Group Size and Cooperation Among Strangers

We study how group size affects cooperation in an infinitely repeated n-player Prisoner`s Dilemma (PD) game. In each repetition of the game, groups of size n ≤ M are randomly and anonymously matched from a fixed population of size M to play the n-player PD stage game. We provide conditions for which the contagious strategy (Kandori, 1992) sustains a social norm of cooperation among all M players. Our main finding is that if agents are sufficiently patient, a social norm of society-wide cooperation becomes easier to sustain under the contagious strategy as n → M.