DP001
Vaccination and Discrimination:
Experimental Evidence under the COVID-19 Pandemic

Shusaku Sasaki, Osaka University
Hirofumi Kurokawa, University of Hyogo

Vaccination and Discrimination:
Experimental Evidence under the COVID-19 Pandemic

Shusaku Sasaki1

Hirofumi Kurokawa2

First Draft: October 11th, 2022

Abstract
In this study, we conduct financially incentivized dictator games with the COVID-19 vaccinated andunvaccinated people in Japan (N=1,578), and ascertain their favorable or hostile attitudes toward eachother, by using ingroup favoritism. We measure ingroup favoritism as the difference in the allocatedamounts between to ingroup members with the same vaccination status and to outgroup members witha different status. Our analyses suggest that the vaccinated people behave more discriminately toward
outgroup members, compared to the unvaccinated people. The vaccinated people show strong ingroupfavoritism, which are shaped mainly by their outgroup bias of decreasing the money amount allocatedto an unvaccinated pair, their outgroup member. In contrast, the unvaccinated people do not exhibit
such the ingroup favoritism, on average. Their outgroup bias is found in the rather opposite directionof the hypothesis, and they tend to increase the amount to a vaccinated pair, their outgroup member.
We find this tendency in particular from the unvaccinated who selected as their non-vaccination reason“I would like to get vaccinated if I could, but I cannot for health or other reasons.” Furthermore, we
confirm significant associations between their ingroup favoritism and attitudes regarding the COVID19 policies, suggesting that the biases would have some degree of social influence in the real world.
This study’s findings can contribute to discovering how to smoothly build cooperative relationships
between vaccinated and unvaccinated people under the current and future pandemics.
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
School of Economics and Management, University of Hyogo, Japan.
1

We obtained ex-ante approval from the ethics committee of Center for Infectious Disease Education andResearch, Osaka University, Japan (2022CRER0114). We also registered the experimental design with the
AEA RCT Registry (Sasaki and Kurokawa, 2022). 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
COVID-19 vaccination is the essential key to restarting socioeconomic activities and ending the social
confusion caused by this pandemic. However, some people are reluctant to receive the vaccine or havenegative attitudes toward the vaccination. Anti-vaccine demonstrations have been held in France,
Germany, Japan, and other countries around the world, and fake news about vaccines have been
flooded on social media.
Although the issue of receiving the vaccine or not has divided people, both the vaccinated andunvaccinated people need to live together in the same society. Since the unvaccinated people do not
obtain immunity from vaccination, the vaccinated need to be careful not to spread the infection to theunvaccinated. The unvaccinated need to agree that the governments allocate taxes to investing theconstruction of environments in which people with vaccination intention can receive the vaccine
immediately. From policy perspectives, it is quite crucial to empirically determine how cooperative or
hostile the vaccinated and unvaccinated people are toward each other.
This experimental study measures ingroup favoritism among the vaccinated and unvaccinatedpeople, examine the characteristics of its distribution, and determine the association between their bias
and real-world attitudes. The ingroup favoritism is captured by the difference in the allocated money
amount between to ingroup members who share group identities (country, religion, political party, etc.)
and to outgroup members 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). It has been reported that the ingroup favoritism appears whenusing social identities, including country, religion, political party, race, and gender (Charness andRustichini 2011; Fershtman and Gneezy 2001; Kranton et al. 2020). Also, the ingroup favoritism has
been observed when using identities that are formed exogenously, including race and gender, as wellas when using those which are formed by people’s endogenous choices (Efferson et al. 2008; Charness
et al. 2014).
We assume that people’s endogenous choice of whether they receive the COVID-19 vaccine or
not will form a group identity, and experimentally measures the ingroup favoritism of the vaccinatedand unvaccinated people toward each other. Jagodics and Szabó (2022) explain, based on psychology
theories, that vaccination status has conditions for shaping a social identity. For example, vaccination
status is socially unneutral since regulations create distinctions based on it, and the membership makes
the vaccinated people perceive that their outgroup threatens the achievement of 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. outgroup2

