Increasing vegetables purchase with a descriptive-norm message: A cluster randomized controlled intervention in two university canteens

Exposure to social norms is a popular way to foster healthy food behavior. Testing the robustness of this effect, we report a field study assessing the impact of a vegetable-related descriptive norm message on vegetables purchase. The first contribution was to rely on a cluster randomized crossover design: Two canteens were randomly selected to display either a vegetable-related or a neutral-behavior norm message. After a first period of data collection, the displays were reversed for a second period: The number of vegetable portions on the main plate were recorded before, during and after the message display (N = 12.994). The second contribution was to test the impact of a message describing vegetables as the normative choice beyond the mere selection of vegetables, on the quantity of vegetables purchased in lunches containing some. Results indicated that the vegetable-related norm message led to a sustained probability of choosing vegetables, contrary to a decrease observed in the control condition. Moreover, students who ordered vegetables ordered a higher quantity when exposed to a vegetable-related message than before whereas quantity declined in the control condition. By treating both canteens as experimental and control and by analyzing both the presence and the amount of vegetables, these results extend and strengthen those previously observed, bringing support for the effectiveness of a descriptive norm message in eliciting healthier food behavior.