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

Reducing greenhouse gas emissions in the fight against climate change requires us to change many aspects of our consumption habits. Previous studies have shown that taking one climate-friendly action can lead to another in a ‘behavioural spillover effect’.

In this paper, the authors examine whether deciding not to eat meat can increase willingness to do more for the environment – and if encouraging vegetarianism can alter this willingness. Specifically, they study how using a ‘social norm nudge’ to encourage people to take up vegetarianism might affect their propensity to then make a donation to a pro-environmental charity.

Overall, they find that choosing not to eat meat increases donations to pro-environmental charities. This suggests that vegetarianism could be an ‘entry point’ for taking additional climate-friendly action.

However, they find that using the social norm nudge reduces donations from the population segment already prone to choosing vegetarian food after they see the nudge. Yet, the nudge led another group to make less carbon-intensive food choices without affecting their donations.

The results suggest that while social norm nudges are effective on specific population segments, they can also reduce the willingness of some groups to do more. This has implications for the design of policies and their delivery as the type of population targeted will influence their effectiveness.

Key points for decision-makers

• ‘Social norm nudges’ are simple messages giving information on what others are doing. They can be effective in shifting behaviours and have been used to foster recycling, promote sustainable diets and so on. However, little is known about whether messages might trigger ‘side effects’ in the form of taking other pro-environmental decisions not specifically targeted by the ‘nudge’.
• The authors used an online randomised control trial to study the side effects of a social norm message promoting vegetarianism, with 2,775 English respondents.
• The authors find evidence of a positive behavioural spillover effect on average: respondents choosing vegetarian food were more likely to give to pro-environmental charities.
• The results indicate that the social norm nudge is effective. Exposure to the message increases the likelihood of choosing a vegetarian item on average.
• However, this hides heterogeneity. The authors only observe a significant reduction in the carbon footprint of food choices for a specific population segment. They also only observe an increase in the likelihood of choosing vegetarian items for another population segment.
• On the latter population segment, they find that the social norm nudge subsequently reduces their donations, outweighing the positive behavioural spillover effect.
• The results confirm the importance of studying side effects and addressing different social-demographic profiles when devising and evaluating behavioural policies.