Friendship, not altruism: an economic theory with cross-cultural applications

Friendship is both ubiquitous and economically important, but neglected in the economic literature. We provide a definition of friendship supported by anthropology research that we believe is plausible, widely accepted, and distinct from altruism. This motivates a game-theoretic model of friendship that provides a characterization of how friendship in a bilateral relationship can explain cooperation in a finite-horizon setting without the aid of altruism or pro-social preferences. We highlight the difference between two key equilibria of our model: opportunistic friendship that is short-lived and driven by material support, and sustained friendship that is long-lived and is distinguished from opportunistic friendship by the provision of support without the expectation of return. Opportunistic friendship seems more likely in environments characterized by economic uncertainty such as in developing countries or immigrant communities. We provide cross-cultural examples of friendship that are consistent with the conditions underpinning opportunistic versus sustained friendship equilibria.