Oops!... I Did It Again: Understanding Mechanisms of Persistence in Prosocial Behavior

We test whether asking individuals to donate blood leads to a persistent change in prosocial behavior, and what the underlying mechanisms are. In a large-scale field experiment, we randomize asking blood donors to turn out, and follow them over up to 18 months. We observe significant behavioral persistence over at least one year. We use naturally occurring rainfall as a second instrument for donor turnout to test whether persistence is due to habit formation (Stigler and Becker, 1977) or a persistent increase in motivation independent of past donation. Our results strongly favor habit formation as the underlying mechanism.