The more the better? Synergies of prosocial interventions and effects on behavioural spillovers.

Incentivising prosocial and pro-environmental behaviours is a sensitive endeavour. While behavioural change is urgently needed to mitigate the consequences of climate change, monetary interventions often have negative side effects. Such interventions are prone to motivation crowding, which can impede lasting positive behavioural change and stimulate negative temporal spillovers to other prosocial behaviours. In this study, we investigate whether implementing monetary interventions as part of policy mixes can mitigate these negative side effects. In an online experiment involving 3782 participants, we test whether the use of nudges that make personal and social norms salient can counteract the motivation-crowding effect and explore the effects of such policy mixes on temporal spillovers. We find that policy mixes of norm-based nudges and monetary incentives are more effective at stimulating engagement in targeted prosocial behaviour than no intervention when controlling for sample characteristics. Analysing the temporal spillover effects of these interventions reveals that policy mixes can alleviate the tendency of monetary incentives to negatively affect subsequent prosocial behaviour. This indicates that norm-based nudges are suitable complements to monetary interventions, facilitating long-lasting positive effects.