members were identified only by number, and the participants themselves had nothing to gain or lose from their choice. Not only did they allocate more money to their instant groupmates, but they preferred to penalize a member of the other group (for example, seven cents for a fellow Klee fan, one cent for a Kandinsky fan) than to benefit both individuals at the expense of the experimenter (nineteen cents for a fellow Klee fan, twenty-five cents for a Kandinsky fan). A preference for one’s group emerges early in life and seems to be something that must be unlearned, not learned. Developmental psychologists have shown that preschoolers profess racist attitudes that would appall their liberal parents, and that even babies prefer to interact with people of the same race and accent.

The psychologists Jim Sidanius and Felicia Pratto have proposed that people, to varying degrees, harbor a motive they call social dominance, though a more intuitive term is tribalism: the desire that social groups be organized into a hierarchy, generally with one’s own group dominant over the others. A social dominance orientation, they show, inclines people to a sweeping array of opinions and values, including patriotism, racism, fate, karma, caste, national destiny, militarism, toughness on crime, and defensiveness of existing arrangements of authority and inequality. An orientation away from social dominance, in contrast, inclines people to humanism, socialism, feminism, universal rights, political progressivism, and the egalitarian and pacifist themes in the Christian Bible.

…and, I suspect, to libertarianism? Perhaps especially, given the other items on that list, to Bleeding Heart Libertarianism?