DP001
Vaccination and Discrimination:
Experimental Evidence during and after the COVID-19 Pandemic（改訂日：2024 年 5 月 16 日）

Shusaku Sasaki, Osaka University
Hirofumi Kurokawa, Kwansei Gakuin University

Vaccination and Discrimination:
Experimental Evidence during and after the COVID-19 Pandemic
Shusaku Sasaki1

Hirofumi Kurokawa2

Third Draft: 16th May 2024

Abstract
The issue of receiving vaccines or not divides people: however, both the vaccinated and unvaccinatedpeople need to live together in the same society. This study conducted financially incentivized dictator
games with about 1,600 COVID-19 vaccinated or unvaccinated people residing throughout Japan andevaluate their cooperative or hostile attitudes toward each other by using ingroup favoritism, which isthe difference in the allocated amounts between to ingroup pairs with the same vaccination status andto outgroup pairs with a different status. Our study’s uniqueness is conducting the first experiment inJanuary-February 2022 and the follow-up experiments in December 2022 and June 2023 to examine
how the attitudes change toward the post-pandemic era. The first experiment suggests that the COVID19 vaccinated people behave more discriminately toward outgroup members, when compared to theunvaccinated people. The vaccinated people show strong ingroup favoritism, which is shaped mainlyby their outgroup bias of decreasing the allocation amount to an unvaccinated pair, their outgroup. In
contrast, the unvaccinated people do not exhibit such ingroup favoritism. Their outgroup bias is foundin the rather opposite direction of the hypothesis, and they tend to increase the allocation amount to avaccinated pair, their outgroup. This tendency is found in particular from the unvaccinated people who
selected as their non-vaccination reason “I would like to get vaccinated if I could, but I cannot for
health or other reasons.” Our follow-up experiments find that the vaccinators’ ingroup favoritism ispersistent over the medium term, while the non-vaccinators’ favorable attitude toward their outgrouppair is temporary, and they gradually exhibit so-called ingroup favoritism.
Keywords: COVID-19, Vaccination status, Discrimination, Altruism, Online experiment
JEL classification: I12, D91, C90

Center for Infectious Disease Education and Research (CiDER), Osaka University, Japan (Corresponding
author: ssasaki.econ@cider.osaka-u.ac.jp).
2
Faculty of Economics, Kwansei Gakuin University, Japan.
1

We obtained ex-ante approvals from the ethics committee of Center for Infectious Disease Education andResearch, Osaka University, Japan (2022CRER0114, 2022CRER1115-2, 2023CRER0515). We also
registered the experimental design with the AEA RCT Registry (Sasaki and Kurokawa, 2022a,b, 2023).
This research is financially supported by Center for Infectious Disease Education and Research, Osaka
University, and Japan Science and Technology Agency [JST PRESTO Grant Number: JPMJPR21R4 (S.,Sasaki)].

1. Introduction
Vaccination is one of essential keys to ending the social confusion caused by pandemics and re-starting
socio-economic activities. However, some people are reluctant to receive the vaccine or have negative
attitudes toward the vaccination. In the COVID-19 pandemic, anti-vaccine demonstrations were held
in France, Germany, Japan, and the other countries around the world, and fake news about vaccines
were flooded on social media. Unvaccinated people also experienced some restrictions when goingout, layoffs from work, etc.
The issue of receiving vaccines or not divides people: however, the vaccinated and unvaccinatedpeople need to live together in the same society. Since the unvaccinated people do not obtain immunityfrom vaccination, the vaccinated need to be careful not to spread the infection to the unvaccinated.
The unvaccinated need to agree that the governments allocate taxes to investing the construction ofenvironments in which people with vaccination intention can receive the vaccine immediately. Frompolicy perspectives, it is crucial to empirically determine how cooperative or hostile the vaccinatedand unvaccinated people are toward each other.
This experimental study measures ingroup favoritism of the COVID-19 vaccinated andunvaccinated people and examine the characteristics of its distribution. Compared to the literature, theuniqueness of this study is in investigating how the ingroup favoritism changes over time. The ingroupfavoritism is captured by the difference in the allocated money amount between to ingroup members
who share group identities (country, religion, political party, and vaccination status) and to outgroupmembers who do not.
The ingroup favoritism has been widely studied as measures of favorable attitudes toward ingroupmembers or hostile attitudes toward outgroup members. When experimenters artificially created a bitof identity and made explicit, based on the minimal group paradigm (Tajfel et al. 1971), people allocate
more money to a paired partner with the same identity than to that with a different identity (Yamagishi
and Mifune 2008; Mifune et al. 2010). The ingroup favoritism appears also when using social identities,
including country, religion, political party, race, and gender (Charness and Rustichini 2011; Fershtman
and Gneezy 2001; Kranton et al. 2020). Also in economics, there has been plenty of research on social
identity since Akerlof and Kranton (2000) published their seminal paper. It reveals that social identity
have significant economic consequences in the labor market (Calvo-Armengol and Jackson, 2004) andhealthcare market (Centola, 2011) through ingroup favoritism.
It is known that the ingroup favoritism appears when using identities that are formed exogenously,
including race and gender, as well as when using identities which are formed by people’s endogenous
choices (Efferson et al. 2008; Charness et al. 2014). We thus assume that people’s endogenous choiceof whether they receive the COVID-19 vaccine or not will form a group identity, and experimentallymeasures the ingroup favoritism of vaccinated and unvaccinated people toward each other. Jagodics
and Szabó (2022) also explain that vaccination status has conditions for shaping a social identity. For2example, vaccination status is socially unneutral since regulations create distinctions based on it, andthe membership makes the vaccinated people perceive that their outgroup threatens the achievementof their goals.
The ingroup favoritism is formed by two types of biases: ingroup bias (a.k.a. ingroup love), whichis the tendency to prefer ingroup members with the same identity, and outgroup bias (a.k.a. outgrouphate, outgroup derogation), which is the tendency to dislike outgroup members with different identities.
The former may improve the performance of the ingroup, providing members with long-term benefits
and increasing their survival probability (Brewer, 1999; Caporael, 2007). The latter may generate
hostile relationships with outgroup members and stimulate competitions (Bornstein, 2003; Halevy etal., 2010; Yzerbyt and Demoulin, 2010). Whether people’s ingroup favoritism emerge mainly due toingroup bias or outgroup bias provides useful insights on how the ingroup favoritism will appear as areal-world behavior.
We conduct financially incentivized online experiments and use the responses to measure ingroupfavoritism, ingroup bias, and outgroup bias among about 1,600 COVID-19 vaccinated or unvaccinatedpeople residing throughout Japan. Specifically, we conduct one main experiment in January-February
2022 (when the program for first- and second-dose vaccinations was almost completed in Japan) andtwo follow-up experiments in December 2022 and June 2023. The second follow-up is conducted justafter downgrading the status of the COVID-19 under the Infectious Diseases Act in Japan. That is, thecurrent study can capture how the attitudes of vaccinated and unvaccinated individuals toward eachother change, over the medium term, toward the post-pandemic era.
The main findings are as follows: The main experiment shows that vaccinated participants exhibit
strong ingroup favoritism and it is shaped by outgroup bias, a hostile attitude toward unvaccinatedpeople. The ingroup favoritism is not observed from the unvaccinated participants; rather, they allocate
more to their out-group member, the vaccinated people. Their favorable attitude toward the outgroupmember is found to be formed by the non-vaccinators who wanted to receive the vaccine but were notable to do so for some health reasons. Further, the two follow-up experiments show that the vaccinators’
ingroup favoritism is maintained over the medium term. In contrast, the non-vaccinators’ favorable
attitude toward their outgroup member is temporary, and they also gradually develop so-called ingroupfavoritism.
This manuscript is organized as follows: Section 2 reviews the literature on infectious diseases,
vaccination, and ingroup favoritism, and specifies the position and contribution of the current study.
Section 3 describes the design of the main experiment and Section 4 presents the experimental results.
Section 5 introduces the two follow-up experiments and presents the results. Section 6 conducts somerobustness checks. In Section 7, we develop the discussion and summarize our conclusions.

32. Literature Review and Positioning
The association between vaccination status and ingroup favoritism has been of general interest. Since
before the COVID-19 pandemic, Korn et al. (2020) conducted a vaccination-framed game experiment,
created hypothetical vaccinated and unvaccinated groups, used their responses in money allocations,and found the ingroup favoritism of the vaccinated group was stronger than that of the unvaccinated.
Weisel (2022) reported the similar trend as Korn et al. (2020), when using the real vaccination statusof whether people received the COVID-19 vaccine or not. Also, Jagodics and Szabo (2022) used ahypothetical third-party dictator game experiment and found strong ingroup favoritism among the
COVID-19 vaccinators. Even when using other indicators than ingroup favoritism, Bor et al. (2022)
found that the COVID-19 vaccinators’ discriminating attitude toward the non-vaccinators was stronger
than that of the non-vaccinators toward the vaccinators.
Economics has also been interested in the significant economic impacts of social identity throughingroup favoritism in the labor market (Calvo-Armengol and Jackson, 2004) and in healthcare market(Centola, 2011). We find economic studies of intergroup cooperation and hostility during the COVID19 pandemic. For example, Bartos et al. (2021) conducted a one-way allocation game experiment intheir financially incentivized online experiment and found emphasizing the threat of this pandemic
rises the hostile attitude toward foreigners in money allocations.
Compared to previous studies, the current study has two features that can add new findings to theliterature. The first feature is that we measure ingroup favoritism as well as its components, ingroupbias and outgroup bias. This feature enables us to provide one possible answer to the puzzle of why asmaller ingroup favoritism has been observed among unvaccinated individuals than among vaccinates
ones. This study presents as one answer the finding that some non-vaccinators allocate generously not
only to non-vaccinators (ingroup members) but also to vaccinators (outgroup members). Therefore,
their ingroup favoritism, which is captured by the difference between the allocation amount to theingroup members and that to the out-group members, is close to zero. This study further empirically
explores the question of why the part of non-vaccinators behaves so generously toward vaccinators.
The second feature is that we track how the ingroup favoritism of vaccinators and non-vaccinatorschanges over the medium term. Capturing the change is essential in particular for prolonged pandemics.
If the strong ingroup favoritism of the vaccinators is temporary, there may be little need for policy
intervention. If it is maintained over the medium to long term, then policy interventions should beconsidered to prevent its negative social consequences. While changes in ingroup favoritism over time
have been examined in controlled laboratory game experiments (Dorrough et al., 2015; Efferson et al.,2008), studies using the realistic context of a pandemic have only captured snapshots at a single point
in time. This study reveals that the ingroup favoritism of vaccinated individuals remains strong overthe medium term. In contrast, the unvaccinated individuals’ favorable attitude toward their outgroupmember is temporary, and they also develop so-called ingroup favoritism over the medium term.
4This study leverages the two features to make one meaningful academic contribution of providing
two possible answers to why the previous studies have observed smaller ingroup favoritism amongunvaccinated individuals. The first candidate is that in the early stages of vaccination promotion, nonvaccinators tend to allocate generously to vaccinated pairs, in addition to unvaccinated pairs, andprevious studies have overlooked this impact on the measurement of ingroup favoritism. The second
candidate, also related to the first, is that late in the pandemic non-vaccinators will show so-called
ingroup favoritism, but previous studies could have been conducted before this time.
3. Main Experiment3.1. Overview
From January 28 to February 1, 2022, we conducted a screening survey through MyVoice.com Ltd.,
which offers online surveys and experiments, and collected a sample of around 7,934 Japanese people
from its registered monitors to match the proportions of a national representative sample in terms ofsex, age, and residential area. In this survey, we set up questions to ascertain their age, sex, residentialarea, nationality, the COVID-19 vaccination status, and future vaccination intentions, in addition to
one hypothetical dictator game in the anonymous condition.
We define as “vaccinated individuals” Japanese people who have completed their first and second
COVID-19 vaccinations and have the intention to receive the additional vaccine soon. The number ofvaccinated individuals is 5,597. We also define as “unvaccinated individuals” those who have notreceived or intend to receive the vaccine. The number of unvaccinated individuals is 1,085.
From February 10 to 14, 2022, we sampled 800 vaccinated individuals and 800 unvaccinatedindividuals from the respondents of the screening survey and conducted the main online experiment,
including financially incentivized dictator games. To compare tendencies among the vaccinated people
with those of the unvaccinated, we ensured equal sample sizes for each group. Here, we adjusted thenumber of response requests so that the ratio of the number of respondents to the number of response
requests does not differ significantly between the vaccinated and unvaccinated samples.
In the dictator game part, we first present, as priming, the COVID-19-related questions, including
whether they have received the COVID-19 vaccine or not. We then present a dictator game five times(see Section 2.2). After the experiment, we set up questions to ascertain reasons for vaccination or
non-vaccination, the COVID-19-related behavioral characteristics and attitudes, and socio-economic
attributes. Concretely, the COVID-19-related questions include attitudes toward the COVID-19related policies (Balancing infectious disease control and socio-economic activities, vaccination
certification, and financial incentives for encouraging vaccination) and dates of the first and second
COVID-19 vaccination uptakes (Vaccinated sample, only).
We followed the pre-registered procedure and excluded from the analysis 22 individuals whose
vaccination status of the first and second doses in the screening survey does not match the status in5this online experiment. Since the duration between the screening survey and the experiment is around
two weeks, it is unlikely that “an unvaccinated individual,” who did not receive the vaccine at thetiming of the screening survey and had no vaccination intention, have started their vaccination duringthat short time. Furthermore, in reality, the vaccinated status cannot change to the unvaccinated status.
Consequently, we obtained valid responses from 796 vaccinated and 782 unvaccinated individuals.1
We need to note that when we conducted the screening survey and the main experiment, almost
all of the Japan’s first- and second-doses vaccination programs were completed. The first- and seconddoses vaccinations for children and teenagers (who are outside the scope of this study) still continued,
while the third-dose vaccination program started among the elderly. As of January 28, 2022, 74.8% of
the Japanese population completed the first and second doses of COVID-19 vaccination; among those
aged 65 and older, the percentage exceeded 92.0%.2 In addition, the nationwide outbreak of Omicronstrains was rapidly spreading, and many local governments requested corporations and individuals to
restrict their socioeconomic activities.
Before starting this study, we obtained approval from a research ethics committee at the Center
for Infectious Disease Education and Research, Osaka University (2022CRER0114). We also preregistered our experimental design and analysis plan to the AEA RCT Registry (Sasaki and Kurokawa,
2022a).
3.2. Dictator Games
Our dictator game experiments have three primary conditions as follows:◼

