Evolution and Human Behavior 23 (2002) 11 – 33

Social sharing and risk reduction
Exploring a computational algorithm for the psychologyof windfall gains
Tatsuya Kamedaa,*, Masanori Takezawab, R. Scott Tindalec,
Christine M. SmithdaDepartment of Behavioral Science, Hokkaido University, Bungakubu, N10 W7 Kita-ku,
Sapporo 060-0810, Japanb
Max Planck Institute for Human Development, Lentzeallee 94, 14195 Berlin, GermanycDepartment of Psychology, Loyola University-Chicago, 6525 N. Sheridan, Chicago, IL 60626, USAd
Department of Psychology, Grand Valley State University, Allendale, MI 49401, USA
Received 24 April 2001; received in revised form 8 June 2001; accepted 17 August 2001

Abstract
Sharing important resources widely beyond direct kin group members is one of the corefeatures characterizing human societies. Moreover, generalized exchange involving many
community members (e.g., meat sharing in bands) seems to be a uniquely human practice.
This paper explores a computational algorithm for the psychology of social sharing that may
underlie such practices, based on the risk-reduction hypothesis in food sharing of Kaplan and
Hill [Curr. Anthropol. 26 (1985) 223]. We predicted that, independent of the amount of effort
actually invested, uncertainty involved in resource acquisition is a key factor that triggers thepsychology of social sharing for both acquirers and nonacquirers of a resource. It was also
predicted that the ‘‘windfall effect’’ is independent of individual preferences as to modern
distributive ideologies. Four multisample/multimethod studies, using Japanese and American
participants, and laboratory as well as vignette experiments, supported these predictions: although
the identical fungible resource (money) was under consideration, different psychological processes
were triggered, depending on the degree of uncertainty involved in the money acquisition.
Implications of the windfall effect for egalitarianism in resource sharing, observed not only in* Corresponding author.
E-mail address: tkameda@let.hokudai.ac.jp (T. Kameda).
1090-5138/02/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved.
PII: S 1 0 9 0 - 5 1 3 8 ( 0 1 ) 0 0 0 8 6 - 1

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T. Kameda et al. / Evolution and Human Behavior 23 (2002) 11–33

hunter – gatherer bands but also in highly industrialized societies, are discussed. D 2002 Elsevier
Science Inc. All rights reserved.
Keywords: Resource sharing; Risk reduction; Computational algorithm; Egalitarianism

1. Introduction
Sharing important resources, such as food, widely beyond direct kin is one of thecore features characterizing human societies. Although a primitive form of food sharing
is known in several primates, including chimpanzees, bonobos, and capuchin monkeys(see de Waal, 1996, for a comprehensive review), no primates other than humans have abroad social sharing system. Food sharing in primates is a complex phenomenon, and its
adaptive origins are most likely manifold (cf. Flack & de Waal, 2000). Nevertheless, as
discussed below, one of the key functions of social sharing may be collective risk
reduction against variability in resource supply. Based on this risk-reduction notion, thispaper aims to shed some light on the ‘‘computational algorithm’’ (Cosmides & Tooby,1992) underlying the psychology of resource sharing. More specifically, by a series ofexperiments, we test a hypothesis that uncertainty involved in resource acquisition
triggers the psychology of sharing for both acquirers and nonacquirers, independent of
the amount of effort actually invested and the individual modern ideologies about
desirable distribution.
1.1. Individual and collective solutions to the problem of variability in food supply
Reducing uncertainty in food acquisition to secure a stable supply has been one of
the most central and universal adaptive problems for many species. In the animal
kingdom, one common strategy to cope with environmental variability in food supply isstorage. For example, small birds such as great tits store fat as body reserves against
unpredictable temperatures and variable feeding day length (Bednekoff, Biebach, &Krebs, 1994; Bednekoff & Krebs, 1995). Many animals also store food reserves in theenvironment. Nutcrackers and jays, for instance, store pine and oak seeds in the autumn
and retrieve them in the following spring to feed themselves and/or their offspring(cf. VanderWall, 1990).
For some types of food resources, however, storage may not be the best way to cope withenvironmental variability. Some group-living species have evolved collective solutions to thisproblem — social exchange and sharing of precious foods within a group. A well-known
example is the blood sharing observed in vampire bats. Wilkinson (1984) observed that theseanimals quite often fail to obtain a blood meal during the night and subsequently beg forblood from other individuals in the daytime roosts. Such a practice takes the form ofreciprocal altruism (Trivers, 1971), providing a social mechanism by which the variability inthe food supply at the individual level is reduced.

