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Blood donations are increasingly important for medical procedures, while meeting demand is challenging. This paper studies the role of spillovers arising from social interactions in the context of voluntary blood donations. We analyze a large-scale intervention among pairs of blood donors who live at the same street address. A quasi-random phone call provides the instrument for identifying the extent to which the propensity to donate spills over within these pairs. Spillovers transmit 41% to 46% of the behavioral impulse from one donor to the peer. This creates a significant social multiplier, ranging between 1.7 and 1.85. There is no evidence that these spillovers lead to intertemporal substitution. Taken together, our findings indicate that policy interventions have a substantially larger effect when targeted towards pairs instead of isolated individuals.

There is a longstanding concern that material rewards might undermine pro-social motivations, thereby leading to a decrease in blood donations. This paper provides an empirical test of how material rewards affect blood donations in a three-month large-scale field experiment and a fifteen-month follow-up period, involving more than 10,000 previous donors. We examine the efficacy of a lottery ticket as a reward vis-a-vis a standard invitation, an appeal, and a free cholesterol test. The offer of a lottery ticket, on average, increases the probability to donate blood during the experiment by 5.6 percentage points over a baseline donation rate of 46%. We find that this effect is driven by less motivated donors. Moreover, no reduction in donations is observed after the experiment.