A systematic review and meta-analysis of the effectiveness of social norms messaging approaches for improving health behaviours in developed countries.

Social norms approaches have been widely applied in health promotion as a cost-effective behaviour-change strategy, but have been little evaluated as a whole. We conducted a pre-registered systematic review and meta-analysis of randomized controlled trials using social norms messaging in developed countries targeted at changing health behaviours among 16+-year-olds to evaluate their effectiveness. Relevant studies were identified through searches in PsycINFO, Medline, Embase, Web of Science, TRIP, Cochrane and grey literature sources. Risk of bias was assessed independently by two reviewers using the Cochrane RoB 2 tool. A random-effects meta-analysis standardized effect sizes to Cohen's d, assessed heterogeneity with I² and applied robust Bayesian meta-analysis to adjust for publication bias. Searches resulted in 89 studies (n = 85,759), which exhibited a small effect of social norms messaging on health behaviours (Cohen's d = 0.1, 95% confidence interval (CI) [0.09, 0.19], P < 0.001). However, this effect disappeared after controlling for publication bias. We conducted moderator analyses, finding no significant differences from the overall effect for different types of social norms message, delivery modalities, health domains or target populations. The review is limited by the lack of studies assessing whether normative information changed participant perceptions, inconsistent use of manipulation checks, and high heterogeneity across studies in terms of target behaviour, population and intervention delivery, affecting the robustness of conclusions. Our analysis suggests that when appropriately controlling for publication bias, social norms messages are not effective at improving health behaviours. Thus, future attempts at improving public health should focus on alternative approaches.