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1.2. Food sharing among the Ache foragers
As in the vampire bat example, most resource sharing beyond direct kin in the animal
kingdom takes the form of reciprocal altruism, viz., enduring exchanges conducted within
particular pairs. Using the language of sociology, this is a restricted exchange (Ekeh, 1974)
where resources flow essentially in a pairwise manner over time. (Of course, a single
individual can engage in restricted exchanges with many individuals; individuals can havebundles of exchange partners.)
However, human social exchanges are not necessarily limited to such pairwise, restrictedexchanges. A particularly illustrative example was provided by Kaplan and Hill’s (1985) and
Kaplan, Hill, and Hurtado’s (1990) observation of the Ache foragers living in lowland
subtropical eastern Paraguay. These researchers found that food transfers among the Acheshow markedly different patterns between hunted games (e.g., peccary, monkey, deer) andcollected resources (e.g., vegetables, fruits). Hunted game, especially when large in package
sizes, tends to be shared widely across many community members beyond the acquirer’s
family. While a substantial portion of collected resources is still given to nonfamily members,
hunted game is much more likely to be the target of communal sharing, in terms of both‘‘depth’’ (the proportion of the food given away to nonfamily members) and ‘‘breadth’’ ofsharing (the number of nonfamily members who receive the share: cf. Gurven, in preparation). Related findings have also been obtained for other hunter–gatherer societies (cf.
Cashdan, 1989; Gibson, 1988; Woodburn, 1982). These observations suggest that theproperties of the resources affect deeply how they may be transferred among community
members. While the principle of kin sharing essentially operates for collected resources,
sharing across the entire community is often observed for hunted game. Notice that the latter
type of sharing not only involves nonkin as recipients, but also seems to signify a generalizedexchange (Ekeh, 1974) or a generalized reciprocity (Sahlins, 1974) involving many
community members simultaneously, rather than a series of pairwise, restricted exchanges(cf. Hawkes, O’Connell, & Blurton Jones, 2001).
Why, then, do these markedly contrasting sharing rules operate for different resources? Kaplan and Hill (1985) explained the difference in terms of the degree of
uncertainty involved in resource acquisition. While provision of collected resources(e.g., vegetables, fruits) is relatively stable, acquisition of meat is a highly variable,
uncertain prospect. On average, there is a 40% chance that an Ache hunter will comeback empty-handed (Kaplan et al., 1990). It is thus essential for them to manage thevariance associated with meat acquisition, securing a stable supply of the resource.
Storage by freezing is an obvious individual solution to reduce the uncertainty, but sucha technique is not readily available in hunter–gatherer societies. Other storage methods
such as drying and smoking meat may result in nutrient loss. Kaplan and Hill argued
that, instead, the sharing system functions as a collective risk-reduction device. By
including more individuals in the risk-pooling group, the variance in food supply
decreases exponentially. Once established and maintained, the generalized exchangesystem (Ekeh, 1974; Sahlins, 1974) that includes many hunters can buffer the variancein the resource supply collectively.

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1.3. Exploring a ‘‘computational algorithm’’ for the psychology of sharing based on the riskreduction notion
Food sharing in hunter–gatherer societies is currently the topic of a vigorous debate, and isunlikely to be explained by any single mechanism (e.g., Gurven, Allen-Arave, Hill, &Hurtado, 2000; Hawkes et al., 2001; Smith & Bliege Bird, 2000; Sosis, 2000; see Winterhalder, 1997, for review). Yet, the risk-reduction notion that focuses on the benefits of ageneralized exchange system is logically straightforward and produces a set of interesting
hypotheses about the psychology of social sharing. That is, if coping with high variance inresource acquisition has been a recurrent adaptive problem in hominid evolution as impliedby the risk-reduction notion, then our minds may have been shaped to respond to varianceinformation quite sensitively. In the following, we explore such a ‘‘computational algorithm’’(Cosmides & Tooby, 1992) that potentially underlies the psychology of social sharing, basedon this risk-reduction notion.
1.3.1. Uncertainty as a key factor for triggering the psychology of social sharing
Casual observation of our everyday behavior suggests that we tend to use windfall
money (obtained by winning lotteries, etc.), more often than money acquired by hardlabor, for social purposes such as treating friends or donations to charities. Although theidentical fungible resource (money) is under consideration in both cases, different
psychological processes seem to be triggered almost automatically, depending on how
the resource is acquired.
Of course, such a tendency may simply reflect a modern ideology of labor theory of value(‘‘money earned without making effort has little value’’). However, the risk-reduction
perspective suggests another possibility. The key factor that triggers such a sharing tendency
may be the uncertainty associated with the acquisition of money per se, rather than theabsence of effort. As Cosmides and Tooby (1992) argued, it may be the case that‘‘information about variance in foraging success should activate different modes of operationof these (computational) algorithms, with high variance due to chance triggering a psychology of sharing’’ (p. 213, parentheses added). We live in modern societies where uncertainty
in resource acquisition is reduced by various social systems (e.g., production technologies),
yet our minds may still be quite sensitive to variance information, as suggested by the riskreduction notion.
1.3.2. Bidirectionality of the windfall effectSecond, for the social sharing system to work as a collective risk-reduction device, theaforementioned ‘‘windfall psychology’’ must be bidirectional. That is, uncertainty in resource
acquisition should trigger the windfall psychology not only for acquirers of a resource but
also for nonacquirers.
Bearing on this point, we recently developed an evolutionary game model about the
emergence of a communal sharing system under uncertainty (Kameda, Takezawa, &Hastie, submitted). This is an n person game model and views social interaction overacquired resource as a process of demand sharing (Hawkes et al., 2001; Peterson, 1993) orT. Kameda et al. / Evolution and Human Behavior 23 (2002) 11–33