hate, 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. Thus, we experimentally measure ingroup bias and outgroup bias, in addition toingroup favoritism.
We conduct a financially incentivized online experiment to measure ingroup favoritism, ingroupbias, and outgroup bias among 1,578 vaccinated or unvaccinated people residing throughout Japan,
as described in Section 2. 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 andthus easy to implement into a nationwide online experiment. Balliet et al. (2014) note that the ingroupfavoritism, which is measured by the dictator game, is relatively small. If we find significant ingroupfavoritism even in such a setting, it will become strong evidence.
Under the COVID-19 pandemic, Bartos et al. (2021) conducted a one-way allocation game, likethe dictator game, in their financially incentivized online experiment and found emphasizing the threatof this pandemic rises the hostile attitude toward foreigners in money allocations. In a psychologicalonline experiment, Jagodics and Szabó (2022) captured how subjects allocate hypothetical monetary
resources between two persons characterized by vaccination status. They report that the vaccinatedpeople prefer to increase the difference in allocation between their ingroup and outgroup members to
disadvantage the outgroup members.
In response to the literature, we design this experiment to be incentivized by real money to capture
ingroup favoritism in situations where participants’ rewards vary depending on their choices in thegame, and ensure sufficient sample size to determine the biases’ characteristics for the unvaccinatedpeople, who are minorities in many societies, 1 as well as the vaccinated, respectively. Furthermore,
we examine the associations between their biases and attitudes regarding the COVID-19 related
policies in the real world, enrich this study’s policy implications, and add new insights to the literature.
2. Experimental Design
2.1. Overview
From January 28 to February 1, 2022, we conducted a screening survey through MyVoice.com Ltd.,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.

13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 the COVID-19-related
behavioral characteristics and attitudes, and socio-economic attributes. Concretely, the COVID-19related questions include attitudes toward the COVID-19-related policies (Balancing infectious
disease control and socio-economic activities, vaccination certification, and financial incentives forencouraging vaccination) and dates of the first and second COVID-19 vaccination uptakes (Vaccinatedsample, 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 inthis 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.
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%. In addition, the nationwide outbreak of Omicron4

strains 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,2022).
2.2. Dictator GamesWe present a dictator game five times. In each game, participants are given an endowment of 100
Japanese Yen in addition to the participation fee (90 JPY). 2 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 notparticipating in this same survey, (2) the participant solely determines the allocation, (3) they are theonly one who can give a share of the money to the paired person, and (4) one of the five experimental
responses will be randomly selected to carry out the allocation after the experiment.
We conduct dictator games in the following five conditions:
Anonymous: A recipient is anonymous for an allocator. The allocator is also anonymous forI.

the recipient.
II.

Private-Ingroup: An allocator is informed that a recipient belongs to their ingroup. The
allocator is anonymous for the recipient.

III.

Private-Outgroup: An allocator is informed that a recipient belongs to their outgroup. The
allocator is anonymous for the recipient.

IV.

Public-Ingroup: An allocator is informed that a recipient belongs to their ingroup. The
recipient is notified of the vaccination status of the allocator.

V.

Public-Outgroup: An allocator is informed that a recipient belongs to their outgroup. The
recipient is notified of the vaccination status of the allocator.

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 show screens for each condition in Appendix A.
[Table 1 is Here]One subject participates in dictator games five times as an allocator. As shown in Table 1, we set

