"The Evolution of Cooperation in Infinitely Repeated Games: Experimental Evidence" with Guillaume Fréchette, October 2006.

We present experimental evidence on the evolution of cooperation in infinitely repeated games as subjects gain experience. We find that cooperation decreases with experience when it cannot be supported as an equilibrium outcome. More interestingly, the converse is not necessarily true: cooperation does not always increase with experience when it can be supported as an equilibrium outcome. Nor is a more stringent condition, risk dominance, sufficient for cooperation to arise. However, subjects do learn to cooperate when the payoff to cooperation and the importance of the future is high enough. These results have important implications for the theory of infinitely repeated games. While we show that cooperation may prevail in infinitely repeated games, the conditions under which it happens are more stringent than the sub-game perfect conditions usually considered.

“Do the Right Thing:” The Effects of Moral Suasion on Cooperation with Ernesto Dal Bó, September 2009.

The use of moral appeals to affect the behavior of others is pervasive (from the pulpit to ethics classes) but little is known about the effects of moral suasion on behavior. In a series of experiments we study whether moral suasion affects behavior in voluntary contribution games and mechanisms by which behavior is altered. We
find that observing a message with a moral standard according to the golden rule or, alternatively, utilitarian philosophy, results in a significant but transitory increase in
contributions above the levels observed for subjects that did not receive a message or received a message that advised them to contribute without a moral rationale. When players have the option of punishing each other after the contribution stage the effect of the moral messages on contributions becomes persistent: punishments and moral messages interact to sustain cooperation. We investigate the mechanism through which moral suasion operates and find it to involve both expectations and preference-shifting effects. These results suggest that the use of moral appeals can be an effective way of promoting cooperation.