Anonymous condition: An allocator is not informed about a recipient’s vaccination status.

◼

Ingroup condition: An allocator is informed that a recipient belongs to their ingroup.

◼

Outgroup condition: An allocator is informed that a recipient belongs to their outgroup.

Here, among the vaccinated group, vaccinated people are ingroup members, and the unvaccinated areoutgroup members. In contrast, among the unvaccinated group, the vaccinated are outgroup members,
and the unvaccinated are ingroup members.
We further set two sub-conditions, Private and Public, respectively for the Ingroup and Outgroupconditions. In the Private condition, a recipient is not informed about the vaccination status of their

The female and regional proportions and average allocations in the hypothetical dictator game experiment among the796 vaccinators of the analysis sample are not different from those among the 5,597 vaccinations of the sample pool.
Although the mean age is around 2 years higher in the analysis sample, we determine that the analysis sample captures
the attitudes of a diverse group of vaccinations from all over Japan. The female and regional proportions, average age,
and average allocations among the 782 non-vaccinators of the analysis sample do not differ from those among the 1,085
non-vaccinators of the sample pool.
2 As of January 28, 2022, the percentage of those who received the first and second doses of COVID-19 vaccination
exceeded 70.0% in many countries of Argentina, Australia, Brazil, Canada, China, France, Germany, Italy, South Korea,
and the United Kingdom, in addition to Japan.
1

6allocator. In the Public condition, the allocator’s vaccination status is informed to a recipient whenproviding them with their money amount. Understanding whether the allocators’ ingroup favoritism
is stronger in the Private or Public condition is useful for predicting how it will appear in the real world
and for considering policy interventions to it.
Consequently, our dictator game experiments have five conditions, including (I: Anonymous) (II:Ingroup-Private) (III: Outgroup-Private) (IV: Ingroup-Public) and (V: Outgroup-Public). We show
screens for each condition in Appendix A. One subject participates in dictator games five times as anallocator. As shown in Table 1, we set randomly the order of the above conditions to create eightgroups in the vaccinated and unvaccinated samples, respectively (Table 1). After we first present the
dictator game in the anonymous condition (I) in all the eight groups, we randomly set the order of theprivate (II, III) and public (IV, V) conditions. Then, within each of the private and public conditions,
we randomly set the order of the ingroup conditions (II, IV) and outgroup conditions (III, V).
Consequently, each of the vaccinated and unvaccinated samples have eight groups. Furthermore, toconsider potential order effects caused by successive participation in the dictator games, we create acontrol group where we present the anonymous dictator game (I) five times. Totally, each of thevaccinated and unvaccinated samples have nine groups, including the control group.
[Table 1 is Here]In this dictator game experiment, participants are financially incentivized as follows: In eachgame, participants are given an endowment of 100 Japanese Yen in addition to the participation fee(90 JPY).3 They are asked to decide how much of the 100 JPY they give to a paired other person.
They are also informed that (1) the paired person is not participating in this same survey, (2) the
participant solely determines the allocation, (3) they are the only one who can give a share of themoney to the paired person, and (4) one of the five experimental responses will be randomly selected
to carry out the allocation after the experiment.
3.3. Definitions and Empirical Hypotheses
Using allocation amounts in the dictator games, ingroup favoritism, ingroup bias, and outgroup bias
are generally defined as follows: ingroup favoritism is a difference between an allocation amount toan ingroup pair and an allocation amount to an outgroup pair, ingroup bias is a difference between an
allocation amount to an ingroup pair and an allocation amount to an anonymous pair, and outgroupbias is a difference between an allocation amount to an outgroup pair and an allocation amount to an
anonymous pair.

3

In February, 2022, one U.S. dollar was on average 115.24 JPY.

7

Ingroup bias and outgroup bias are the background factors of ingroup favoritism. Specifically,
ingroup bias represents a favorable attitude toward ingroup members, while outgroup bias representsa hostile attitude toward outgroups. We use the ingroup and outgroup biases to examine which attitude
primarily shapes the ingroup favoritism.
Our empirical hypotheses for the ingroup favoritism and ingroup bias are that they are positive,
respectively. That is, the allocations to ingroup pairs are expected to be higher than those to anonymousor outgroup pairs. In contrast, it is hypothesized that the outgroup bias is negative. The allocations tooutgroup pairs are expected to be lower than those to anonymous pairs.
For the measurement, previous studies have employed various experimental games, including the
dictator game, ultimatum game, and trust game (Balliet et al. 2014; Lane et al. 2016). This study
employs the dictator game, considering its advantages of being non-interactive and thus easy to
implement into a nationwide online experiment. Balliet et al. (2014) note that the ingroup favoritism,
which is measured by the dictator game, is relatively small. If we find significant ingroup favoritism
even in such a setting, it will become strong evidence.
3.4. Analytical Procedure
Our experimental design enables us to conduct both between- and within-individual comparisons. The
between-analysis measures the biases, by focusing on responses in the second dictator game andcomparing them across groups. The within-analysis measures the biases for each individual in thetreatment groups, by using their responses from the first to fifth games and making within-individual
differences.
The advantage of the between-comparison is that random assignment allows us to estimate thebiases under the causal inference framework. However, since this comparison cannot directly calculate
out the biases for each individual, subsample size greatly impacts estimated results when examining
associations between the biases and some categories based on survey responses. The within-individualcomparison can compensate for this disadvantage. However, the biases measured by using withinindividual differences could be influenced by the order of the five conditions. By conducting both thebetween-analysis and the within-analysis, we can carefully consider the shortcomings of each analysis
and investigate the direction and degree of the biases While many past studies on ingroup favoritism
employed the between-analysis, an increasing number of studies have employed the within-analysis
approach in recent years (Kranton et al., 2021; Bartos et al., 2021).
Specifically, first, we conduct the between-analysis and test the existence of ingroup favoritism,
ingroup bias, and outgroup bias in each group of vaccinated and unvaccinated individuals. In addition,
we examine how the biases differ between vaccinated and unvaccinated individuals. For robustness
check, we control for covariates, address socially desirable bias and experimenter demand effect, andcompare the biases between the private and public conditions. We further conduct the within-analysis
8and check whether individuals’ average tendencies of ingroup favoritism, ingroup bias, and outgroupbias are consistent with those observed in the between-analysis.
Second, in the within-analysis, we check the relationship between the biases and the reasons for
vaccination or non-vaccination. By so doing, we enrich our discussion of exploring background factors
for the biases.
3.5. Descriptive Statistics and Balance Check
Since the between-analysis basically focuses on the allocated amount in the second dictator game, thepairs of (1) and (2), (3) and (4), (5) and (6), (7) and (8), (10) and (11), (12) and (13), (14) and (15),and (16) and (17) in Table 1 are identical in their condition, respectively. In the meta-analysis (Balliet
et al., 2014), d of the ingroup favoritism is 0.32. When we calculate the necessary sample size under
the condition of power=0.8 and alpha=0.05, it becomes 155 for each group. Therefore, we obtained asample size of approximately 160 for each pair.
[Table 2 is Here]For balance checks, we examine homogeneity between the control group and the above four pairsof treatment groups in each of the vaccinated and unvaccinated samples (Table 2). Using the criterionof a 5% pre-registered significance level, we test for differences across the groups in terms of age, sex,family composition, household annual income, educational years and baseline altruism (responses tothe hypothetical dictator game in the screening survey and the first dictator game with the anonymouscondition in the main experiment). Although we find a minor difference in marital status, the groups
are homogeneous in almost all respects.
However, we need to note that when using a 10% significance level, we find a difference in the
allocation amount of the first dictator game with the anonymous condition between the control andthe pair of (7) and (8), the public and outgroup condition. Because we find no significant difference
between the two in that of the hypothetical game in the screening survey, which is used in our stratified
randomization, such a difference appears possibly by chance. However, we consider the phenomenon,
use the difference between the allocation amounts of the second and first dictator games, and conduct
our analysis of between-group comparisons. This procedure allows us to directly consider potentiallyremaining differences across groups in baseline altruism and other unobserved characteristics. We
announced the possibility of employing this procedure in our pre-registration.
Finally, we use control groups of the vaccinated and unvaccinated samples and compare their
attributes. The vaccinated group shows higher average age, higher percentage of marriage, higher
annual household income, longer educational years, and a larger number of family members living
together compared to the unvaccinated group. Baseline altruism, which is captured by the allocations
9