2

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

5

randomly the order of the above conditions to create eight groups in the vaccinated and unvaccinatedsamples, 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 the private (II, III) and public (IV, V) conditions.
Then, within each of the private and public conditions, we randomly set the order of the ingroupconditions (II, IV) and outgroup conditions (III, V). Consequently, each of the vaccinated andunvaccinated samples have eight groups. Furthermore, to consider potential order effects caused bysuccessive participation in the dictator games, we create a control group where we present theanonymous dictator game (I) five times. Totally, each of the vaccinated and unvaccinated samples
have nine groups, including the control group.
2.3. Biases’ Definitions and Empirical Hypotheses
We define ingroup favoritism, ingroup bias, and outgroup bias using allocation amounts in the dictator
games as follows:
Ingroup favoritism = allocation amount to an ingroup pair – allocation amount to an outgroup pair
Ingroup bias = allocation amount to an ingroup pair – allocation amount to an anonymous pair
Outgroup bias = allocation amount to an outgroup pair – allocation amount to an anonymous pair
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.
We further use the public and private conditions and the background factors of ingroup favoritism.
This approach is based on Ockenfels and Webner (2014). Under the public condition, allocators know
the vaccination status of recipients, and the recipient also knows the allocators’ vaccination status. If
the ingroup bias is greater in the public than in the private condition, we interpret that the allocators
give higher money amounts in consideration of reputation from ingroup recipients (Guala et al. 2013;
Ockenfels and Webner 2014). If the outgroup bias is greater to the positive direction in the public thanin the private condition (i.e., the outgroup bias gets weakened in the public condition), we interpret
this as the allocators refraining from allocating lower money amounts in consideration of reputationfrom outgroup recipients. If there is no difference between the two conditions, we interpret this as the
ingroup favoritism based on something other than reputation mechanisms, including just preferences(Chen and Li 2009; Klor and Shayo 2010).
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2.4. Analytical Procedure
Our experimental design enables us to conduct both between- and within-individual comparisons. We
can measure the biases, by focusing on responses in the second dictator game and comparing themacross groups. Also, we can measure the biases for each individual in the treatment 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. Also, to explore
background factors for the biases, we examine how the biases change between when the vaccination
status of the allocator is not informed to the recipient (the private condition) and when it is informed
to them (the public condition).
Second, we conduct the within-analysis and check whether individuals’ average tendencies ofingroup favoritism, ingroup bias, and outgroup bias are consistent with those observed in the betweenanalysis. To explore other background factors for the biases, we also check the relationship between
the biases and the reasons for vaccination or non-vaccination. Furthermore, to enrich the discussion
on the policy implications of this study, we examine how the biases are associated with their realworld attitudes. Specifically, in both the vaccinated and unvaccinated group, we examine theassociation between their biases and attitudes toward the COVID-19-related policies. In the vaccinatedgroup, we also examine the association between their biases and the timing of COVID-19 vaccination.
2.5. 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 a7

sample size of around 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 (e.g.,socially desirable bias and experimenter demand effect). 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, captured by the allocations in thescreening survey and in the first dictator game, do not differ significantly between the two groups after
controlling for the effects of attribute variables.
3. Between-analysis Results
3.1. Ingroup Favoritism, Ingroup Bias, and Outgroup Bias
[Figure 1 is Here]
We perform group comparisons by using the allocations of the second and first dictator games and8measure ingroup favoritism, ingroup bias, and outgroup bias. 3 On the left side of Figure 1, we present
the result of estimating the vaccinated 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 showsthat the vaccinated group 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. The result also indicates that this ingroup favoritism may be generated by the outgroupbias 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 is relatively
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 thatthe 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 same tendencies in the biases after directly controlling
for the effects of attribute variables in regression analysis. As already described in Section 2.5, ouranalysis considers to some degree the effects of unobserved individual characteristics, including
socially desirable bias and experimenter demand effect, by using the difference between the allocationsof the second game and the first game as the dependent variable. Furthermore, in Appendix B, we still
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.

3

9

observe the similar tendencies in the biases when adding variables that partially capture the sociallydesirable bias and experimenter demand effect to the estimation and directly controlling their effects.
3.2. Comparison between Vaccinated and Unvaccinated Groups
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.
3.3. Comparison between Private and Public Conditions
We examine how the degrees of the ingroup favoritism, ingroup bias, and outgroup bias differ between
the private and public conditions. As described in Section 2.3, if the ingroup bias is greater in the
public condition than in the private condition, allocators will give a higher amount with consideration
for the reputation from a recipient of the same group. Also, if the outgroup bias is larger in the positive
direction in the public condition than in the private condition (i.e., the outgroup bias gets weakened in
the public condition), allocators will refrain from allocating a lower amount with consideration for the
reputation from a recipient of the different group. The direction of the private-public difference iningroup favoritism will depend on which is larger, the difference in ingroup bias or the difference inoutgroup bias.
[Table 4 is Here]In Table 4, we present results of estimating the models in Appendix C. The estimated results do
not support the existence of differences between the private and public conditions for all of the ingroupfavoritism, ingroup bias, and outgroup bias in both the vaccinated and unvaccinated groups. Also, the10results do not support the possibility that the reputation mechanism may shape the ingroup favoritism
and outgroup bias of the vaccinated people. Their biases may arise from causes other than themechanism.

