Effects of social norms vs health claims on purchasing behaviour: Reducing high-salt foods and promoting fruit and vegetables.

OBJECTIVES: This study aimed to compare the impact of health claims and social norms messages on consumer purchases of fruits, vegetables, and salt-related products in a supermarket setting. STUDY DESIGN: A crossover study was conducted in 12 Portuguese supermarkets, randomized into control and intervention groups. METHODS: Each intervention supermarket was exposed to two message types - health claims and social norms - delivered via audio and posters over six weeks (three weeks focusing on salt reduction, followed by three weeks promoting fruits and vegetables). Sales data were classified into three food groups: fruit, vegetables, legumes, and soup (FVL-S); high-salt; and low-salt products. Relative weekly sales (weight) were calculated. Data were analysed using Kruskal-Wallis tests and logistic regression models. RESULTS: Health claim messages were positively associated with increased purchases of FVL-S (High vs. Low tertile: OR = 2.9, 95% CI: 1.09-7.74). Social norm messages were associated with higher sales of unsalted/low-salt products (>P50 vs. <P50: OR = 3.8, 95% CI: 1.61-8.90). No significant effects were observed for high-salt products. CONCLUSIONS: In supermarket settings, health claims appear more effective in promoting purchases of fruit, vegetables, legumes, and soup, whereas social norm messages may be more effective in encouraging unsalted/low-salt food choices. These findings underscore the need to tailor public health communication strategies to specific dietary behaviours and product categories.