in the screening survey and in the first dictator game, do not differ significantly between the two groups
after controlling for the influences of attribute variables. This does not support the possibility that thevaccinated participants are more altruistic than the unvaccinated ones.
4. Main Results
4.1. Ingroup Favoritism, Ingroup Bias, and Outgroup Bias
4.1.1. Between-Analysis[Figure 1 is Here]
We use the data of the main experiment, perform group comparison, and measure ingroup favoritism,
ingroup bias, and outgroup bias.4 On the left side of Figure 1, we present the result of estimating thevaccinated group’s biases by comparing the ingroup condition group (1, 2, 5, and 6 in Table 1),
outgroup condition group (3, 4, 7, and 8), and control group (9). The result shows that the vaccinatedgroup has the strong ingroup favoritism. The direction of their ingroup favoritism is positive as
hypothesized, and its size is 8.21 JPY (Cohen’s d = 0.46), which is statistically significant at the 1%
level. This effect size is large, considering that d of the ingroup favoritism in the meta-analysis is 0.32
(Balliet et al., 2014). The result also indicates that this ingroup favoritism may be generated by theoutgroup bias rather than the ingroup bias. The size of the ingroup bias is small (1.69 JPY, Cohen’s d= 0.11) and not statistically significant even at the 10% level, while the size of the outgroup bias isrelatively large (-6.53 JPY, Cohen’s d = 0.39) and statistically significant at the 1% level.
That is, the vaccinated people tend to increase their allocations to a paired person more when they
are paired with another vaccinated person (ingroup members), compared to when they are paired withan unvaccinated person (outgroup members). This tendency is occurring as a consequence of disliking
the outgroup members rather than as that of preferring the ingroup members.
On the right side of Figure 1, we present the result of estimating the unvaccinated group’s biases
by comparing the ingroup condition group (10, 11, 14, and15), outgroup condition group (12, 13, 16,
and 17), and control group (18). The result does not suggest that the unvaccinated group has the
ingroup favoritism, but support the hypothesis of the ingroup bias. Its size is 2.91 JPY (Cohen’s d =0.17) and statistically significant at the 5% level.
Despite accepting the hypothesis of the ingroup bias, we cannot observe the ingroup favoritism,
because the outgroup bias has the opposite direction of the hypothesis that it is negative. Its size is
4.04 JPY (Cohen’s d = 0.22), showing a positive sign. If we re-establish the opposite hypothesis that
The pairs of (1) and (2), (3) and (4), (5) and (6), (7) and (8), (10) and (11), (12) and (13), (14) and (15), and (16) and(17) are identical in their condition, respectively. In addition, the first analysis does not separate the Private and Publicconditions, we regard each pair of (1)(2)(5) and (6), (3)(4)(7) and (8), (10)(11)(14) and (15), and (12)(13)(16) and (17)
as identical in their condition.
4

10

the out-group bias is positive, it is statistically supported at the 5% significance level. This implies thatthe unvaccinated people increase the amount allocated to an out-group member, a vaccinated person,compared to an anonymous paired person.
That is, the unvaccinated people increase their allocations compared to an anonymous paired
person, both when they are paired with a similarly unvaccinated person, an ingroup member, and when
they are paired with a vaccinated person, an outgroup member. Thus, we do not find the ingroupfavoritism of favoring the unvaccinated pair compared to the vaccinated pair.
In Appendix B, we robustly observe the similar tendencies in the biases after directly controlling
for the effects of attribute variables in regression analysis. We still find the similar tendencies in thebiases, when adding variables, darkness and normative consciousness, that partially capture thesocially desirable bias and experimenter demand effect to the estimation and controlling their effects.5
Furthermore, we confirm that these biases do not differ between the private and public conditions. It
indicates that the vaccinators’ ingroup favoritism strongly exists regardless of whether information on
their vaccination status is made public or not.
4.1.2. Comparison between Vaccinators and Non-vaccinators
We statistically check how the ingroup favoritism, ingroup bias, and outgroup bias differ between thevaccinated and unvaccinated groups, by employing a Difference-in-Difference method and conducting
regression analysis (we present the details of our estimated models in Appendix C).
[Table 3 is Here]Column 1 in Table 3 shows the parameter for the difference in the ingroup favoritism between
the vaccinated and unvaccinated groups is 9.3543 (S.E. = 2.3543; p-value = 0.0000). This implies thatthe ingroup favoritism is stronger among the vaccinated group than the unvaccinated group, and thatthe vaccinated people are more likely to favor an ingroup member over an outgroup member. Column
2 does not reject the null hypothesis that the difference in the ingroup bias between the vaccinated andunvaccinated groups is zero, but shows that their difference in the outgroup bias is -10.4841 (S.E. =1.9313; p-value = 0.0000). We do not find differences between the vaccinated and unvaccinated groups
in their favorable attitudes toward ingroup members: however, their hostile attitudes toward outgroupmembers are much stronger among the vaccinated group than those in the unvaccinated.
This direct comparison also addresses the concern on the socially desirable bias and experimenter
demand effect. If the experimental design of this study induces the bias and effect and these primarily

We capture the participants’ darkness by using their match to the behavior, “If I can never be found by others, I will
do bad things (littering, violating parking laws, etc.).” In addition, we capture their normative consciousness by using
their match to the behavior, “I will never interrupt someone in line.”
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form ingroup favoritism, then the ingroup favoritism must be found equally both in the vaccinated andunvaccinated participants. Differences in the attributes between both groups may produce different
responses to the socially desirable bias and the experimenter demand effect: however, this possibility
is directly addressed by controlling for the attributes in the regression analysis. Thus, the heterogeneity
in the ingroup favoritism between both groups implies that the socially desirable bias and experimenter
demand effect is not crucial for our findings.
4.1.3. Within-Analysis
We investigate the existence of ingroup favoritism, ingroup bias, and outgroup bias also in the withinanalysis. In our experimental design, 1,261 participants assigned to the treatment groups join the
dictator games in all different conditions. Thus, we can calculate out ingroup favoritism, ingroup bias,
and outgroup bias for each participant, by making within-individual differences.
The vaccinated group’s ingroup favoritism, ingroup bias, and outgroup bias are 7.92 (0.67), 1.29
(0.62), and -6.63 (0.79), 6 respectively. Their tendencies are consistent with those in the between
analysis. The existence of the ingroup favoritism is statistically significantly supported, and it could
be shaped by the outgroup bias rather than the ingroup bias.
The unvaccinated group’s ingroup favoritism, ingroup bias, and outgroup bias are are 0.42 (0.63),
0.51 (0.66), and 0.10 (0.72), respectively. The tendencies are also consistent with those in the between
analysis, in that the existence of the ingroup favoritism is not supported and the outgroup bias is notobserved in the hypothesized direction. Also, in that the ingroup favoritism and outgroup bias are
greater in the hypothesized direction among the vaccinated group than among the unvaccinated group,
the results in the within-analysis are consistent with those in the between-analysis. However, on
average, we do not find from the within-analysis that the outgroup bias of the unvaccinated groupshows a significantly positive direction, which is the exact opposite of the hypothesis.
4.2. Heterogeneity by Reasons
Why do the vaccinated and unvaccinated people show the tendencies for the biases as observed? Here,we conduct the within-analysis and challenge the question by focusing on the participants’ reasons fortheir vaccination or non-vaccination. The advantage of the within-analysis is that this allows direct
examination of the relationship between the individuals’ biases and several responses captured in thesurvey, including the reasons.
In this experiment, we presented to the vaccinated group the question, “Please choose only one
reason that most closely matches the reason why you received the vaccine,” after the dictator games.
Among 636 vaccinated people (without the control group), 443 (69.7%) selected the first reason,

6

The values in parentheses are standard errors.

12

“Vaccination makes it less likely that I will develop the disease even if I am infected, and thus I canavoid serious illness or sequelae.” 72 (11.3%) selected the second reason, “Vaccination makes it lesslikely that I will develop the disease even if I am infected and thus contribute to the stability of thehealthcare delivery system and the maintenance of socioeconomic activities.” 107 (16.8%) selected
the third reason, “Vaccination prevents me from spreading the infection to those who are at high riskof serious illness or who are not vaccinated,” and only 14 (2.2%) selected the fourth reason, “others.”
Similarly, we presented to the unvaccinated group the question, “Please choose only one reason
that most closely matches the reason why you do not receive the vaccine.” Among 625 unvaccinatedpeople, 100 (16.0%) selected the first reason, “I would like to get vaccinated if I could, but I cannot
for health or other reasons.” 447 (71.5%) selected the second reason, “I do not want to get vaccinatedanyway in the first place,” and 78 (12.5%) selected the third reason, “others.”[Figure 2 is Here]Figure 2 shows the biases’ values by reasons. We find no substantial differences in the biases ofthe vaccinated group across their selected reasons. Their first reason would be relatively selfish, while
the second and third reasons would be altruistic. If the vaccinated people consider the positive
externality of vaccination and discriminate against the unvaccinated pairs, then ingroup favoritism
should be greater in the groups that chose the second and third reasons. However, the results in Figure
2 do not support the explanation, because the vaccinated people exhibit the strong ingroup favoritism,no matter whether they received the vaccine for selfish or altruistic reason.
In contrast, we find quite substantial differences in the direction and degree of the biases of theunvaccinated group across their reasons. Among the subgroup selecting the first reason, as in the
between-analysis, their outgroup bias has the opposite direction of the hypothesis, a positive sign.
Consequently, in this within-analysis, their ingroup favoritism also has the opposite direction of the
hypothesis, a negative sign. This implies that the unvaccinated people with this reason increase theallocation amount more when paired with a vaccinated person, their outgroup member, compared towhen paired with an unvaccinated person, their ingroup member. We may interpret this tendency as asign of their appreciation for those who received the vaccination on their behalf or as a sign of theirperception that the vaccinated people are the true ingroup members for them.
Among the other subgroup selecting the second reason, their ingroup favoritism has the
hypothesized direction, a positive sign. This result implies that, unlike the unvaccinated people
selecting the first reason, those who selected this second reason increase the allocation amount morewhen paired with an unvaccinated person compared to when paired with a vaccinated person. However,the level of their ingroup favoritism (1.54) is relatively small and around one-fifth of the average levelof that of the vaccinated group (7.92). In this way, our analysis by reasons allowed us to get partially13