4. Within-analysis Results
4.1. Ingroup Favoritism, Ingroup Bias, and Outgroup Bias
From here on, we conduct the within-analysis. 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.
[Figure 2 is Here]The left-hand side of Figure 2 presents the average biases of the vaccinated and unvaccinatedgroups, which are calculated by using the within-individual differences. The tendencies in the biasesof the vaccinated group are almost 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.
Similarly, the tendencies in the biases of the unvaccinated group are consistent with those in the
between analysis, in that the existence of the ingroup favoritism is not supported and the outgroup bias
is not observed 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 unvaccinatedgroup, 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. Analysis by Reasons
Why do the vaccinated and unvaccinated people show the tendencies for the biases as observed? In
Section 3.3, the between-analysis with the private and public conditions did not support the reputation
mechanism. Here, we retry this question by focusing on the participants’ reasons for their vaccination
or non-vaccination. The advantage of the within-analysis is that this allows direct examination of therelationship between the individuals’ biases and several responses in the survey, 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,
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“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.”
The remaining part of Figure 2 shows the biases’ values by reasons. We find no substantial
differences in the biases of the vaccinated group across their selected reasons. Their first reason would
be relatively selfish, while the second and third reasons would be altruistic. The results in Figure 2
indicates the possibility that, no matter whether the vaccinated people received the vaccine for selfish
or altruistic reason, they have the strong ingroup favoritism based on the outgroup bias.
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 partiallycloser to the background mechanism of the biases of the unvaccinated group.
4.3. 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
12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 5 is Here]
Panel A of Table 5 shows that the vaccinated people with stronger ingroup favoritisms are more
likely to agree with relaxing the behavioral restrictions of people whose vaccination record can beverified. This association becomes statistically significant, especially when making them imagine asituation in which the number of infected people is decreasing. In addition, those with stronger ingroupfavoritism are more likely to receive the vaccine at an earlier date. Although this association becomes
weakened when controlling for age and other attribute variables, it remains statistically significant at
the 10% level.
Panel B of Table 5 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 6 is Here]
Panel A of Table 6 shows that, among the unvaccinated people selecting the first reason, theiringroup favoritism is negatively and significantly associated with all the three opinions regarding the
COVID-19 policies. Given the distributional characteristic, the unvaccinated people who allocatedmore to the vaccinated than the unvaccinated tend to prioritize infectious disease control oversocioeconomic 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 ofTable 5 (models with no covariate), we find that, compared to the unvaccinated people, the vaccinatedtend to favor infectious disease control, the relaxation of behavioral restrictions, and the promotional
measure with financial rewards. That is, the unvaccinated people who selected the first reason and13have ingroup favoritism in the opposite direction of the hypothesis are more likely to share the same
opinions with the vaccinated.
Panel B of Table 6 shows that, among the unvaccinated people selecting the second reason, theiringroup 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 the unvaccinated thanthe vaccinated tend to prioritize socioeconomic activities over infectious disease control and disagreewith the promotional measure with financial rewards. Their attitudes are completely opposite to thoseof the vaccinated people and the unvaccinated who selected the first reason.
5. Discussion and Conclusions
We conducted incentivized dictator game experiments and presented the following findings. First, thevaccinated people have strong ingroup favoritism. This ingroup favoritism could be generated mainlyby their outgroup bias of decreasing the amount allocated to the unvaccinated people, their outgroupmembers, not by the ingroup bias of increasing the amount allocated to the vaccinated people, theiringroup members. The superficial tendency of ingroup favoritism is consistent with the hypothesis andresults 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 COVID19 vaccination. Jagodics and Szabó (2022) also use a hypothetical scenario experiment and report that
the COVID-19 vaccinated people prefer to increase the difference in allocation between their ingroupand outgroup members to disadvantage the outgroup members. In contrast to the vaccinated people,
the unvaccinated do not have ingroup favoritism, on average. Among them, the outgroup bias is foundin the opposite direction of the hypothesis, and they tend to allocate more money to a vaccinated pair,their outgroup members, than to an anonymous pair.
Second, we take advantage of ensuring a sufficient sample size of the unvaccinated people andexamine how the biases vary depending on their reason for not receiving the COVID-19 vaccine, toexplore the background factors of their seemingly strange biases. The ingroup favoritism and outgroupbias are observed in the opposite directions of the hypotheses, as in the above average result, 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 the first place,” the biases are observed as hypothesized, and they tend to allocate
more money amount to an unvaccinated pair, an ingroup member, than to a vaccinated pair, anoutgroup member. The attitudes of the former subgroup could be interpreted as a sign of theirappreciation for people who received the vaccine on their behalf or as a sign of their perception thatthe vaccinated people are the true ingroup members for them.
Third, respectively among the vaccinated and unvaccinated groups, the biases are associated with14

