Side messages, side effects: Justifying infection prevention but fuelling social polarisation

As infectious diseases spread, governments and specialised agencies typically release a main message conveying the recommended preventive action, accompanied by side messages justifying the action. In this study, we explored how side messages influence intentions to engage in infection prevention behaviours, attitudes toward people who do versus do not support infection prevention measures, and evaluations of the poster. We conducted three repeated cross-sectional online surveys in Japan and the United States in September, November, and December 2024. Participants were presented with posters promoting hand washing and mask wearing that included one of five side messages: self/other protection, public health, social norms, self/other protection plus norms, or public health plus norms. Meta-analyses of the three surveys showed side-message effects on intentions varied by country and baseline compliance: in Japan, for individuals who initially had high compliance tendencies, side messages combining social norms with additional justifications (self/other protection or public health) promoted behavioural intent, whereas in the United States, side messages that justified public health outcomes were most effective in promoting behavioural intent. We found no evidence that side-message content altered intergroup bias, defined as the difference in attitudes towards people who share one's own views versus those holding opposing views regarding infection prevention measures. However, the posters were perceived as coercive and stressful, particularly among participants with low baseline compliance, indicating that campaign designers may consider crafting messages that encourage adherence to infection prevention measures while taking care not to increase criticism of non-compliant individuals, who are often in the minority.