Nudging Cooperation in Environmental Social Dilemmas: Moral versus Social Appeals in Waste Sorting

Many pro-environmental actions are environmental social dilemmas where cooperation is often conditional, and people are more willing to contribute when they believe that others are doing their part. We treat household waste sorting as a paradigmatic case. It is a necessary first step for effective recycling that helps tackle climate change, yet it is often neglected because of time costs and low perceived cooperation. This study investigates whether social nudges (highlighting what others do) and moral nudges (appealing to personal norms) can promote waste sorting, and whether perceived cooperation moderates their effectiveness. In a between-subjects experiment, 123 US participants were randomly assigned to a fictitious group and completed a computerized waste-sorting task. Choosing to recycle involved a time cost, as each choice triggered a short waiting animation mirroring the opportunity costs of sorting waste in everyday life, whereas discarding the remaining waste in the mixed bin was immediate. At the end of each round, participants received visual feedback on the amount of waste recycled by the group, indicating high or low cooperation. Bayesian regression models revealed that cooperation level strongly predicted behavior. Moreover, recycling increased under the moral nudge condition only when participants believed that others were contributing very little, with this effect being especially pronounced among those who already endorsed strong pro-environmental personal norms. These findings suggest that moral nudges can serve as a low-cost tool to promote waste sorting by activating existing personal norms in unsupportive social environments.