Vaccination as a social contract.

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 ( = 1,032) confirmed the overall social contract effect. In a fourth experiment ( = 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.