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Table 1
Four behavioral strategies in the evolutionary game model of the emergence of a communal sharing system under
uncertainty (Kameda et al., submitted)
When in the nonacquirer role
When in the acquirer role

Demanding communal sharing

Granting anotheracquirer’s ownership

Provisioning as a common property
Claiming private ownershipcommunal sharer
egoistsaintbourgeois

tolerated theft (Bliege Bird & Bird, 1997; Blurton Jones, 1987) — a process where
nonacquirers essentially scrounge the resource. As shown in Table 1, the model assumed
four behavioral strategies specifying (a) how to behave when an individual happens to be
an acquirer of resource in a group and (b) how to behave when the individual happens to
be a nonacquirer. The four behavioral strategies differ in their underlying ‘‘ideology’’ about
what to do with the resource (to be privatized or to be shared) when in the acquirer role
and when in the nonacquirer role, respectively (thus 2  2 = 4).
For example, ‘‘communal sharers’’ (see Table 1) are the purest endorsers of thewindfall psychology; they provision the windfall gain as a common property when in
the acquirer role, and demand sharing when in the nonacquirer role. On the other hand,‘‘egoists’’ and ‘‘saints’’ (cf. Table 1) are unidirectional. Their windfall psychology istriggered only when in the nonacquirer role (egoists) or when in the acquirer role (saints),
but not in both times. (‘‘Bourgeois’’ in Table 1 are totally free from the windfall
psychology.) The evolutionary game analysis revealed that, when resource acquisition is
uncertain, the ‘‘communal sharing’’ strategy is a unique Evolutionarily Stable Strategy
under a wide range of parameters, overcoming various free-rider problems in enforcementof the sharing norm (see Kameda et al., submitted, for details). In other words, in terms ofindividual fitness, the windfall effect is predicted to be bidirectional — individuals should
be susceptible to the windfall psychology when in the nonacquirer role as well as in theacquirer role.
1.3.3. Independence from modern distributive ideologies
Third, if uncertainty in resource acquisition is indeed the key factor that triggers thepsychology of social sharing, the aforementioned windfall effect should function independently of modern distributive ideologies. We have already argued that a modern
ideology of labor theory of value (‘‘money earned without making effort has little value’’)will not mediate this effect. The same argument goes for other modern distributive
ideologies. For example, the social psychological literature has repeatedly shown individual differences as to ‘‘fair’’ distributive principles (e.g., Deutsch, 1985; Mellers &Baron, 1993). Some people endorse an equality rule as the fairest principle, while others
endorse an equity rule, which demands a balance between individual inputs andoutcomes, as their prime distributive justice system. However, as in the labor value
ideology, we argue that such individual differences will not mediate the windfall effect.
We predict that both equality rule endorsers and equity rule endorsers will be equally

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susceptible to the windfall psychology under conditions where uncertainty in resource
acquisition is high.
1.4. Previous studies and the present research
There are several empirical studies in the decision-making literature that are relevant to theargument we have made thus far (Arkes et al., 1994; Henderson & Peterson, 1992; McLean
Parks et al., 1996). Among them, the Arkes et al. (1994) study is particularly pertinent to thepresent study. By a series of questionnaire surveys and experiments, these researchers showedthat windfall gains were spent more readily than other types of assets. Arkes et al. argued that,
because of their unanticipated nature, windfall gains are likely to be placed in a different‘‘mental account’’ (Thaler, 1990) than other types of predictable income (e.g., monthly
salary). For example, the windfall money may go into a ‘‘fun money’’ account that theindividual has a much higher ‘‘marginal propensity to consume’’ (Keynes, 1936). AlthoughArkes et al. did not distinguish potentially different usage of windfall money conceptually(for personal fun or for social sharing), their argument that a defining characteristic of awindfall gain is its unanticipa-ted status is close to our theoretical perspective focusing on
uncertainty in resource acquisition.
The present research recasts such a windfall effect more systematically from an evolutionary viewpoint. With the risk-reduction notion as a theoretical guide, we focus on
uncertainty as a critical factor for triggering the psychology of social sharing. In this paper,
we report four studies to test the computational algorithm that should characterize thepsychology of sharing, viz., (a) variance information being a key that triggers the communal
sharing psychology, (b) bidirectionality of the windfall effect, and (c) independence of thewindfall effect from modern distributive ideologies. Studies 1 and 2 are scenario experiments
in which we manipulated the degree of uncertainty involved in resource acquisition,
independent of the amount of effort actually invested. Study 3 aims to replicate and extend
results of Studies 1 and 2 using American as well as Japanese samples. Study 4 is a laboratory
experiment in which we attempted a different manipulation of uncertainty and also assessed
participants’ actual (not imaginary) sharing behaviors directly. Thus, as a package, the fourstudies constitute a multisample/multimethod examination of the windfall effect from an
evolutionary perspective.