their attitudes toward COVID-19-related policies, indicating that the biases would have some degreeof social influence in the real world. The vaccinated people with stronger ingroup favoritism are more
likely to agree with relaxing the behavioral restrictions for those whose vaccination records can beverified and have received the vaccine at an earlier date. As with the second finding, the associations
for the unvaccinated people vary substantially depending on their non-vaccination reasons. Among theunvaccinated with the first reason, those who allocate more to the vaccinated are more likely to agreewith prioritizing infectious disease control over socioeconomic 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 the unvaccinated are more
likely to disagree with prioritizing infectious disease control and the above vaccination promotion
measure.
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.
Some vaccinated people may feel as if almost all the unvaccinated people have a hostile attitude
toward vaccinators, because they see lots of unvaccinated people supporting and spreading fake news
about the COVID-19 vaccine through social media. However, this belief is not supported as far as we
investigate by using ingroup favoritism, ingroup bias, and outgroup bias. On average, the vaccinatedpeople, rather than the unvaccinated, 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 isessential for them to realize their hostile attitudes and consider diverse reasons and behaviors of theunvaccinated people to build cooperative relationships between the two groups smoothly.

15

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17

Figure 1. Ingroup Favoritism, Ingroup Bias, and Outgroup Bias (from the Between-analysis)

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.

18

Figure 2. Ingroup Favoritism, Ingroup Bias, and Outgroup Bias (from the Within-analysis)

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

19

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. Private-Ingroup (An allocator is informed that arecipient belongs to their ingroup. The allocator is anonymous for the recipient), III. Private-Outgroup (An allocator isinformed that a recipient belongs to their outgroup. The allocator is anonymous for the recipient), IV. Public-Ingroup(An allocator is informed that a recipient belongs to their ingroup. The recipient is notified of the vaccination status of
the allocator), and V. Public-Outgroup (An allocator is informed that a recipient belongs to their outgroup. The recipient
is notified of the vaccination status of the allocator).

20

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=159

N=160

N=796

N=160

Control (Anonymous)

N=157

N=160

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

Female dummy

0.51

0.50

0.49

0.50

0.46

0.50

0.48

0.50

0.43

0.50
11.47

Age

49.34

11.97

49.32

12.75

51.19

11.85

50.01

12.55

50.88

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

S.D.

Female dummy

0.49

0.50

0.48

0.50

0.49

0.50

0.49

0.50

0.50

0.50
11.99

Age

43.32

12.61

42.71

11.84

44.16

13.28

44.54

12.08

43.26

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

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.

21

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***(1.0609)

Vaccinated sample×Outgroup condition-10.4841***(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, 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.

22

Table 4. 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, 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. The results also do not support thepossibility that the reputation mechanism may shape the ingroup favoritism and outgroup bias of the vaccinated people. Their biases may arise from causes other than the mechanism.

23

Table 5. Associations with COVID-19-policy Related Attitudes (1)Panel A:
Vaccinated sample, N=636

(1)

(2)

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

(3)

(4)

(5)

(6)

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

0-10

(7)

(9)

(10)

(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 vaccinatedvaccinated

1-5

1-5

(8)

1-5

1-5

Number of days

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

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

(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 spreading
the infection is shrinking

0-10-0.0285***(0.0031)1-5-0.0082**(0.0035)

1-5-0.0080**(0.0032)

1-5

1-5-0.0089*(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
4.5654***6.1727***2.2434***
3.1999***2.2933***(0.0892)(1.1976)(0.0346)(0.6532)(0.0321)
0.024
0.073
0.014
0.046
0.016
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)

I agree with offering financial rewards
to vaccinated people.
Both to newly and previously
Only to newly vaccinatedvaccinatedCovariates

Constant term

24

-0.1230*(0.0647)

Table 6. Associations with COVID-19-policy Related Attitudes (2)Panel A:(1)

Unvaccinated sample
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.

