Behavioural spillovers unpacked: estimating the side effects of social norm nudges

Fighting the climate crisis requires changing many aspects of our consumption habits. Previous studies show that a first pro-environmental action can lead to another. But does this spillover effect persist when nudges foster the initial action? We model the mechanisms leading nudges to alter such behavioural spillovers. In an online experiment (n=2775), we test if encouraging vegetarianism with a social norm nudge alters environmental donations. The nudge is effective in increasing intentions to choose vegetarian food. Using machine learning, we find that a subgroup drives this effect. We also see a positive spillover effect: choosing vegetarian food increases donations. However, the nudge crowds out this spillover effect for the subgroup identified with machine learning. Our results suggest that social norm nudges are effective but crowd out people’s willingness to do more.