2. Studies 1 and 2
Studies 1 and 2 were vignette experiments in which the uncertainty (outcome
variance) associated with resource (money) acquisition and the amount of effort actuallyinvested were independently manipulated. Participants received a booklet containing aseries of hypothetical scenarios concerning various resource acquisition situations, andfor each of the situations, stated their willingness to share (WTS) when in the acquirerrole (Study 1) or their willingness to demand sharing when in the nonacquirer role
(Study 2).

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2.1. Method
2.1.1. ParticipantsParticipants in Study 1 were 91 (34 male and 57 female) undergraduate students enrolled in
an introductory psychology class at Sapporo Gakuin University, Japan. Participants in Study
2 were 70 (23 male and 47 female) undergraduate students enrolled in an introductory social
psychology class at Shukutoku University, Japan.
2.1.2. Experimental design
We used a within-subject design with three conditions regarding resource acquisitionmode. Participants were provided with hypothetical scenarios in which they (Study 1) ora friend (Study 2) obtained some money, either (a) contingent on investing substantial
effort; (b) unexpectedly (i.e., high-outcome variance due to low contingency between
effort and outcome) but after investing substantial effort; or (c) unexpectedly with almost
no effort. These conditions are referred to as (a) certain/high-effort, (b) uncertain/higheffort, and (c) uncertain/low-effort conditions, respectively. As implied by the riskreduction perspective, our main theoretical focus is with the comparison of participants’sharing tendencies between the certain/high-effort and the uncertain/high-effort conditions. Example scenarios for these conditions are provided in Table 2.
To control for potentially confounding thematic effects in scenario use, we initiallyprepared six prototype situations (like the ‘‘prize giveaway’’ situation in the above
example). We first picked two of these prototype situations randomly for each of thethree conditions and added the necessary modifications (i.e., degree of effort anduncertainty) to the texts. Then, using a Latin Square design (Winer, 1971), we
permutated the assignment order. This procedure yielded six different variations in
combinations of the prototype situations to the conditions. A booklet with one of the six
variations was assigned to each participant randomly. (Notice that each prototype
situation appeared only once in a single booklet; each participant saw, for example,
the ‘‘prize giveaway’’ situation only once under one of the conditions in the booklet.)Table 2
Example scenarios (‘‘prize giveaway’’) used in Study 1
Certain/high-effort condition
An acquaintance requested you to fill out application forms for a prize giveaway. It was a tedious job to fill out theform. You completed 50 forms in total. Your acquaintance paid you US$100 for this service.
Uncertain/high-effort condition
You decided to apply for a prize giveaway. Although it was a tedious job to fill out the application forms, youcompleted 50 of them to increase the chance to win. Later, you found that you won a prize of US$100.
Uncertain/low-effort condition
You decided to apply for a prize giveaway and submitted one application form. Later, you found that you won a
prize of US$100.

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Thus, the confounding thematic effects that might accrue from particular combinations ofconditions and situations were controlled experimentally.
2.1.3. Procedure
The studies were run in large classrooms. Participants received a booklet containing the six
hypothetical scenarios. For each scenario, participants were asked to rate their WTS themoney with a friend (Study 1) or the extent to which they would demand some share from afriend (Study 2) on three 7-point scales. The items used in Study 1 asked: (a) how likely it
was that they would buy their friend something with the money; (b) how reluctant they would
be to treat their friend with the money if asked to do so (reversed item); and (c) how obliged
they would feel toward buying their friend something with the money. These questions weremodified accordingly in Study 2.
At the end of the booklet, participants were asked to answer additional questions that
identified their primary distributive principles (e.g., equality, equity) through a series ofchoices about various allocation schemes (Ohtsubo, Kameda, & Kimura, 1996).
2.1.4. Experimental hypotheses
Our experimental hypotheses were as follows.
Hypothesis 1: Independent of the amount of effort invested, money obtained
unexpectedly in a high-outcome variance situation will be more likely to be sharedwith others than money obtained in a low-outcome variance situation (Study 1).
Hypothesis 2: Independent of the amount of effort invested, money that another obtainedin a high-outcome variance situation will be more likely to be demanded for social sharing
than money that another obtained in a low-outcome variance situation (Study 2).
Hypothesis 3: These ‘‘windfall effects’’ will be obtained even if we statistically control
for participants’ individual attitudes toward distributive rules (Studies 1 and 2).
2.2. Results
2.2.1. Social sharing as a function of uncertainty
Hypotheses 1 and 2 predict that social sharing should be facilitated by the uncertainty
involved in money acquisition, even though the amount of effort invested is identical. If thesehypotheses are correct, then there should be a significant difference in the sharing tendency
between the certain/high-effort and the uncertain/high-effort conditions.
Figs. 1 and 2 display mean sharing tendencies in the three conditions in Study 1(WTS) and in Study 2 (willingness to advocate sharing, WTAS), respectively. For thesemeans, each participant’s responses to the two scenarios in each condition were averaged
to yield his/her sharing tendency in the condition. Also, since the three response scales
to measure the sharing tendency were correlated with each other (average Cronbach’s
a = .76 in Study 1 and .70 in Study 2), we aggregated them into a composite score.
Means in the figures are based on these composite scores. A higher score indicates