closer to the background mechanism of the biases of the unvaccinated group.
5. Follow-up Experiments
The main experiment shows that the vaccinated participants exhibit strong ingroup favoritism and thisbias is shaped by outgroup bias, which is a hostile attitude toward non-vaccinators. Ingroup favoritism
is not observed among the unvaccinated participants, but rather they allocate more to their outgroupmembers, the vaccinators.
Is their attitude temporary or persistent? If the ingroup favoritism of vaccinators is maintained
over the medium term, we will need to consider policy interventions, while taking into account how
the ingroup favoritism appears as a real behavior in society. To answer this question, we conduct two
follow-up experiments in December 2022 and June 2023 to investigate how the tendencies of ingroupfavoritism, ingroup bias, and outgroup bias change over the medium term.
In December 2022, the infection stage more calmed down and socio-economic activities weremore active, compared to January-February 2022, when the main experiment had been conducted.
June 2023 was just after the Japanese government had downgraded the status of the COVID-19 underthe Infectious Diseases Act and thus the society entered an after-pandemic era.
5.1. First Follow-up Experiment
We conduct the first follow-up experiment from 16 to 21 December 2022 while partially adding new
sample (vaccinated sample: 606 follow-ups and 194 added ones, unvaccinated sample: 574 followups and 226 added ones). The definitions of the vaccinated and unvaccinated participants are the sameas in the main experiment. The vaccinated participants are defined as those who received two or more
vaccinations as of the February-2022 survey, while the unvaccinated participants are defined as those
who never received any vaccination. Of course, their vaccination status changes between the main
experiment and the first follow-up experiment as follows: The vaccinated participants consist of 53
people who were vaccinated twice, 275 people who were vaccinated three times, 385 people who were
vaccinated four times, and 87 people who were vaccinated five or more times as of December 2022.
Almost all of the unvaccinated participants (792 of 800) remain unvaccinated, and only 8 are newly
vaccinated. That is, the follow-up for the vaccinated sample can capture how their ingroup favoritism
changes with increasing heterogeneity in the number of their vaccinations. The follow-up for theunvaccinated sample can capture how their ingroup favoritism changes while almost all of themremain unvaccinated.
In the first follow-up experiment, the financially incentivized dictator games have five conditions,
including (I: Anonymous) (II: Ingroup-December2022) (III: Outgroup-December2022) (IV: IngroupFebruary2022) and (V: Outgroup-February2022). Considering the increase in vaccination coverage inJapan from February to December 2022, we change vaccinated pairs in II and III conditions to those14

who received three or more vaccinations as of December 2022. We keep the pairs’ information of IV
and V conditions the same in the main experiment. In IV and V conditions, vaccinated pairs are those
who received two or more vaccinations as of February 2022, and unvaccinated pairs are those who
never received a vaccination as of this timing. As shown in Table Appendix D1, we set the order ofthe five conditions so that we could prioritize capturing ingroup favoritism for pairs with updated
information. The random assignment of the first follow-up experiment is completely independent of
the random assignment of the main experiment. We conduct stratified randomization for the followup sample by the assigned group in the main experiment, while we conduct it for the added sample byage and baseline altruism. For the first follow-up experiment, we obtained approval from the research
ethics committee at the Center for Infectious Disease Education and Research, Osaka University,(2022CRER1115-2).
5.2. Second Follow-up Experiment
We follow the similar procedure and conduct the second follow-up experiment from 23 to 28 June
2023 (vaccinated sample: 665 follow-ups and 135 added ones, unvaccinated sample: 686 follow-ups
and 114 added ones). As of this timing, the vaccinated participants consist of 60 people who were
vaccinated twice, 190 people who were vaccinated three times, 328 people who were vaccinated fourtimes, and 222 people who were vaccinated five or more times. Still, almost all of the unvaccinatedparticipants (791 of 800) remain unvaccinated.
In the second follow-up experiment, the financially incentivized dictator games have three
conditions, including (I: Anonymous) (II: Ingroup-June2023) and (III: Outgroup- June2023). Since
there exist a significant number of people who remain vaccinated three times, in II and III conditions,vaccinated pairs are defined as those who received three or more vaccinations as of June 2023, andunvaccinated pairs are defined as those who never received a vaccination as of this timing. The groupassignment is shown in Table Appendix D2. Also for the second follow-up experiment, we obtained
approval from the research ethics committee at the Center for Infectious Disease Education andResearch, Osaka University (2023CRER0515).
5.3. Follow-up Results[Figure 3 is Here]The upper part of Figure 3 shows that the vaccinated participants continue to have a certain degree ofstrong ingroup favoritism. The size of the ingroup favoritism at the first follow-up is 7.66 JPY (Cohen’s
d=0.46), statistically significant at the 1% level. The ingroup bias remains small (1.41 JPY, Cohen’s
d=0.10) and is not statistically significant even at the 10% level. The outgroup bias is still large (-6.25
JPY, Cohen’s d=0.37) and statistically significant at the 1% level. The direction and size of the ingroup15

favoritism is almost identical to that at the time of the February’s main experiment.
At the second follow-up, ingroup favoritism is almost unchanged in size, at 6.20 JPY (Cohen’s
d=0.41, p<.01). The size of the outgroup bias, the primary source of the ingroup favoritism, is alsounchanged at -4.98 JPY (Cohen’s d=0.34, p<.01).
The bottom part of Figure 3 shows that the biases of the unvaccinated participants considerablychange over time. At the first follow-up, there is no significant difference in the ingroup bias, while
the favorable attitude toward the outgroup members disappears and is close to zero. Consequently,
ingroup favoritism begins to exhibit a positive sign as hypothesized, although it is not statisticallysignificant even at the 10% level.
At the second follow-up, the tendency is more pronounced, with the outgroup bias being negative(-2.41 JPY, Cohen’s d=0.16, p<.01) as hypothesized. Thus, the size of the ingroup favoritism is 5.83
JPY (Cohen’s d=0.36), which is closer to that of the vaccinated participants, and this is also statisticallysignificant at the 1% level.
The second follow-up experiment is conducted just after the Japanese government downgraded
the status of the COVID-19 under the Infectious Diseases Act. That is, the above results indicate thateven if the society enters an after-pandemic era, the vaccinators’ discriminating attitude toward nonvaccinators could strongly persist. On the contrary, the non-vaccinators’ favorable attitude toward theiroutgroup member is temporary, and they also gradually develop so-called ingroup favoritism.
6. Address of Concerns
In this section, we address two concerns. The first concern is that the outgroup bias of vaccinators may
be greater because many of them imagine that their anonymous pair is vaccinated. It might be possible
because the vaccinators are the majority in Japanese society. If so, the outgroup bias is the difference
between the allocation amounts to the unvaccinated pair and to the vaccinated pair and is almost equal
to the inverse of ingroup favoritism. The ingroup bias is the difference between the allocation amounts
to the vaccinated pair and to the similar vaccinator pair, which is almost zero.
As explained in Section 2.5, our analysis uses the within-individual difference in allocations forthe second and first dictator games as the outcome measure. This procedure enables us to control forthe effects of individuals’ time-invariant unobserved characteristics, which could include what they
imagine about their anonymous pair.
In addition to the above procedure, we conduct an additional validation. In the first follow-up
experiment, we asked the question, “To date, what percentage of the Japanese population do you thinkhas received three or more doses of the COVID-19 vaccine?” We divide the vaccinated group into two
subgroups: those who answered that 51% or more have received the vaccines and those who answeredthat 50% or less have done. The former subgroup is more likely to assume their anonymous pairs as
vaccinators than the latter. When estimating the ingroup and outgroup biases for each, we find that the16ingroup bias could be weakly dependent on the subjective proportion, while the outgroup bias is foundto be robustly large regardless of the proportion, thereby rejecting that the first concern determines our
results (See Table Appendix E1).
The second concern is that the results in Section 4.3 may be affected by samples that dropped out
or replenished during the follow-up process. For example, many of the non-vaccinators who increased
their allocations to vaccinated pairs in the main experiment did not participate in the follow-ups, which
could critically affect the change in attitudes of non-vaccinators.
To address this concern, we estimate ingroup favoritism, ingroup bias, and outgroup bias for eachexperiment, restricting the sample to the 493 vaccinators and 495 non-vaccinators who participated in
all three experiments. The estimated results are consistent with those reported in Section 4.3, rejecting
the possibility that the second concern determines our results (See Table Appendix E2).
7. Discussion and Conclusions
We conducted financially incentivized dictator game experiments and presented the following findings.
First, the COVID-19 vaccinated people have strong ingroup favoritism. This ingroup favoritism could
be generated mainly by the outgroup bias of decreasing the amount allocated to the unvaccinated pairs,
their outgroup members, not by the ingroup bias of increasing the amount allocated to the vaccinatedpairs, their ingroup members. The tendency of ingroup favoritism is consistent with the hypothesis
and results of previous studies, in which the ingroup favoritism was formed mainly by ingroup bias
rather than outgroup bias (Balliet et al. 2014). The current results may be unique to the context of the
COVID-19 vaccination.
In contrast to the vaccinated people, the unvaccinated do not show ingroup favoritism, on average.
Among them, the ingroup bias is positive and consistent with the hypothesis; however, the outgroupbias is also positive in the opposite direction of the hypothesis, and they tend to allocate more money
to the vaccinated pairs, their outgroup members, than to the anonymous pairs. Consequently, we do
not observe ingroup favoritism, which is captured by the difference in the amounts allocated to ingroupmembers and outgroup members. The previous studies have also reported smaller ingroup favoritism
among non-vaccinators compared to vaccinators (Jagodics and Szabo, 2022; Korn et al., 2020; Weisel,2022), but to our knowledge, few have empirically addressed the underlying mechanisms.
Second, we further take advantage of ensuring a sufficient sample size of the unvaccinated people
and examine how the biases vary depending on their reason for not receiving the COVID-19 vaccine,
to explore the background factors of their seemingly strange biases. The ingroup favoritism andoutgroup bias are observed in the opposite directions of the hypotheses among the unvaccinated who
selected the reason, “I would like to get vaccinated if I could, but I cannot for health or other reasons.”
On the contrary, among those who selected the reason, “I do not want to get vaccinated anyway in thefirst place,” the biases are observed as hypothesized, and they tend to allocate more money amount to17the unvaccinated pairs, their ingroup members, than to the vaccinated pairs, their outgroup members.
The attitudes of the former subgroup could be interpreted as a sign of their appreciation for people
who received the vaccine on their behalf or as a sign of their perception that the vaccinated people are
the true ingroup members for them.
Favorable attitudes toward outgroup members have been observed among racial, religious, andsexual minorities in previous studies (Axt et al., 2018). The unvaccinated people are also a minority
in Japan, and this study is common to the previous studies in this respect. The tendencies in the biasesof the unvaccinated people are heterogeneous depending on their non-vaccination reasons, with someexhibiting hostile attitudes toward outgroup members, the vaccinated. However, their level of hostility
is smaller than that of the vaccinated.
In Appendix F, respectively among the vaccinated and unvaccinated groups, we find the biases
are associated with their attitudes toward COVID-19-related policies, indicating that the biases would
have some degree of social influence in the real world. The vaccinated people with stronger ingroupfavoritism are more likely to agree with relaxing the behavioral restrictions for those whose
vaccination records can be verified and have received the vaccine at an earlier date. As with the second
finding, the associations for the unvaccinated group largely vary depending on their non-vaccination
reasons. Among the unvaccinated with the first reason, those who allocate more to the vaccinated are
more likely to agree with prioritizing infectious disease control over socio-economic activities,
relaxing the behavioral restrictions by vaccination records, and promoting vaccination by financial
rewards. In contrast, among the unvaccinated with the second reason, those who allocate more to theunvaccinated are more likely to disagree with prioritizing infectious disease control and the above
vaccination promotion measure.
Third, we conduct two follow-up experiments in December 2022 and June 2023 to track how the
ingroup favoritism of the vaccinated and unvaccinated people changes over the medium term. If the
ingroup favoritism of the vaccinators is temporary, there may be little need for policy intervention. If
it is maintained over the medium term, then policy interventions should be considered to prevent its
negative social consequences. The second follow-up experiment is conducted just after the Japanese
government downgraded the status of the COVID-19 under the Infectious Diseases Act. Our findings
indicate that even if the society enters an after-pandemic era, the vaccinators’ ingroup favoritism could
strongly persist. In contrast, the non-vaccinators’ favorable attitude toward their outgroup member is
temporary, and they gradually exhibit so-called ingroup favoritism. It should be noted that the ingroupfavoritism of the vaccinators is consistently shaped by the outgroup bias, while that of the nonvaccinators is largely dependent on the ingroup bias, and in this respect they differ.
The primary academic contribution of this study is that it provides two possible answers to why
existing studies have observed smaller ingroup favoritism among unvaccinated individuals. The firstcandidate is that in the early stages of vaccination promotion, non-vaccinators tend to allocate
18generously to vaccinators, and previous studies have overlooked this effect on the measurement ofingroup favoritism. The second candidate, also related to the first, is that late in the pandemic nonvaccinators will show so-called ingroup favoritism, but previous studies were conducted before thistime.
This study also has policy implications when we look at the fact that the ingroup favoritism of
the vaccinated people, in contrast to the unvaccinated, is characterized by their hostile attitude toward
the outgroup members. This factor poses a risk to the stable management of society during a pandemic.
Some vaccinated people may feel as if almost all the unvaccinated people have a hostile attitude towardvaccinators, because they see lots of unvaccinated people supporting and spreading fake news about
the COVID-19 vaccine through social media, etc. However, this belief is not supported as far as we
investigate by using ingroup favoritism, ingroup bias, and outgroup bias. On average, the vaccinatedpeople behave more discriminately toward their outgroup members. In many countries and regions
where the vaccinated people are the majority (e.g., Argentina, Australia, Brazil, Canada, China, France,
Germany, Italy, Japan, South Korea, and the United Kingdom), it is essential for them to realize theirown hostile attitudes and consider diverse reasons and behaviors of the unvaccinated people to build
cooperative relationships between the two groups smoothly.

