Vaccination as a social contract

Abstract

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.

Keywords: generosity; reciprocity; vaccine advocacy; vaccine decision making; vaccine hesitancy.

Fig. 1.

Illustration of the experimental setting (A) and procedure (B) used to test whether vaccination is a social contract. (A) After learning whether the other person vaccinated or did not vaccinate in the experimental game, the participants (Center) allocated money between oneself and four other people, respectively, who were either vaccinated or not and belonged to the in-group (black) or an out-group (gray). Changes to a baseline measure indicated changes in generosity. A.1 describes the situation in which vaccination is a social contract: vaccinated individuals allocate more money to vaccinated others and less money to nonvaccinated others (social contract hypothesis). In A.1, changes in generosity apply to all others alike, i.e., changes are independent from the others’ group memberships. A.2, however, describes a case where the decision maker changes his or her generosity only with regard to in-group but not to out-group members (conditional social contract). This pattern would indicate that vaccination is not a social contract.

Fig. 2.

Forest plot of mean changes in generosity (generosity conditional to own and other decision and group membership, relative to the unconditional baseline). Positive values indicate an increase in generosity, whereas negative values indicate a decrease in generosity. Absolute values of unconditional baseline generosity can be found in SI Appendix, Table S8. The pattern of results shows that the interaction effect between participants’ vaccination decisions and others’ vaccination decisions (social contract hypothesis) was mainly driven by a reduction of generosity from vaccinated participants toward nonvaccinated others. Nonvaccinated participants also showed a reduction of generosity toward nonvaccinated others, but this effect was smaller than among vaccinated participants. An increase in generosity was less pronounced and was only shown from vaccinated participants toward vaccinated in-group members. Note: Changes in generosity refer to standardized changes. The experimental materials were comparable and varied only in little details as described in the methods section; thus, the three experiments can be understood as conceptual replications. The overall effects were calculated using a random effects model for meta-analysis. Q and I² were used for heterogeneity assessment among the studies. CIs refer to 95% confidence intervals. N_{Experiment 1} = 117, N_{Experiment 2} = 372, N_{Experiment 3} = 444.

Fig. 3.

Forest plot displaying tests of the social contract and conditional social contract. Hypotheses across three experiments. The results show evidence for the social contract hypothesis as indicated by the significant interaction effect of the participant’s and the other’s vaccination decision (Top). There was no evidence for the conditional social contract hypothesis as indicated by the nonsignificant interaction effect (Middle) between participant’s and other’s decision and group membership – this supports the idea of vaccination being a social contract irrespective of the individuals’ group memberships. The manipulation check (Bottom) confirmed that the groups were indeed perceived as distinct groups by showing a significant main effect for group membership. Note: SI Appendix, Fig. S1 in the supplement displays all remaining main effects and interaction effects of the analysis. The experimental materials were comparable and varied only in little details as described in Materials and Methods; thus, the three experiments can be understood as conceptual replications. The figure display betas, calculated from mixed effects regressions, and overall effects using a random effects model for metaanalysis. Q and I² were used for heterogeneity assessment.

Fig. 4.

Testing the social contract hypothesis in a task framed as neutral or as vaccination decision and depending on the perception of vaccination as a social contract. The figure displays changes in generosity as a function of participant’s vaccination decision (columns), other’s vaccination decision (lines), framing (Top vs. Bottom), and perceiving vaccination as a moral obligation (x axis) in experiment 4. Conditional generosity is indicated by the black line being above the gray line (increased generosity toward vaccinated and decreased generosity toward unvaccinated individuals). When framed as a vaccination decision, vaccinated participants were less generous to nonvaccinated others the more they perceived vaccination as a social contract; likewise, generosity toward vaccinated others increased with increasing perception of vaccination as a social contract (Lower Right quadrant). Regardless of framing, nonvaccinated participants again did not differentiate between vaccinated and nonvaccinated others, irrespective of their belief of vaccination as social contract (Lower Left quadrant). This pattern again supports the idea of vaccination as a social contract. Note: Confidence bands represent 95% CI. n = 1,212.