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Fig. 1. Participants’ mean WTS (with standard deviations in parentheses) as a function of uncertainty involved in
the acquisition of money and the amount of effort actually invested (Study 1).

greater sharing tendency. (Since no effects involving participant’s sex were obtained, thedata were collapsed across sex in the figures.)A repeated-measure analysis of variance (ANOVA) yielded a highly significant main effectfor the resource acquisition mode [ F(2,178) = 14.77, P < .001, in Study 1 and F(2,138) = 34.71,
P < .001, in Study 2]. Furthermore, Scheffé post hoc test revealed that the main effect was
mainly due to the difference in sharing tendency between the uncertain/high-effort and thecertain/high-effort conditions [ F(2,178) = 7.81, P < .01, in Study 1 and F(2,138) = 8.01,
P < .01, in Study 2]. The difference between the uncertain/high-effort and the uncertain/loweffort conditions was marginally significant in Study 1 [ F(2,178) = 2.45, P = .09] andsignificant in Study 2 [ F(2,138) = 5.26, P < .01].
To summarize, even though the effort invested was identical, money obtained unexpectedly in a high-outcome variance situation was more likely to be shared (Study 1) and more
likely to be demanded for sharing (Study 2) than money obtained in a low-outcome variancesituation. Thus, Hypotheses 1 and 2 were supported.
2.2.2. Independence of the windfall effect from modern distributive ideologies
Although these results support our argument that uncertainty involved in resource
acquisition triggers the psychology of social sharing, it might be argued that this effect isqualified by one’s ideology about resource distribution. The social psychological literature on
distributive justice has repeatedly shown that people’s attitudes toward distributive rules

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Fig. 2. Participants’ mean WTAS (with standard deviations in parentheses) as a function of uncertainty involved in
the acquisition of money and the amount of effort actually invested (Study 2).

differ substantially from each other (see Deutsch, 1985; Mellers & Baron, 1993, for acomprehensive review). Some people prefer the equity rule emphasizing a balance between
individual inputs and distributive outcomes, whereas others prefer the equality rule as theirprime distributive justice system.
To see if such individual differences in distributive ideologies may qualify the windfall
effect, we classified participants into equity rule endorsers or equality rule endorsers based on
their choice patterns about desirable distributions (Ohtsubo et al., 1996), and then added thisideology factor to the original ANOVA.
A 2 (Distributive Ideology)  3 (Resource Acquisition Mode) ANOVA with the second
factor as repeated measures again revealed a main effect for the resource acquisitionmode [ F(2,166) = 9.40, P < .01, in Study 1 and F(2,124) = 27.00, P < .001, in Study 2]. A
main effect for the distributive ideology was also significant in Study 1 [ F(1,83) = 4.94,
P < .05], indicating that equality rule endorsers were more willing to share (mean = 3.19)
than equity rule endorsers (mean = 2.56), but this effect failed to reach significance inStudy 2 [ F(1,62) = 1.79, ns]. However, most important to our concern here, nointeraction effect was obtained in either study [ F(2,166) = 1.90, ns, in Study 1 andF(2,124) = 0.03, ns, in Study 2]. Responses to uncertainty on the part of equality ruleendorsers were indistinguishable from those of equity rule endorsers, as predicted byHypothesis 3.

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2.3. Discussion
The results of Studies 1 and 2 supported all three hypotheses about uncertainty andresource sharing. Even though the amount of effort invested was identical, obtaining money
in a high-outcome variance situation was more likely to trigger a ‘‘psychology of social
sharing’’ than obtaining it in a low-variance situation, with an essentially isomorphic
pattern of results for both the acquirer (Study 1) and nonacquirer (Study 2) roles.
Furthermore, this windfall effect was observed even after statistically controlling formodern distributive ideologies.
Although these results are consistent with our reasoning, several methodologicallimitations should be considered. First, these were vignette studies using imaginary
scenarios and did not assess participants’ actual sharing behavior directly. Second, although
we attempted to control confounding thematic effects in scenario use experimentally (bycreating different stimulus patterns via Latin Square design), some uncontrolled features
might have biased the results. We will address these two problems that essentially accrue
from scenario use in Study 4.
Thirdly, the social psychological literature on distributive justice has reported crosscultural differences in preferred distributive principles (e.g., Bond, Leung, & Wan, 1982;
Kashima, Siegal, Tanaka, & Isaka, 1988; Siegal & Shwalb, 1985). For example, Bond et al.
(1982) showed that Chinese preferred equality in reward allocation to a larger extent thandid Americans; Kashima et al. (1988) and Siegal and Shwalb (1985) reported acorresponding difference between Japanese and Australians. Given such cross-culturaldifferences, the focal windfall effect may also be culture- or society-specific. Thus, inStudy 3, we conducted a cross-cultural validation of the windfall effect, using Japanese andAmerican samples.

