Will there be blood? Incentives and displacement effects in pro-social behavior

We present evidence from nearly 14,000 American Red Cross blood drives and from a natural field experiment showing that economic incentives have a positive effect on blood donations without increasing the fraction of donors who are ineligible to donate. The effect increases with the incentive's economic value. However, a substantial proportion of the increase in donations is explained by donors leaving neighboring drives without incentives to attend drives with incentives; this displacement also increases with the economic value of the incentive. We conclude that extrinsic incentives stimulate prosocial behavior, but unless displacement effects are considered, the effect may be overestimated.