Most vaccines protect both the vaccinated individual and the society by reducing the transmission of infectious diseases. In order to eliminate infectious diseases, individuals need to consider social welfare beyond mere self-interest—regardless of ethnic, religious, or national group borders. It has therefore been proposed that vaccination poses a social contract in which individuals are morally obliged to get vaccinated. However, little is known about whether individuals indeed act upon this social contract. If so, vaccinated individuals should reciprocate by being more generous to a vaccinated other. On the contrary, if the other doesn’t vaccinate and violates the social contract, generosity should decline. Three preregistered experiments investigated how a person’s own vaccination behavior, others’ vaccination behavior, and others’ group membership influenced a person’s generosity toward respective others. The experiments consistently showed that especially compliant (i.e., vaccinated) individuals showed less generosity toward nonvaccinated individuals. This effect was independent of the others’ group membership, suggesting an unconditional moral principle. An internal metaanalysis (n= 1,032) confirmed the overall social contract effect. In a fourth experiment (n= 1,212), this pattern was especially pronounced among vaccinated individuals who perceived vaccination as a moral obligation. It is concluded that vaccination is a social contract in which cooperation is the morally right choice. Individuals act upon the social contract, and more so the stronger they perceive it as a moral obligation. Emphasizing the social contract could be a promising intervention to increase vaccine uptake, prevent free riding, and, eventually, support the elimination of infectious diseases.

The current experiment shows that communicating and rewarding "small wins" may increase individuals' willingness to act in the group's interest. Intergroup processes deserve further attention and investigation as potential strategies for improving vaccine communication and advocacy. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

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SummaryAnnual vaccination is the most effective way to prevent seasonal influenza. However, globally, the recommendations vary from country to country, ranging from universal recommendations, risk-group-specific recommendations, to no recommendation at all. Due to high diversity both in recommendation practice and country-specific preconditions, it is difficult to determine the effect of different recommendations on vaccine uptake. This incentivised laboratory experiment (N = 288) tests the behavioural consequences of different recommendations in a repeated interactive vaccination game. The participants are part of heterogeneous groups, comprised of low-and high-risk type of players. They receive either a universal, risk-group-specific or no recommendation prior to their vaccination decisions. Results show that individuals are sensitive to the recommendations. In detail, a risk-group-specific recommendation increases vaccine uptake of high-risk types. However, at the same time, it decreases vaccine uptake of low-risk types. The results imply that when the proportion of low-risk types in a population is considerably larger than the high-risk group, a risk-group-specific (vs. universal) recommendation comes at the cost of decreased social benefit of vaccination due to the overall lower vaccine uptake. Policy decision-making should therefore complement epidemiological considerations with potential positive and negative behavioural consequences of vaccination recommendations.

We run several experiments which allow us to compare cooperation under perfect and imperfect information in a centralized and decentralized punishment regime. Under perfect and extremely noisy information, aggregate behavior does not differ between institutions. Under intermediate noise, punishment escalates in the decentralized peer-to-peer punishment regime which badly affects efficiency while sustaining cooperation for longer. Only decentralized punishment is often directed at cooperators (perverse punishment). We report several, sometimes subtle, differences in punishment behavior, and how contributions react.