3. Study 3
The purpose of Study 3 was twofold. First, we aimed to replicate the previous twostudies using a different (but again Japanese) sample. Second, we also administered thesame questionnaire to an American sample to see if there were cross-cultural/societal
differences in the sharing tendencies under uncertainty. Our experimental hypotheses wereidentical to Hypotheses 1–3 in Studies 1 and 2. If the Japanese and American samples inStudy 3 show a conceptually converging pattern with each other and also replicate theresults of Studies 1 and 2, then our confidence in the robustness of the windfall effect will
be heightened substantially.
3.1. Method
3.1.1. Participants
Japanese participants were 88 (70 male and 18 female) undergraduate students enrolled in
an introductory social psychology class at Mie University. American participants were 162

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T. Kameda et al. / Evolution and Human Behavior 23 (2002) 11–33(49 male and 113 female) undergraduate students enrolled in introductory psychology classes
at Loyola University-Chicago and Grand Valley State University.
3.1.2. Experimental design and procedure
Experimental design and procedure were identical to Studies 1 and 2 except thatparticipants experienced both the acquirer and nonacquirer roles in the same questionnaire.
Again to control for potentially confounding thematic effects in scenario use, we used a Latin
Square design in assigning six prototype situations to six conditions, i.e., 3 (Resource
Acquisition Mode)  2 (Acquirer or Nonacquirer), and added necessary modifications to thetexts. This procedure yielded six different variations in combinations of the prototype
situations to the conditions.
Another minor modification from Studies 1 and 2 was that, in addition to the
distributive ideologies, we also assessed participants’ attitudes concerning labor values
at the end of the questionnaire. Participants responded to three attitudinal items on labor
values: ‘‘money earned without making effort has little value,’’ ‘‘hard work is honorable,’’ and ‘‘the value of a thing is determined by how much effort you have made toobtain it.’’
The questionnaire for the American sample was translated by one of the authors fromJapanese to English and then back-translated into Japanese by an independent person who
was ignorant of the research purpose. We then checked the final version against the original
and found them to be identical in meaning. Comparability of the Japanese questionnaire
and the American questionnaire was thus assured.
3.2. Results
Again, no effects involving participant’s sex were obtained. Thus, responses werecollapsed across sex in the following analyses.
3.2.1. Social sharing as a function of uncertainty
3.2.1.1. Willingness to share. Fig. 3 displays mean ‘‘WTS’’ scores when in the acquirer rolein the Japanese and American samples. A 2 (Country)  3 (Resource Acquisition Mode)
ANOVA with the second factor as repeated measures yielded a significant main effect for
country [ F(1,244) = 5.80, P < .05]. As can be seen from the figure, the Japanese sample
tended to be more ‘‘generous’’ than the American sample, essentially replicating the previous
finding that Japanese prefer equal distributions more than Americans (cf. Bond et al., 1982).
More important to our concern here, a main effect for the resource acquisition mode was
highly significant [ F(2,488) = 29.33, P < .001]. The interaction effect was also significant[ F(2,488) = 10.54, P < .001], reflecting a greater differential impact of resource acquisitionmode in the Japanese sample than in the American sample. However, planned contrasts
separately conducted for each sample revealed that American participants’ mean WTS wassignificantly higher in the uncertain/high-effort condition than in the certain/high-effort
condition [ F(1,314) = 4.10, P < .05]. This comparison was also significant in the Japanese

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Fig. 3. Japanese and American participants’ mean WTS (with standard deviations in parentheses) when in theacquirer role (Study 3).

sample [ F(1,174) = 22.76, P < .001]. Therefore, our Hypothesis 1 was supported for both
Japanese and American samples.
3.2.1.2. Willingness to advocate sharing. Fig. 4 displays mean ‘‘WTAS’’ scores when in
the nonacquirer role in the Japanese and American samples. A 2 (Country)  3(Resource Acquisition Mode) ANOVA with the second factor as repeated measures
yielded a significant main effect for country [ F(1,248) = 19.67, P < .001]. The Japanesesample tended to be more ‘‘demanding’’ than the American sample—a mirror image of
their greater ‘‘WTS’’ tendencies.1 More importantly, a main effect for the resource
acquisition mode was highly significant [ F(2,496) = 39.28, P < .001]. Although theinteraction effect was significant [ F(2,496) = 14.06, P < .001], planned contrasts separately conducted for each sample again revealed that American participants’ mean
WTAS was higher in the uncertain/high-effort condition than in the certain/high-effort

1
The editors pointed out that the difference in sample structure about sex (predominance of males in theJapanese sample and females in the American sample) might be responsible for the observed cross-national
differences. To assess this possibility, we reanalyzed data separately for each sex. The separate analyses again
yielded significant cross-national differences. Collapsed across the resource acquisition mode, Japanese males
were more willing to share (mean = 3.29) and more willing to advocate sharing (mean = 3.08) than American
males [mean = 2.87, F(1,114) = 3.32, P = .07; mean = 2.31, F(1,117) = 6.46, P < .02]. Likewise, Japanese females
were more willing to share (mean = 3.37) and more willing to advocate sharing (mean = 3.17) than American
females [mean = 2.88, F(1,127) = 3.31, P = .07; mean = 2.22, F(1,128) = 11.84, P < .001]. Thus, the cross-national
differences in the overall sharing tendencies between the Japanese and American samples held for each sex.