19

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Figure 1. Ingroup Favoritism, Ingroup Bias, and Outgroup Bias

Notes: Vertical lines in the graph represent 95% confidence intervals. On the left side of Figure 1, we present the resultof estimating the vaccinated group’s biases by comparing the ingroup condition group, outgroup condition group, andcontrol group. The result shows that the vaccinated group has the ingroup favoritism. The direction of their ingroupfavoritism is positive as hypothesized, and its size is 8.21 JPY (Cohen’s d = 0.46), which is statistically significant at
the 1% level. This result also indicates that this ingroup favoritism may be generated by the outgroup bias rather than
the ingroup bias. The size of the ingroup bias is small (1.69 JPY, Cohen’s d = 0.11) and not statistically significant evenat the 10% level, while the size of the outgroup bias is relatively large (-6.53 JPY, Cohen’s d = 0.39) and statisticallysignificant at the 1% level. Similarly, on the right side of Figure 1, we present the result of estimating the unvaccinatedgroup’s biases by comparing the ingroup condition group, outgroup condition group, and control group. The result does
not suggest that the unvaccinated group has the ingroup favoritism, but support the hypothesis of the ingroup bias. Its
size is 2.91 JPY (Cohen’s d = 0.17) and statistically significant at the 5% level. Despite accepting the hypothesis of the
ingroup bias, we cannot observe the ingroup favoritism, because the outgroup bias has the opposite direction of the
hypothesis that it is negative. Its size is 4.04 JPY (Cohen’s d = 0.22), showing a positive sign. If we re-establish the
opposite hypothesis that the out-group bias is positive, it is statistically supported at the 5% significance level.

22Figure 2. Heterogeneity by Reasons

Note: Vertical lines in the graph represent 95% confidence intervals.

23

Figure 3. Medium-term Transitions in Ingroup Favoritism, Ingroup Bias, and Outgroup Bias

Note: Vertical lines in the graph represent 95% confidence intervals.

24

Table 1. Group Assignment

Vaccinated sample (N=800)
(1)

(2)

(3)

N=160

(4)

(5)

N=160

(6)

(7)

N=160

(8)
N=160

(9)
N=160

Order

N=80

N=80

N=80

N=80

N=80

N=80

N=80

N=80

1

I

I

I

I

I

I

I

I

I

2

II

II

III

III

IV

IV

V

V

I

3

III

III

II

II

V

V

IV

IV

I

4

IV

V

IV

V

II

III

II

III

I

5

V

IV

V

IV

III

II

III

II

I

(16)

(17)

(18)

Unvaccinated sample (N=800)
(10)

(11)

(12)

N=160

(13)

(14)

N=160

(15)
N=160

N=160

N=160

Order

N=80

N=80

N=80

N=80

N=80

N=80

N=80

N=80

1

I

I

I

I

I

I

I

I

I

2

II

II

III

III

IV

IV

V

V

I

3

III

III

II

II

V

V

IV

IV

I

4

IV

V

IV

V

II

III

II

III

I

5

V

IV

V

IV

III

II

III

II

I

Notes: We conduct dictator games in the following five conditions: I. Anonymous (A recipient is anonymous for an
allocator. The allocator is also anonymous for the recipient), II. Ingroup-Private (An allocator is informed that arecipient belongs to their ingroup. The allocator is anonymous for the recipient), III. Outgroup-Private (An allocator isinformed that a recipient belongs to their outgroup. The allocator is anonymous for the recipient), IV. Ingroup-Public(An allocator is informed that a recipient belongs to their ingroup. The recipient is notified of the vaccination status of
the allocator), and V. Outgroup-Public (An allocator is informed that a recipient belongs to their outgroup. The recipient
is notified of the vaccination status of the allocator).

25

Table 2. Descriptive Statistics

Private & Ingroup

Private & Outgroup

Public & Ingroup

Public & Outgroup

II

III

IV

V

I

（１）（２）（３）（４）（５）（６）（７）（８）（９）

Vaccinated sample

N=160

N=796

N=159

N=160

Control (Anonymous)

N=157

N=160

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

Female dummy

0.51

0.50

0.49

0.50

0.46

0.50

0.48

0.50

0.43

S.D.
0.50

Age

49.34

11.97

49.32

12.75

51.19

11.85

50.01

12.55

50.88

11.47

Married dummy

0.65

0.48

0.61

0.49

0.62

0.49

0.70

0.46

0.58

0.49

Number of family members

1.84

1.30

2.00

1.41

1.85

1.33

1.98

1.44

2.02

1.49

Household income

671.02424.30686.25425.91629.81

412.12627.50320.06631.03408.34

No income information dummy

0.14

0.35

0.14

0.35

0.12

0.32

0.13

0.34

0.14

0.35

Educational years

14.50

1.98

14.55

2.17

14.59

1.90

14.93

2.09

14.75

1.80

Allocation in the screening survey

40.56

34.66

37.80

35.23

43.50

37.85

40.38

34.53

39.00

35.08

Allocation in the first dictator game

21.50

28.42

22.77

25.78

23.69

26.53

26.88

26.53

21.56

26.24

Private & Ingroup

Private & Outgroup

Public & Ingroup

Public & Outgroup

II

III

IV

V

I

（１０）（１１）（１２）（１３）（１４）（１５）（１６）（１７）（１８）

Unvaccinated sample

N=152

N=782

N=157

N=158

Control (Anonymous)

N=158

N=157

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

Female dummy

0.49

0.50

0.48

0.50

0.49

0.50

0.49

0.50

0.50

S.D.
0.50

Age

43.32

12.61

42.71

11.84

44.16

13.28

44.54

12.08

43.26

11.99

Married dummy

0.39

0.49

0.33

0.47

0.40

0.49

0.44

0.50

0.39

0.49

Number of family members

1.73

1.27

1.73

1.42

1.74

1.29

1.67

1.40

1.94

1.46

Household income

525.86292.41504.54335.03487.74329.18534.83357.50509.17353.83

No income information dummy

0.18

0.39

0.22

0.41

0.22

0.41

0.23

0.42

0.23

0.42

Educational years

14.12

2.19

14.11

2.05

13.64

2.18

14.20

2.49

14.01

2.18

Allocation in the screening survey

36.51

35.33

36.75

36.54

38.23

34.63

36.58

37.40

34.78

33.18

Allocation in the first dictator game

26.84

31.04

21.02

26.68

26.77

30.33

23.73

29.07

24.33

27.83

Note: Some participants did not answered annual household income. We imputed the average amout of the income for such respondents while considering that they did not answer it byusing the variable of no income information.

Notes: Some participants did not answer annual household income. We imputed the average amount of the income forsuch participants, at the same time considering that they did not answer it by using the variable of no income information.
For balance checks, we examine homogeneity between the control group and the above four pairs of treatment groups
in each of the vaccinated and unvaccinated samples. Using the criterion of a 5% pre-registered significance level, we
test for differences across the groups in terms of age, sex, family composition, household annual income, educational
years and baseline altruism (responses to the hypothetical dictator game in the screening survey and the first dictator
game with the anonymous condition in the main experiment). Although we find a minor difference in marital status,
the groups are homogeneous in almost all respects.

26

Table 3. Comparison between Vaccinated and Unvaccinated Groups

Tests for

Vaccinated sample

Ingroup condition

Vaccinated sample×Ingroup condition

(1)

(2)

Ingroup Favoritism

Ingroup Bias andOutgroup Bias

-7.7650***

2.5386(0.7456)(2.1227)-1.15132.8943(1.1289)(1.5681)9.3543***-1.1447(1.2966)(2.8542)

Outgroup condition

4.0277***

Vaccinated sample×Outgroup condition-10.4841***(1.0609)(2.0010)

Covariates

YES

YES

10.3841**4.5465(3.6959)(3.8168)

Number of observations

1,261

1,578

R-squared

0.046

0.040

Constant term

Notes : Cluster robust standard errors at region level in parentheses; *** p<0.01, ** p<0.05, * p<0.1

Notes: Cluster robust standard errors at region level in parentheses. *** p<0.01, ** p<0.05, and * p<0.10. Covariates
include female dummy, age, married dummy, number of family members, household income, no income information
dummy, and educational years. Column 1 shows that the parameter for the difference in the ingroup favoritism between
the vaccinated and unvaccinated groups is 9.3543 (S.E. = 2.3543; p-value = 0.0000). This implies that the ingroupfavoritism is stronger among the vaccinated group than the unvaccinated group, and that the vaccinated people are more
likely to favor their ingroup member over the outgroup member. Column 2 does not reject the null hypothesis that thedifference in the ingroup bias between the vaccinated and unvaccinated groups is zero, but shows that their differencein the outgroup bias is -10.4841 (S.E. = 1.9313; p-value = 0.0000). We do not find differences between the vaccinatedand unvaccinated groups in their favorable attitudes toward ingroup members, while their hostile attitudes toward
outgroup members are much stronger among the vaccinated group than those in the unvaccinated group.

