punished. As a consequence, the existence of an opportunity for costly punishment causes a large increase in cooperation levels … opportunity almost complete cooperation can be achieved and maintained although, under the standard assumptions of rationality and … selfishness, there should be no cooperation at all. We also show that free riding causes strong negative emotions among …

We examine how conditional cooperation is related to the material payoffs in a Sequential Prisoner's Dilemma experiment … the material loss to the first-mover when the second-mover defects in response to cooperation. We find that few second …