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Fig. 4. Japanese and American participants’ mean WTAS (with standard deviations in parentheses) when in the
nonacquirer role (Study 3).

condition [ F(1,322) = 5.19, P < .05]. This comparison was also significant in the Japanesesample [ F(1,174) = 16.80, P < .001]. These results support our Hypothesis 2.
3.2.2. Independence of the windfall effect from modern distributive/labor ideologies
3.2.2.1. Distributive ideologies. As in Studies 1 and 2, we categorized participants into‘‘equality rule endorsers’’ or ‘‘equity rule endorsers’’ based on their choices about desirable
distributions (Ohtsubo et al., 1996), and added the ideology factor to the original ANOVAs.
Separate 2 (Country)  2 (Distributive Ideology)  3 (Resource Acquisition Mode) mixed
ANOVAs on the ‘‘WTS’’ and ‘‘WTAS’’ scores replicated the results of the earlier ANOVAs.
The main effects for the resource acquisition mode [ F(2,436) = 16.36, P < .001, for WTS andF(2,444) = 23.33, P < .001, for WTAS] and the Country  Resource Acquisition Modeinteraction [ F(2,436) = 3.76, P < .05, for WTS and F(2,444) = 5.68, P < .01, for WTAS] weresignificant. However, more important to our concern here, no interaction effect was observed
involving distributive ideology and the resource acquisition mode [ F(2,436) = 0.16, ns, for
WTS and F(2,444) = 0.66, ns, for WTAS]. As predicted by Hypothesis 3, the sharing patternin response to the uncertainty level was indistinguishable between the equality rule endorsers
and equity rule endorsers.
3.2.2.2. Labor values. A similar analysis was also conducted with the data on attitudes
toward labor values. For the Japanese and American samples separately, participants weresplit into categories with relatively positive or negative attitudes toward the labor value

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25

ideology (50 percentile in each sample was used as a cutoff point). Separate 2 (Country)  2(Labor Value Ideology)  3 (Resource Acquisition Mode) mixed ANOVAs on the ‘‘WTS’’and ‘‘WTAS’’ scores again yielded the main effect for the resource acquisition mode
[ F(2,484) = 19.33, P < .001, for WTS and F(2,492) = 25.15, P < .001, for WTAS] and theCountry  Resource Acquisition Mode interaction [ F(2,436) = 7.78, P < .01, for WTS andF(2,492) = 10.90, P < .001, for WTAS]. However, no interaction effect involving labor value
ideology, and the resource acquisition mode was significant [ F(2,484) = 0.77, ns, for WTS
and F(2,492) = 1.16, ns, for WTAS]. In conjunction with the absence of interaction effect withthe distributive ideology just mentioned, these patterns support our Hypothesis 3 that thewindfall effect is independent of modern sharing ideologies.
3.3. Discussion
On both the ‘‘WTS’’ and the ‘‘WTAS’’ scores, the Japanese sample showed a greater
sharing tendency than the American sample, which may reflect cultural/societal differenceson the value of equality as observed in the previous studies (e.g., Bond et al., 1982). Thisimplies that socialization in different cultures indeed has sizable and perhaps prime impacts
on people’s actual sharing behaviors. However, besides the cultural/societal differences, it
should be noted that the identical manipulations about the degree of uncertainty involved inresource acquisition had the same directional impacts across the two countries. In bothcountries, money obtained unexpectedly in a high-outcome variance situation was more
likely to be shared and more likely to be demanded for sharing than money obtained in a lowoutcome variance situation. This suggests that the windfall effect, albeit subtler than the
cross-cultural/societal effects, may indeed be a universal psychological or behavioral
phenomenon. Furthermore, the results indicate that such a windfall effect was not qualified
by modern distributive ideologies or labor values.
The converging results of the three studies certainly heighten our confidence about therobustness of the windfall effect, yet another methodological problem, as outlined earlier,still remains. That is, the three studies reported so far all used hypothetical scenarios anddid not directly assess participants’ actual sharing behavior. Moreover, the use of
scenarios, in principle, can never be free from the possibility of confounding thematiceffects, even though we tried to control them experimentally via a Latin Square design(see Lanza, 1990, for a methodological discussion about how to enhance the ecologicalvalidity of vignette experiments). Thus, in Study 4, we attempted to replicate the results ofStudies 1 through 3 conceptually using a laboratory experiment that did not rely on
hypothetical scenarios.

