Spillovers of Prosocial Motivation: Evidence from an Intervention Study on Blood Donors

Spillovers of prosocial motivation are crucial for the formation of social capital. They facilitate interactions among individuals and create social multipliers that amplify the effects of policy interventions. We conducted a large-scale intervention study among dyads of blood donors to investigate whether social ties lead to motivational spillovers in the decision to donate. The intervention is a randomized phone call making donors aware of a current shortage of their blood type and serving us as an instrument for identifying motivational spillovers. About 40% of a donor's motivation spills over to the other donor, creating a significant social multiplier of 1.78.