Social Norms and Free-Riding in Influenza Vaccine Decisions: An Online Experiment

Objective

‘Nudges’ based on social norm messages impact decision-making in an assortment of economic and health contexts. Whether and how this impact differs at various social norm intensities, however, remains unclear. Furthermore, in public good games, free-riding behaviour may misalign with social norm adherence after certain social norm intensities. Perceived social norms and perceived free-riding threshold can potentially affect decision-making. We aim to identify the causal impact of different social norm intensities on stated and revealed preferences for a classical health public good game: influenza vaccination. This will allow us to understand how perceived social norms and perceived herd immunity, the threshold where free-riding should occur, impact the relationship between social norms and prevention preferences.

Methods

We conducted an online experiment using the UK-based Prolific Academic platform with n=1,216 participants randomly assigned to a control group (no explicit social norm) or to one of seven social norm treatment groups, where participants were asked to imagine that a certain proportion (i.e. 10%, 25%, 50%, 65%, 75%, 85%, or 95%) of people in their environment normally get the flu vaccine. Stated preferences were measured as self-reported vaccination likelihood. Directly revealed preferences were measured as (a) opening an online map showing nearby private flu jab providers’ locations; (b) time spent looking at this map; and (c) downloading a calendar reminder to vaccinate. Perceived social norm, perceived herd immunity thresholds, attitudes, preferences, behavioural and socio-economic characteristics were also measured. We used linear regressions, logistic and double hurdle models to analyse the impact of social norms on stated flu vaccination likelihood; revealed online map and calendar reminder interest; and map viewing time.

Results

Social norms significantly monotonically increased stated vaccination likelihood from the 50% treatment level and above after controlling for perceived social norms and perceived herd immunity. Social norms positively impacted map interest and map viewing time at all treatment levels. This effect increased until the 75% treatment level; individuals treated with greater social norm intensities exhibited free-riding behaviour. Similar behaviour was demonstrated for calendar reminder interest, with positive social norm impact peaking at 85%, followed by free-riding behaviour. Participants who received higher (lower) social norm treatment than their perceived social norm had significantly lower (higher) stated vaccination likelihood. Those who received higher (lower) social norm treatment than their perceived herd immunity threshold had significantly lower (higher) stated and revealed vaccination preferences.

Conclusion

Our results suggest that social norms impact revealed preferences for influenza vaccination in a non-monotonic manner. People significantly follow the social norm of getting vaccinated only when the social norm is below 75%, the herd immunity threshold. Free-riding begins beyond this social norm intensity. Perceived social norms and perceived herd immunity play important roles in explaining the heterogeneity of vaccination decisions: an induced social norm higher than the perceived social norm and/or perceived herd immunity threshold signal a lower infection risk and could lead individuals to free-ride and forego vaccination. These findings suggest that policymakers should exercise caution when designing behavioural interventions using social norm messages to ‘nudge’ vaccination.