  We explore the impact of mutual altruism among the players belonging to the
same set -- their tribe -- in a partition of all players in arbitrary strategic
games upon the quality of equilibria attained. To this end, we introduce the
notion of a {\tau}-tribal extension of an arbitrary strategic game, in which
players' subjective cost functions are updated to reflect this, and the
associated Price of Tribalism, which is the ratio of the social welfare of the
worst Nash equilibrium of the tribal extension to that of the optimum of social
welfare. We show that in a well-known game of friendship cliques, network
contribution games as well as atomic linear congestion games, the Price of
Tribalism is higher than the Price of Anarchy of either the purely selfish
players or fully altruistic players (i.e. ones who seek to maximise the social
welfare). This phenomenon is observed under a variety of equilibrium concepts.
In each instance, we present upper bounds on the Price of Tribalism that match
the lower bounds established by our example.
