Solving the Puzzle of Prosociality∗

Homo sapiens is the only species in which we observe extensive cooperation among large numbers of genetically unrelated individuals. Incompatible approaches to explaining cooperation among humans have been offered by sociologists, biologists, and economists. None is wholly successful. Each discipline, moreover, has ignored basic insights of the others. This paper explains cooperation by combining central contributions of these disciplines, developing a model of cultural evolution in which we use (a) the sociological concept of the internalization of norms to explain cultural transmission; (b) the biological concepts of vertical and oblique transmission to model the interaction of cultural and biological adaptation; and (c) the economic concepts of rational action and the replicator dynamic to model the interaction between self-interested and altruistic behavior.