In January-February 2022, we conducted financially incentivized dictator games with those with two COVID-19 vaccinations and those with zero vaccination in Japan (N=1,578), and ascertained their favorable or hostile attitudes toward each other, by using ingroup favoritism. We measured ingroup favoritism as the difference in the allocated amounts between to ingroup members with the same vaccination status and to outgroup members with a different status. Our analyses suggested that the vaccinated people behaved more discriminately toward outgroup members, compared to the unvaccinated people. The vaccinated people showed strong ingroup favoritism, which were shaped mainly by their outgroup bias of decreasing the money amount allocated to an unvaccinated pair, their outgroup member. In contrast, the unvaccinated people did not exhibit such the ingroup favoritism, on average. Their outgroup bias was found in the rather opposite direction of the hypothesis, and they tended to increase the amount to a vaccinated pair, their outgroup member. We found this tendency in particular from the unvaccinated who selected as their non-vaccination reason “I would like to get vaccinated if I could, but I cannot for health or other reasons.” Furthermore, we confirmed significant associations between their ingroup favoritism and attitudes regarding the COVID-19 policies, suggesting that the biases would have some degree of social influence in the real world.
In December 2022, we will conduct a follow-up experiment primarily on the above participants and examine how the ingroup favoritism, ingroup bias, and outgroup bias of vaccinated and unvaccinated participants change during the period from January-February to December, 2022. We will investigate whether the vaccinated people’s discrimination against the unvaccinated is temporary or persistent, from which we will derive policy implications.