27

Appendix A. Experimental Screens
Appendix Figure 1. Screens for Vaccinated Sample
I. Anonymous (for Vaccinated Sample)

You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“This money allocation was decided by an anonymous Japanese monitor.”
In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Anonymous you: 0 JPY, An anonymous pair: 100 JPY⚫ Anonymous you: 10 JPY, An anonymous pair: 90 JPY⚫ Anonymous you: 20 JPY, An anonymous pair: 80 JPY⚫ Anonymous you: 30 JPY, An anonymous pair: 70 JPY⚫ Anonymous you: 40 JPY, An anonymous pair: 60 JPY⚫ Anonymous you: 50 JPY, An anonymous pair: 50 JPY⚫ Anonymous you: 60 JPY, An anonymous pair: 40 JPY⚫ Anonymous you: 70 JPY, An anonymous pair: 30 JPY⚫ Anonymous you: 80 JPY, An anonymous pair: 20 JPY⚫ Anonymous you: 90 JPY, An anonymous pair: 10 JPY⚫ Anonymous you: 100 JPY, An anonymous pair: 0 JPY

28

II. Ingroup-Private (for Vaccinated Sample)

Again, please read the following instructions carefully and determine your choice.
You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
⚫ He/she has already completed the first and second COVID-19 vaccinations. He/she answered
to receive additional vaccinations in the future.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“This money allocation was decided by an anonymous Japanese monitor.”

In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Anonymous you: 0 JPY, A vaccinated pair: 100 JPY⚫ Anonymous you: 10 JPY, A vaccinated pair: 90 JPY⚫ Anonymous you: 20 JPY, A vaccinated pair: 80 JPY⚫ Anonymous you: 30 JPY, A vaccinated pair: 70 JPY⚫ Anonymous you: 40 JPY, A vaccinated pair: 60 JPY⚫ Anonymous you: 50 JPY, A vaccinated pair: 50 JPY⚫ Anonymous you: 60 JPY, A vaccinated pair: 40 JPY⚫ Anonymous you: 70 JPY, A vaccinated pair: 30 JPY⚫ Anonymous you: 80 JPY, A vaccinated pair: 20 JPY⚫ Anonymous you: 90 JPY, A vaccinated pair: 10 JPY⚫ Anonymous you: 100 JPY, A vaccinated pair: 0 JPY

29

III. Outgroup-Private (for Vaccinated Sample)

Again, please read the following instructions carefully and determine your choice.
You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
⚫ He/she has never received the COVID-19 vaccination. He/she answered not to receive any
future vaccinations.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“This money allocation was decided by an anonymous Japanese monitor.”

In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Anonymous you: 0 JPY, An unvaccinated pair: 100 JPY⚫ Anonymous you: 10 JPY, An unvaccinated pair: 90 JPY⚫ Anonymous you: 20 JPY, An unvaccinated pair: 80 JPY⚫ Anonymous you: 30 JPY, An unvaccinated pair: 70 JPY⚫ Anonymous you: 40 JPY, An unvaccinated pair: 60 JPY⚫ Anonymous you: 50 JPY, An unvaccinated pair: 50 JPY⚫ Anonymous you: 60 JPY, An unvaccinated pair: 40 JPY⚫ Anonymous you: 70 JPY, An unvaccinated pair: 30 JPY⚫ Anonymous you: 80 JPY, An unvaccinated pair: 20 JPY⚫ Anonymous you: 90 JPY An unvaccinated pair: 10 JPY⚫ Anonymous you: 100 JPY, An unvaccinated pair: 0 JPY

30

IV. Ingroup-Public (for Vaccinated Sample)

Again, please read the following instructions carefully and determine your choice.
You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
⚫ He/she has already completed the first and second COVID-19 vaccinations. He/she answered
to receive additional vaccinations in the future.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“The money allocation was decided by a Japanese monitor who, like you, have completed thefirst and second COVID-19 vaccinations. He/she answered to receive additional vaccinations in
the future.”
In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Vaccinated you: 0 JPY, A vaccinated pair: 100 JPY⚫ Vaccinated you: 10 JPY, A vaccinated pair: 90 JPY⚫ Vaccinated you: 20 JPY, A vaccinated pair: 80 JPY⚫ Vaccinated you: 30 JPY, A vaccinated pair: 70 JPY⚫ Vaccinated you: 40 JPY, A vaccinated pair: 60 JPY⚫ Vaccinated you: 50 JPY, A vaccinated pair: 50 JPY⚫ Vaccinated you: 60 JPY, A vaccinated pair: 40 JPY⚫ Vaccinated you: 70 JPY, A vaccinated pair: 30 JPY⚫ Vaccinated you: 80 JPY, A vaccinated pair: 20 JPY⚫ Vaccinated you: 90 JPY, A vaccinated pair: 10 JPY⚫ Vaccinated you: 100 JPY, A vaccinated pair: 0 JPY

31

V. Outgroup-Public (for Vaccinated Sample)

Again, please read the following instructions carefully and determine your choice.
You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
⚫ He/she has never received the COVID-19 vaccination. He/she answered not to receive any
future vaccinations.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“The money allocation was decided by a Japanese monitor who, unlike you, have completed thefirst and second COVID-19 vaccinations. He/she answered to receive additional vaccinations in
the future.”
In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.

*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Vaccinated you: 0 JPY, An unvaccinated pair: 100 JPY⚫ Vaccinated you: 10 JPY, An unvaccinated pair: 90 JPY⚫ Vaccinated you: 20 JPY, An unvaccinated pair: 80 JPY⚫ Vaccinated you: 30 JPY, An unvaccinated pair: 70 JPY⚫ Vaccinated you: 40 JPY, An unvaccinated pair: 60 JPY⚫ Vaccinated you: 50 JPY, An unvaccinated pair: 50 JPY⚫ Vaccinated you: 60 JPY, An unvaccinated pair: 40 JPY⚫ Vaccinated you: 70 JPY, An unvaccinated pair: 30 JPY⚫ Vaccinated you: 80 JPY, An unvaccinated pair: 20 JPY⚫ Vaccinated you: 90 JPY, An unvaccinated pair: 10 JPY⚫ Vaccinated you: 100 JPY, An unvaccinated pair: 0 JPY

32

Appendix Figure 2. Screens for Unvaccinated Sample
I. Anonymous (for Unvaccinated Sample)

You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“This money allocation was decided by an anonymous Japanese monitor.”
In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Anonymous you: 0 JPY, An anonymous pair: 100 JPY⚫ Anonymous you: 10 JPY, An anonymous pair: 90 JPY⚫ Anonymous you: 20 JPY, An anonymous pair: 80 JPY⚫ Anonymous you: 30 JPY, An anonymous pair: 70 JPY⚫ Anonymous you: 40 JPY, An anonymous pair: 60 JPY⚫ Anonymous you: 50 JPY, An anonymous pair: 50 JPY⚫ Anonymous you: 60 JPY, An anonymous pair: 40 JPY⚫ Anonymous you: 70 JPY, An anonymous pair: 30 JPY⚫ Anonymous you: 80 JPY, An anonymous pair: 20 JPY⚫ Anonymous you: 90 JPY, An anonymous pair: 10 JPY⚫ Anonymous you: 100 JPY, An anonymous pair: 0 JPY

33

II. Ingroup-Private (for Unvaccinated Sample)

Again, please read the following instructions carefully and determine your choice.
You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
⚫ He/she has never received the COVID-19 vaccination. He/she answered not to receive any
future vaccinations.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“This money allocation was decided by an anonymous Japanese monitor.”

In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Anonymous you: 0 JPY, An unvaccinated pair: 100 JPY⚫ Anonymous you: 10 JPY, An unvaccinated pair: 90 JPY⚫ Anonymous you: 20 JPY, An unvaccinated pair: 80 JPY⚫ Anonymous you: 30 JPY, An unvaccinated pair: 70 JPY⚫ Anonymous you: 40 JPY, An unvaccinated pair: 60 JPY⚫ Anonymous you: 50 JPY, An unvaccinated pair: 50 JPY⚫ Anonymous you: 60 JPY, An unvaccinated pair: 40 JPY⚫ Anonymous you: 70 JPY, An unvaccinated pair: 30 JPY⚫ Anonymous you: 80 JPY, An unvaccinated pair: 20 JPY⚫ Anonymous you: 90 JPY An unvaccinated pair: 10 JPY⚫ Anonymous you: 100 JPY, An unvaccinated pair: 0 JPY

34

III. Outgroup-Private (for Unvaccinated Sample)

Again, please read the following instructions carefully and determine your choice.
You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
⚫ He/she has already completed the first and second COVID-19 vaccinations. He/she answered
to receive additional vaccinations in the future.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“This money allocation was decided by an anonymous Japanese monitor.”

In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Anonymous you: 0 JPY, A vaccinated pair: 100 JPY⚫ Anonymous you: 10 JPY, A vaccinated pair: 90 JPY⚫ Anonymous you: 20 JPY, A vaccinated pair: 80 JPY⚫ Anonymous you: 30 JPY, A vaccinated pair: 70 JPY⚫ Anonymous you: 40 JPY, A vaccinated pair: 60 JPY⚫ Anonymous you: 50 JPY, A vaccinated pair: 50 JPY⚫ Anonymous you: 60 JPY, A vaccinated pair: 40 JPY⚫ Anonymous you: 70 JPY, A vaccinated pair: 30 JPY⚫ Anonymous you: 80 JPY, A vaccinated pair: 20 JPY⚫ Anonymous you: 90 JPY, A vaccinated pair: 10 JPY⚫ Anonymous you: 100 JPY, A vaccinated pair: 0 JPY

35

IV. Ingroup-Public (for Unvaccinated Sample)

Again, please read the following instructions carefully and determine your choice.
You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).
You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
⚫ He/she has never received the COVID-19 vaccination. He/she answered not to receive any
future vaccinations.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“The money allocation was decided by a Japanese monitor who, like you, have never received the
COVID-19 vaccination. He/she answered not to receive any future vaccinations.”
In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Vaccinated you: 0 JPY, An unvaccinated pair: 100 JPY⚫ Vaccinated you: 10 JPY, An unvaccinated pair: 90 JPY⚫ Vaccinated you: 20 JPY, An unvaccinated pair: 80 JPY⚫ Vaccinated you: 30 JPY, An unvaccinated pair: 70 JPY⚫ Vaccinated you: 40 JPY, An unvaccinated pair: 60 JPY⚫ Vaccinated you: 50 JPY, An unvaccinated pair: 50 JPY⚫ Vaccinated you: 60 JPY, An unvaccinated pair: 40 JPY⚫ Vaccinated you: 70 JPY, An unvaccinated pair: 30 JPY⚫ Vaccinated you: 80 JPY, An unvaccinated pair: 20 JPY⚫ Vaccinated you: 90 JPY, An unvaccinated pair: 10 JPY⚫ Vaccinated you: 100 JPY, An unvaccinated pair: 0 JPY

36

V. Outgroup-Public (for Unvaccinated Sample)

Again, please read the following instructions carefully and determine your choice.
You have now received another reward of 100 JPY, in addition to the survey reward (90 JPY).

You can share some of that 100 JPY with someone who has the following characteristics:⚫ He/she is another Japanese monitor registered with the same company.
⚫ He/she is not participating in this experiment.
⚫ He/she has already completed the first and second COVID-19 vaccinations. He/she answered
to receive additional vaccinations in the future.
You are solely responsible for deciding which allocation of 100 JPY will be divided. Also, you are theonly one who can share the money with him/her, no one else.
When informing him/her of the allocation result, the following message will be attached.
“The money allocation was decided by a Japanese monitor who, unlike you, have never received
the COVID-19 vaccination. He/she answered not to receive any future vaccinations.”
In this situation, how will you divide the 100 JPY? Choose only one allocation that you prefer the most.
*If, as a result of a random drawing, this response is selected out of the five responses, the allocation iscarried out according to the procedure described above.
*You cannot change your choice, so please make your choice after careful consideration.

