  Incentives have surprisingly inconsistent effects when it comes to
encouraging people to behave prosocially. Classical economic theory, according
to which a specific behavior becomes more prevalent when it is rewarded,
struggles to explain why incentives sometimes backfire. More recent theories
therefore posit a reputational cost offsetting the benefits of receiving an
incentive -- yet unexplained effects of incentives remain, for instance across
incentive types and countries. We propose that social norms can offer an
explanation for these inconsistencies. Ultimately, social norms determine the
reputational costs or benefits resulting from a given behavior, and thus
variation in the effect of incentives may reflect variation in norms. We
implemented a formal model of prosocial behavior integrating social norms,
which we empirically tested on the real-world prosocial behavior of blood
donation. Blood donation is essential for many life-saving medical procedures,
but also presents an ideal testing ground for our theory: Various incentive
policies for blood donors exist across countries, enabling a comparative
approach. Our preregistered analyses reveal that social norms can indeed
account for the varying effects of financial and time incentives on
individual-level blood donation behavior across 28 European countries.
Incentives are associated with higher levels of prosociality when norms
regarding the incentive are more positive. The results indicate that social
norms play an important role in explaining the relationship between incentives
and prosocial behavior. More generally, our approach highlights the potential
of integrating theory from across the economic and behavioral sciences to
generate novel insights, with tangible consequences for policy-making.
