Humans are widely considered to be susceptible to incentives, which are frequently employed to encourage specific behaviors. However, incentives have surprisingly inconsistent effects when used to motivate prosocial behavior – sometimes producing no behavioral change or even backfiring. To explain these inconsistencies, we extended a prominent image-based model of prosocial behavior, based on the idea that social norms shape the reputational consequences of receiving incentives. We tested the key predictions of this model by examining the blood donation behavior of 26,000 individuals from 28 European countries. Our preregistered analyses revealed that social norms can indeed predict how incentives, either in the form of financial payments or time off work, relate to individual-level blood donation behavior. Incentives are associated with higher levels of prosociality if they align with existing norms. The results suggest that humans may not be universally persuaded by incentives to behave prosocially, but that the effectiveness of incentives depends on social norms. © 2023