⚫ Vaccinated you: 0 JPY, A vaccinated pair: 100 JPY⚫ Vaccinated you: 10 JPY, A vaccinated pair: 90 JPY⚫ Vaccinated you: 20 JPY, A vaccinated pair: 80 JPY⚫ Vaccinated you: 30 JPY, A vaccinated pair: 70 JPY⚫ Vaccinated you: 40 JPY, A vaccinated pair: 60 JPY⚫ Vaccinated you: 50 JPY, A vaccinated pair: 50 JPY⚫ Vaccinated you: 60 JPY, A vaccinated pair: 40 JPY⚫ Vaccinated you: 70 JPY, A vaccinated pair: 30 JPY⚫ Vaccinated you: 80 JPY, A vaccinated pair: 20 JPY⚫ Vaccinated you: 90 JPY, A vaccinated pair: 10 JPY⚫ Vaccinated you: 100 JPY, A vaccinated pair: 0 JPY

37

Appendix B. Robustness Check
Table Appendix B1. With Covariates
(1)

(2)

(3)

(4)

(5)

Vaccinated sample
Tests forIngroup condition

Ingroup Favoritism

Age

Married dummy

Number of family members

Household income

No income information dummy

Educational years(8)

Ingroup Bias andOutgroup Bias

Ingroup Favoritism

Ingroup Bias andOutgroup Bias

8.2749***1.7803

1.8493-1.1711-1.1293

2.8985*2.7039(1.4229)(1.4215)(1.5493)(1.5570)(1.5557)(1.5513)(1.6570)(1.6603)-6.4530***-6.4240***

4.0232**

3.7648**(1.3583)(1.3605)(1.5077)(1.4978)-1.9075-1.4995-1.6687-1.3467-3.1992**-3.1197*-2.6782**-2.4288*(1.7003)(1.6307)(1.3905)(1.3079)(1.5675)(1.6312)(1.2656)(1.3343)-0.0970-0.0817-0.0810-0.0720-0.0800-0.0626-0.0558-0.0410(0.0647)(0.0632)(0.0539)(0.0529)(0.0717)(0.0689)(0.0622)(0.0592)-1.0811-1.2499-0.8543-0.9469

0.68520.55920.19330.0665(1.4539)(1.4460)(1.3212)(1.3333)(1.8156)(1.7860)(1.5103)(1.4983)-0.1280-0.0779

0.0201

0.0538-0.1593-0.1465-0.2086-0.1866(0.4914)(0.4776)(0.4459)(0.4396)(0.6392)(0.6579)(0.4925)(0.5037)-0.0004-0.0004

0.00000.0000-0.0016-0.0018-0.0004-0.0005(0.0018)(0.0018)(0.0015)(0.0015)(0.0034)(0.0033)(0.0024)(0.0024)-2.4492-2.2934-1.7459-1.6735

2.67032.33172.8361**2.5888*(2.5115)(2.5712)(2.2149)(2.2268)(1.6261)(1.6898)(1.3522)(1.4034)0.08840.07360.0001-0.0036-0.3760-0.4664-0.2927-0.3414(0.3937)(0.3943)(0.2961)(0.2969)(0.3153)(0.3165)(0.2585)(0.2543)

Darkness

Normative consciousness

Constant term

(7)8.2258***Outgroup condition

Female dummy(6)

Unvaccinated sample

0.81730.6124-0.7517-0.3062(0.5416)(0.4967)(0.8788)(0.7550)-0.4779-0.2982-1.6244*-1.4196**(0.4865)(0.4426)(0.8404)(0.6722)-0.9912-1.6165

5.02514.269112.3075**

21.7177**

5.3872

12.4898*(5.5417)(5.8127)(4.6589)(4.9188)(5.6700)(8.6411)(4.7253)(6.9922)

Number of observations

636

636

796

796

625

625

782

782

R-squared

0.065

0.069

0.058

0.060

0.015

0.023

0.018

0.024

Notes : Cluster robust standard errors at region level in parentheses; *** p<0.01, ** p<0.05, * p<0.1

38

Table Appendix B2. Comparison between Private and Public Conditions

3V

4V

5V

3U

Vaccinated sample
Tests for Ingroup Favoritism

Public & Ingroup condition

Ingroup Bias

Public condition

5U

Unvaccinated sample
Outgroup Bias

Ingroup Favoritism

Ingroup Bias

0.6281-0.20311.13600.3833(3.9666)(1.4256)(3.5341)(1.4132)

Public & Outgroup condition

Ingroup condition

4U

Outgroup Bias

-0.3857-1.1570(3.2126)(2.7834)

7.9155***-1.7407(1.7245)(1.7900)-0.5397-1.0713(3.1622)(2.7790)

Covariates

YES

YES

YES

YES

YES

YES

Constant term

-0.8084

6.3141-1.6998

12.7715**

4.056820.0942(5.3216)(8.2498)(5.1400)(4.2562)(9.7910)(12.4886)

Number of observations

636

320

316

625

310

315

R-squared

0.066

0.017

0.022

0.015

0.008

0.040

Notes : Cluster robust standard errors at region level in parentheses; *** p<0.01, ** p<0.05, * p<0.1
Notes: Cluster robust standard errors at region level in parentheses. *** p<0.01, ** p<0.05, and * p<0.10. The estimated results do not support the existence of differences between theprivate and public conditions for all of the ingroup favoritism, ingroup bias, and outgroup bias in both the vaccinated and unvaccinated groups.

39Appendix C. Estimation Models
Appendix C.1. Comparison between Private and Public Conditions
The estimated equation for ingroup favoritism (1) is as follow. This estimation uses the samples
excluding the control groups. The baseline is the Outgroup condition, and the parameter of interest is𝛽3
𝑌 = 𝛽0 + 𝛽1 × 𝐼𝑛𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 + 𝛽2 × 𝑃𝑢𝑏𝑙𝑖𝑐 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 + 𝛽3 × 𝑃𝑢𝑏𝑙𝑖𝑐 𝐼𝑛𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛+ 𝐶𝑜𝑛𝑡𝑟𝑜𝑙𝑠 (1)The estimated equation for ingroup bias is as follow (2). This estimation uses the samples in the
Ingroup conditions among the vaccinated and unvaccinated groups, respectively. The baseline is thePrivate-Ingroup condition.
𝑌 = 𝛽0 + 𝛽1 × 𝑃𝑢𝑏𝑙𝑖𝑐 𝐼𝑛𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 + 𝐶𝑜𝑛𝑡𝑟𝑜𝑙𝑠 (2)The estimated equation for outgroup bias (3) is as follow. This estimation uses the samples in the
Outgroup conditions among the vaccinated and unvaccinated groups, respectively. The baseline is thePrivate-Outgroup condition.
𝑌 = 𝛽0 + 𝛽1 × 𝑃𝑢𝑏𝑙𝑖𝑐 𝑂𝑢𝑡𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 + 𝐶𝑜𝑛𝑡𝑟𝑜𝑙𝑠 (3)Appendix C.2. Comparison between Vaccinated and Unvaccinated Groups
The estimation for ingroup favoritism uses the samples excluding the two control groups, and theestimated equation (4) is as follows:
𝑌 = 𝛽0 + 𝛽1 × 𝑉𝑎𝑐𝑐𝑖𝑛𝑎𝑡𝑒𝑑 𝑔𝑟𝑜𝑢𝑝 + 𝛽2 × 𝐼𝑛𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛+ 𝛽3 × 𝑉𝑎𝑐𝑐𝑖𝑛𝑎𝑡𝑒𝑑 𝑔𝑟𝑜𝑢𝑝 × 𝐼𝑛𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 + 𝐶𝑜𝑛𝑡𝑟𝑜𝑙𝑠 (4)The baseline is the Outgroup condition in the unvaccinated group. 𝛽3 captures how different the
ingroup favoritism in the vaccinated group is from that in the unvaccinated group (𝛽2 ).
The estimation for ingroups bias and outgroup bias uses all samples in the vaccinated andunvaccinated groups at the same time, and the estimated equation (5) is as follows:
𝑌 = 𝛽0 + 𝛽1 × 𝑉𝑎𝑐𝑐𝑖𝑛𝑎𝑡𝑒𝑑 𝑔𝑟𝑜𝑢𝑝 + 𝛽2 × 𝐼𝑛𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛+ 𝛽3 × 𝑉𝑎𝑐𝑐𝑖𝑛𝑎𝑡𝑒𝑑 𝑔𝑟𝑜𝑢𝑝 × 𝐼𝑛𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 + 𝛽4 × 𝑂𝑢𝑡𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑡𝑖𝑜𝑛+ 𝛽5 × 𝑉𝑎𝑐𝑐𝑖𝑛𝑎𝑡𝑒𝑑 𝑔𝑟𝑜𝑢𝑝 × 𝑂𝑢𝑡𝑔𝑟𝑜𝑢𝑝 𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛 + 𝐶𝑜𝑛𝑡𝑟𝑜𝑙𝑠 (5)40The baseline is the anonymous condition in the unvaccinated group. The parameters of our interest are
𝛽3 and 𝛽5 . 𝛽3 captures how different the ingroup bias in the vaccinated group is from that in theunvaccinated group (𝛽2 ). Also, 𝛽5 captures how different the ingroup bias in the vaccinated group isfrom that in the unvaccinated group (𝛽4 ).

41

Appendix D. Follow-up Experiments: Group Assignment
Table D1. 1st Follow-up Experiment: Group Assignment
Vaccinated people, N=800
(1)

(2)

(3)

(4)

(5)

Order

N=160

N=160

N=160

N=160

N=160

1

I

I

I

I

I

2

II

II

III

III

I

3

III

III

II

II

I

4

IV

V

IV

V

I

5

V

IV

V

IV

I

Unvaccinated sample (N=800)
(6)

(7)

(8)

(9)

(10)

Order

N=160

N=160

N=160

N=160

N=160

1

I

I

I

I

I

2

II

II

III

III

I

3

III

III

II

II

I

4

IV

V

IV

V

I

5

V

IV

V

IV

I

Notes: We conduct dictator games in the following five conditions: I. Anonymous, II: Ingroup-December2022, III:
Outgroup-December2022, IV: Ingroup-February2022, and V: Outgroup- February2022.

42

Table D2. 2nd Follow-up Experiment: Group Assignment

Vaccinated people, N=800
(1)

(2)

(3)

Order

N=320

N=320

N=160

1

I

I

I

2

II

III

I

3

III

II

I

Unvaccinated sample (N=800)
(4)

(5)

(6)

Order

N=320

N=320

N=160

1

I

I

I

2

II

III

I

3

III

II

I

Notes: We conduct dictator games in the following five conditions: I. Anonymous, II: Ingroup-June2023, and III:
Outgroup- June2023.

