The Effect of Small Quasi-Cash Rewards on Prosocial Behavior: The Case of Voluntary Plasma Donations

To better understand how prosocial behavior can be effectively encouraged, we study four unresolved issues: whether small rewards also motivate prosocial behavior, how short-term effects differ from longer-run behavioral adjustments, whether incentives induce substitution away from other prosocial activities, and whether reminders about existing rewards meaningfully influence behavior. We examine these issues in a long-run randomized field experiment across 12 donation centers of the Belgian Red Cross-Flanders. Donors normally receive one thank-you-coupon (worth €1.5) for a blood donation and two coupons for a plasma donation. Informing donors about the existing rewards increases plasma donations by 4.6% during the nine-month information period, although this effect fades with repeated reminders. Rewarding plasma donors with an extra coupon raises plasma donations by an additional 7.1% during the six-month reward period; this effect grows during the reward period but disappears immediately afterward. We also document a substitution effect; the plasma reward reduces blood donations by 3.3%. Survey evidence suggests that small rewards act as signals of increased need or as gifts eliciting reciprocity and that our findings may extend to volunteering decisions more broadly. This paper was accepted by Marie Claire Villeval, Behavioral Economics and Decision Analysis. Funding: This paper is based on research funded by the Belgian Red Cross-Flanders; the content reflects solely the views of the authors and not those of the Red Cross. Supplemental Material: The online appendix and data files are available at https://doi.org/10.1287/mnsc.2024.08081 .