Camilla Sandri

In the lab, in both one-shot interactions and first rounds of repeated games, subjects turn out to cooperate significantly more than the well-known, classical Homo Oeconomicus model predicts. Behavioural economics has persuasively shown that this ‘irrational’ rate of cooperation is compatible with the presence of reciprocity on the part of some of the individuals involved in the group. At the same time, a sizeable proportion of players act selfishly, failing to cooperate from the outset. However, so far we lack theoretical models accounting for such stable coexistence of free riders and reciprocators. Our work, by means of an evolutionary analysis of the Prisoner’s Dilemma (PD) game, provides an endogenisation of such motivational heterogeneity and, under certain conditions, sheds light on the evolutionary stability of two-type populations consisting of positive proportions of egoists and reciprocators. Key words: Free riding, reciprocity, cooperation, evolutionary game theory. JEL Classification: B41, C73, C91, Z13.