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We conducted a field experiment with the American Red Cross (ARC) to study the effects of economic incentives on volunteer activities. The experiment was designed to assess local and short-term effects as well as spatial and temporal substitution, heterogeneity, and spillovers. Subjects offered $5, $10, and $15 gift cards to give blood were more likely to donate and more so for the higher reward values. The incentives also led to spatial displacement and a short-term shift in the timing of donation activity, but they had no long-term effects. Many of the effects were also heterogeneous in the population. We also detected a spillover effect whereby informing some individuals of rewards through official ARC channels led others who were not officially informed to be more likely to donate. Thus, the effect of incentives on prosocial behavior includes not only the immediate local effects but also spatial displacement, social …