43

Appendix E. Address of Concerns
Table Appendix E1. The Concern on Anonymous Pairs

Vaccinated sample(1)
Ingroup Bias andOutgroup Bias

Tests forIngroup condition(2)

3.27463.3247*(1.9738)(1.9593)-5.2166***-5.1570**(1.9400)(1.9417)1.85732.0779(2.5799)(2.5717)-3.3870-3.6018(2.9590)(2.9501)-1.9219-2.0279(2.9145)(2.8831)NO

YES-0.81080.3586(1.6588)(4.1127)

Number of observations

800

800

R-squared

0.050

0.055

Outgroup condition

Subjective prop of vaccinated people is 50% or more

Ingroup condition×Subjective prop is 50% ore more

Outgroup condition×Subjective prop is 50% ore more

Covariates
Constant term

Notes: Cluster robust standard errors at region level in parentheses. *** p<0.01, ** p<0.05, and * p<0.10. Covariates
include female dummy, age, married dummy, number of family members, household income, no income information
dummy, and educational years. In the first follow-up experiment, we asked the question, “To date, what percentage of
the Japanese population do you think has received three or more doses of the COVID-19 vaccine?” We divide thevaccinated group into two subgroups: those who answered that 51% or more have received the vaccines and those who
answered that 50% or less have done.

44Table Appendix E2. The Concern on Sample Selection

VaccinatedIngroup Favoritism

Ingroup Bias

Outgroup Bias

Feb-22Ingroup Bias

Outgroup Bias

Jun-23Full

Limited

Full

Limited

Full

Limited

8.21

8.23

7.66

7.20

6.20

5.81

(1.37)(1.28)(1.28)(1.61)(1.10)(1.10)

1.69

0.96

1.41

0.98

1.23

0.27

(1.53)(1.80)(1.53)(1.95)(1.17)(1.21)

-6.53

-7.26

-6.25

-6.23

-4.98

-5.54

(1.62)(1.83)(1.53)(1.98)(1.12)(1.34)

UnvaccinatedIngroup Favoritism

Dec-22

Feb-22

Dec-22Jun-23Full

Limited

Full

Limited

Full

Limited

-1.13

-1.38

2.53

0.59

5.83

3.92

(1.55)(2.28)(1.50)(1.81)(1.53)(1.35)

2.91

3.40

2.16

0.06

3.43

3.60

(1.70)(1.78)(1.69)(2.05)(1.48)(0.97)

4.04

4.77

-0.38

-0.53

-2.41

-0.33

(1.80)(2.22)(1.69)(2.07)(1.14)(1.16)

Note: Standard deviations in parentheses.

45

Appendix F. COVID-19-policy Related Attitudes
We finally investigate whether and how ingroup favoritism is associated with opinions and behaviors
regarding the COVID-19 related policies in the real world. If we find a significant association between
the two, it implies that the biases could have some degree of social influence. We look at the opinion
regarding infectious disease control and socioeconomic activities, that regarding the relaxation of
behavioral restrictions, that regarding measures to promote vaccination, and only for the vaccinatedgroup, the actual timing of their vaccination.
[Table Appendix F1 is Here]
Panel A of Table Appendix F1 shows that the vaccinated people with stronger ingroupfavoritisms are more likely to agree with relaxing the behavioral restrictions of people whose
vaccination record can be verified. This association becomes statistically significant, especially when
making them imagine a situation in which the number of infected people is decreasing. In addition,
those with stronger ingroup favoritism are more likely to receive the vaccine at an earlier date.
Although this association becomes weakened when controlling for age and other attribute variables, itremains statistically significant at the 10% level.
Panel B of Table Appendix F1 shows that, among the unvaccinated people, negative associations
exist between their ingroup favoritism and all the opinion variables. Here we need to note that the
distribution of their ingroup favoritism varies for non-vaccination reasons. The ingroup favoritism isdistributed in the direction that the amount allocated to an outgroup member is higher than the amountallocated to an ingroup member, among the subgroup that selected the reason, “I would like to get
vaccinated if I could, but I cannot for health or other reasons.” Conversely, it is distributed in the
direction that the amount allocated to an ingroup member is higher than the amount allocated to an
outgroup member, among the other subgroup that selected the reason, “I do not want to get vaccinatedanyway in the first place.”[Table Appendix F2 is Here]
Panel A of Table Appendix F2 shows that, among the unvaccinated people selecting the firstreason, their ingroup favoritism is negatively and significantly associated with all the three opinions
regarding the COVID-19 policies. Given the distributional characteristic, the unvaccinated people who
allocated more to the vaccinated than the unvaccinated tend to prioritize infectious disease control
over socioeconomic activities, compared to those who do not. They also tend to agree with relaxing
behavioral restrictions for people with a verifiable vaccination record and with promotional measuresthat provide financial rewards for vaccinators. Looking at the constant terms in the odd columns of46

Table Appendix F1 (models with no covariate), we find that, compared to the unvaccinated people,
the vaccinated tend to favor infectious disease control, the relaxation of behavioral restrictions, andthe promotional measure with financial rewards. That is, the unvaccinated people who selected thefirst reason and have ingroup favoritism in the opposite direction of the hypothesis are more likely toshare the same opinions with the vaccinated.
Panel B of Table Appendix F2 shows that, among the unvaccinated people selecting the second
reason, their ingroup favoritism is negatively and significantly associated with the opinions regarding
infectious disease control and socioeconomic activities and regarding measures to promote vaccination.
Given the distributional characteristic, the unvaccinated people who allocated more to theunvaccinated than the vaccinated tend to prioritize socioeconomic activities over infectious disease
control and disagree with the promotional measure with financial rewards. Their attitudes arecompletely opposite to those of the vaccinated people and the unvaccinated who selected the firstreason.

47

Table Appendix F1. Associations with COVID-19-policy Related Attitudes (1)Panel A:
Vaccinated sample, N=636

(2)

(1)

I think we should
prioritize infectious disease
control over socioeconomic
activities.

(6)

(5)

I agree with relaxing the behavioral restrictions
when a vaccination record can be verified.
In areas where
In areas where
the infection is shrinking
the infection is spreading

(8)

(7)

(10)

(9)

(11)

(12)

I agree with offering financial rewards
The period from January 1,
to vaccinated people.
2021 to the vaccination date
Both to newly and previously
Only to newly vaccinatedvaccinatedNumber of days

1-5

1-5

1-5

1-5

0-10

Scales:(4)(3)-0.1230*(0.0647)

Ingroup Favoritism

0.0015(0.0082)-0.0012(0.0079)

0.0054(0.0035)

0.0055(0.0036)0.0046***(0.0014)0.0048**(0.0017)-0.0049(0.0029)-0.0036(0.0031)-0.0004(0.0016)0.0012(0.0020)-0.2080***(0.0581)

CovariatesNO
6.1111***(0.1142)0.000

YES
5.5520***(1.2428)
0.041

NO
3.2390***(0.0502)
0.008

YES
3.4002**(1.0290)0.036

NO3.3029***(0.0625)
0.006

YES
3.0753***(0.5175)
0.031

NO
2.5074***(0.0336)
0.004

YES
3.1294***(0.4585)0.035

NO
3.5578***(0.0309)
0.000

YES
5.1891***(0.6999)
0.061

YES
NO
211.8462*** 200.2294***(22.2803)(2.9190)
0.181
0.006

(13)

(14)

(15)

(16)

(17)

(18)

(19)

(20)

(21)

(22)

Constant term
R-squared
Panel B:
Unvaccinated sample, N=625

I think we should
prioritize infectious disease
control over socioeconomic
activities.
Scales:I agree with relaxing the behavioral restrictions
when a vaccination record can be verified.
In areas where
In areas where
the infection is shrinking
the infection is spreading

1-5

1-5

1-5

1-5

0-10

I agree with offering financial rewards
to vaccinated people.
Both to newly and previously
Only to newly vaccinatedvaccinated-0.0089*(0.0043)-0.0085*(0.0041)-0.0105***(0.0027)-0.0106***(0.0022)-0.0116***(0.0025)-0.0120***(0.0021)

NO
YES
NO
YES
NO
2.2933***3.1999***2.2434***6.1727***4.5654***(0.0321)(0.6532)(0.0346)(1.1976)(0.0892)
0.016
0.046
0.014
0.073
0.024
R-squared
Notes : Cluster robust standard errors at region level in parentheses; *** p<0.01, ** p<0.05, * p<0.1

YES
3.7687***(0.4133)
0.041

NO
2.2044***(0.0315)0.020

YES
3.8664***(0.5807)0.075

NO2.3088***(0.0221)
0.022

YES
4.4526***(0.5598)
0.097

Ingroup Favoritism-0.0284***(0.0047)-0.0285***(0.0031)-0.0082**(0.0035)-0.0080**(0.0032)

Covariates

Constant term

48

Table Appendix F2. Associations with COVID-19-policy Related Attitudes (2)Panel A:
(2)

(1)

Unvaccinated sample

(8)

(7)

(4)

(3)

(12)

(11)

(14)

(13)

with Reason UV1, N=100"Because I would like to get
vaccinated if I could, but I cannot
for health or other reasons."

I think we should
prioritize infectious disease
control over socioeconomic
activities.

Ingroup Favoritism

Covariates
Constant term
R-squared

In areas where
the infection is spreading

In areas where
the infection is shrinking

Only to newly vaccinatedBoth to newly and previously
vaccinated

1-5

1-5

1-5

1-5

0-10

Scales:I agree with offering financial rewards
to vaccinated people.

I agree with relaxing the behavioral restrictions
when a vaccination record can be verified.

-0.0349**-0.0247**-0.0105***-0.0129***-0.0092**-0.0114***-0.0037-0.0107**-0.0055-0.0125**(0.0115)(0.0089)(0.0030)(0.0028)(0.0032)(0.0031)(0.0066)(0.0041)(0.0081)(0.0052)

NO

YES

NO

YES

NO

YES

NO

YES

NO

YES

5.5469***10.9626***

2.3969***1.32892.4621***2.42602.2049***3.2280*2.3376***2.6191(0.3762)(3.0817)(0.0559)(0.9781)(0.0504)(1.4104)(0.1114)(1.5150)(0.0898)(1.5903)

0.058

0.174

0.046

0.147

0.031

0.075

0.004

0.176

0.007

0.173

(15)

(16)

(17)

(18)

(21)

(22)

(25)

(26)

(27)

(28)

Panel B:

Unvaccinated sample
with Reason UV2, N=447"Because I do not want to get
vaccinated anyway in the firstplace."

I think we should
prioritize infectious disease
control over socioeconomic
activities.

Ingroup Favoritism

Covariates

In areas where
the infection is spreading

In areas where
the infection is shrinking

Only to newly vaccinatedBoth to newly and previously
vaccinated
1-5

1-5

1-5

1-5

0-10

Scales:I agree with offering financial rewards
to vaccinated people.

I agree with relaxing the behavioral restrictions
when a vaccination record can be verified.

-0.0170**-0.0180**-0.0074-0.0066-0.0093-0.0079-0.0158***-0.0141***-0.0164***-0.0147***(0.0059)(0.0055)(0.0043)(0.0042)(0.0054)(0.0052)(0.0032)(0.0029)(0.0031)(0.0031)

NO

YES

NO

YES

NO

YES

NO

YES

NO

YES

4.3082***4.4948**2.2172***

3.1287**2.2895***

3.8372***2.2100***4.3892***2.3027***4.7774***(0.0686)(1.5640)(0.0484)(1.0180)(0.0489)(0.6215)(0.0225)(0.5199)(0.0290)(0.5327)

0.015
0.036
0.010
0.054
0.007
R-squared
Notes : Cluster robust standard errors at region level in parentheses; *** p<0.01, ** p<0.05, * p<0.1

0.042

0.039

0.092

0.039

0.105

Constant term4950