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 climate-friendly action can lead to another. Does deciding not to eat meat increase our willingness to do more for the environment? Can encouraging vegetarianism alter this willingness? Using an online randomised control trial, we study the side effects of a social norm nudge promoting vegetarianism on environmental donations. We develop an experimental design to estimate these side effects and a utility maximisation framework to understand their mechanisms. Using an instrumental variable, we find that choosing not to eat meat increases donations to pro-environmental charities. We use machine learning to find that the social norm nudge crowds out donations from the population segment prone to choosing vegetarian food after seeing the nudge. However, the nudge led another group to make less carbon-intensive food choices without affecting their donations. Our results suggest that whilst social norm nudges are effective on specific population segments, they can also reduce the willingness of some groups to do more.