Doing it once is good, doing it twice is even better. On the dynamics of altruistic behavior

We study a series of dictator games repeated a number of times at considerably large time intervals. The experimental design is such that reputation and learning effects can be ruled out. Treatments differ with respect to the number of repetitions, the time span between repetitions and observability of behavior. We observe in all treatments a strong tendency towards more selfish behavior over the course of the repeated experiment. We argue that this behavior can be rationalized if giving in dictator games is driven by a social norm that approves repeated gifts more than a single altruistic act. We report experimental evidence for the existence of such a norm using the norm elicitation method introduced by Krupka and Weber (2013).