Scales:Ingroup Favoritism

Covariates
Constant term
R-squared

(2)

(3)

(4)

(7)

(8)

(11)

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

(12)

(13)

(14)

I agree with offering financial rewards
to vaccinated people.

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
-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.

Scales:Ingroup Favoritism

Covariates

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

0-10

In areas where
the infection is shrinking

1-5

I agree with offering financial rewards
to vaccinated people.
Only to newly vaccinated

1-5

Both to newly and previously
vaccinated

1-5

1-5

-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)

0.007
0.054
0.010
0.036
0.015
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 term

25

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

26

II. Private-Ingroup (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

27

III. Private-Outgroup (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

28

IV. Public-Ingroup (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

29

V. Public-Outgroup (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

30

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

31

II. Private-Ingroup (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

32

III. Private-Outgroup (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

33

IV. Public-Ingroup (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

34

V. Public-Outgroup (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

35

Appendix B. Robustness Check
As shown in Table Appendix A, we robustly observe the same tendencies in the biases after directly
controlling for the effects of attribute variables in regression analysis. As already described in Section
2.5, our analysis considers some degree of effects of unobserved individual characteristics, including
socially desirable bias and experimenter demand effect, by using the difference between the allocationsof the second game and the first game as the dependent variable. Furthermore, we still find the similartendencies in the biases, when adding variables, darkness and normative consciousness, that partially
capture the socially desirable bias and experimenter demand effect to the estimation and controlling
their effects. 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 inline.”

36

Table Appendix 1. Robustness Check
(1)

(2)

(3)

(4)

(5)

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

Vaccinated 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<0.137Appendix C. Estimation Models
Appendix C.1. Comparison between Vaccinated and Unvaccinated Groups
The estimation for ingroup favoritism uses the samples excluding the two control groups, and theestimated equation (1) is as follows:
𝑌𝑌 = 𝛽𝛽0 + 𝛽𝛽1 × 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 + 𝛽𝛽2 × 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐+ 𝛽𝛽3 × 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 × 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 (1)The baseline is the Outgroup condition in the unvaccinated group. 𝛽𝛽3 captures how different the
ingroup favoritism in the vaccinated group is from that in the unvaccinated group (𝛽𝛽2 ).

The estimation for ingroups bias and outgroup bias uses all samples in the vaccinated andunvaccinated groups at the same time, and the estimated equation (2) is as follows:
𝑌𝑌 = 𝛽𝛽0 + 𝛽𝛽1 × 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 + 𝛽𝛽2 × 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐+ 𝛽𝛽3 × 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔𝑔 × 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝛽𝛽4 × 𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐+ 𝛽𝛽5 × 𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉𝑉 𝑔𝑔𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 × 𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 (2)The baseline is the anonymous condition in the unvaccinated group. The parameters of our interest are𝛽𝛽3 and 𝛽𝛽5 . 𝛽𝛽3 captures how different the ingroup bias in the vaccinated group is from that in theunvaccinated group (𝛽𝛽2 ). Also, 𝛽𝛽5 captures how different the ingroup bias in the vaccinated group isfrom that in the unvaccinated group (𝛽𝛽4 ).

Appendix C.2. Comparison between Private and Public Conditions
The estimated equation for ingroup favoritism (3) is as follow. This estimation uses the samples
excluding the control groups. The baseline is the Outgroup condition, and the parameter of interest is
𝛽𝛽3

𝑌𝑌 = 𝛽𝛽0 + 𝛽𝛽1 × 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝛽𝛽2 × 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝛽𝛽3 × 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐+ 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 (3)The estimated equation for ingroup bias is as follow (4). This estimation uses the samples in the
Ingroup conditions among the vaccinated and unvaccinated groups, respectively. The baseline is thePrivate-Ingroup condition.
𝑌𝑌 = 𝛽𝛽0 + 𝛽𝛽1 × 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼𝐼 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 (4)38The estimated equation for outgroup bias (5) 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.
𝑌𝑌 = 𝛽𝛽0 + 𝛽𝛽1 × 𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂𝑂 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 (5)39