4. Study 4
In Study 4, we tested Hypotheses 1 and 3 (sharing behavior when in the acquirer role),
with a laboratory experiment using a Japanese sample. If a conceptually parallel pattern was
obtained in the laboratory experiment as well, our concern that the windfall effect may be an

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T. Kameda et al. / Evolution and Human Behavior 23 (2002) 11–33

artifact arising from some thematic or other confounding factors associated with scenario use
would be reduced.
4.1. Methods
4.1.1. ParticipantsParticipants in Study 4 were 65 (41 male and 24 female) undergraduate students enrolled in
introductory psychology classes at Hokkaido University, Japan.
4.1.2. Experimental design
There were two conditions in which participants received a fixed amount of money either
in a deterministic manner (‘‘certain condition’’) or in a stochastic manner where high variance
was associated with the money acquisition (‘‘uncertain condition’’ hereafter). The amount ofeffort invested to obtain the money was kept identical across the two conditions. Participants
were assigned to one of the conditions randomly.
4.1.3. Procedure
Upon arrival, each participant was seated in a private booth and received further
instructions individually via a computer. Participants were told that the purpose of theexperiment was to investigate the effects of monetary reward on cognitive performance, andthat they would be asked to work on 30 arithmetic problems individually. Before they actually
started working, the reward for solving one problem correctly was decided either in a
deterministic manner (certain condition) or in a stochastic manner (uncertain condition).
In the certain condition, participants were told, ‘‘as a unit reward per problem, we have
five conditions ranging from ¥5 to ¥25. You have been assigned to the ¥25 per problem
condition.’’ In the uncertain condition, participant’s unit reward was determined by ‘‘using aroulette wheel of fortune’’ with five slots ranging from ¥5 to ¥25; however, in actuality, theroulette was preset to always stop at the ¥25 slot. Thus, these two conditions differed withrespect to how the participants acquired their entitlement to the advantageous ¥25-unit
reward, either assigned by the experimenter or by pure chance. Notice that the modern
notion of property rights makes no distinction about the legitimacy of the entitledownership between these two conditions. However, if our minds are sensitive to varianceinformation associated with resource acquisition, then participant’s subjective legitimacy ofmonopolizing the money should be weakened, leading to more social sharing in theuncertain condition.
After the unit reward was determined this way, participants started working on the 30
arithmetic problems on the computer. They were instructed that their performance speed
would be timed and that they could not proceed to the next problem until they had answered
the current problem correctly. Thus, all participants received an identical amount(¥750 = US$7) at the end as a total reward. The average working time required for solving
30 problems correctly was 18.7 min (S.D. = 4.8 min).
After completing the calculation task, participants were told that their experimental
reward would be ¥750. While waiting for others to finish, each participant received a

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27

Fig. 5. Distributions of individual donations as a function of uncertainty involved in the acquisition of money(Study 4).

message from the experimenter soliciting a donation of some money from the reward‘‘to help participants in another, unrelated experiment.’’ Notice that this request wasmade to help individuals who had no prior histories of repeated interaction with theparticipant. Except that those individuals were also undergraduate students of Hokkaido
University (a common in-group), no enduring pairwise relations with the participant(e.g., friendship), such as those found in restricted exchange, existed. Thus, theexperimental situation featured core elements of generalized exchange, in terms of boththe number of recipients and the absence of enduring pairwise relations.2 The solicitation
for a donation was made through a chat window on the computer screen, and aparticipant was able to input a figure from ¥0 to ¥750 that he/she thought wasappropriate for this request. The amount of the donation that a participant made served
as an index of the sharing tendency in the respective conditions. A couple of minutes
after the solicitation, participants received a postsession questionnaire concerning theirdistributive ideologies and attitudes toward labor values. Participants were then debriefed, paid, and dismissed.

2

In the preceding three studies, participants were asked about their sharing tendencies toward a ‘‘friend.’’ As
the reviewers pointed out, this procedure caused some mismatch between our theoretical argument and theexperiments: the sharing context in Studies 1 through 3 featured a restricted exchange with a single individual
having a history of repeated interaction, rather than a generalized exchange. Study 4 addressed this criticism bytesting whether the windfall effect observed in the preceding three studies could be replicated in a sharing context
that incorporated core features of generalized exchange.

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T. Kameda et al. / Evolution and Human Behavior 23 (2002) 11–33

4.2. Results and discussion
4.2.1. Social sharing as a function of uncertainty
Fig. 5 displays distributions of individual donations in the two conditions. Participants inthe uncertain condition shared more than those in the certain condition. A Mann–Whitney U
test (the distributions in Fig. 5 were skewed and the usual normality assumption was not met)
revealed that the difference was significant, z = 1.81, P < .05 (one-tailed).
Since participants in both conditions worked on the identical calculation task, there is no
reason to believe that they invested different amounts of effort across conditions. Indeed, there
was no sizable difference in mean working time, mean = 19.5 min in the certain condition andmean = 18.0 min in the uncertain condition, t(63) = 1.31, ns. Furthermore, a logistic regressionof the amount of donation3 revealed only a significant effect of the resource acquisition mode[c2(1, n = 63) = 6.98, P < .01]. The working time had no effect [c2(1, n = 63) = 1.83, ns].
Thus, our Hypothesis 1 was supported in a laboratory experiment as well; independent of
the amount of effort invested, uncertainty involved in resource acquisition facilitated thepsychology of social sharing.
4.2.2. Relation to modern sharing ideologies
4.2.2.1. Distributive ideologies. As in previous studies, we categorized participants into‘‘equality rule endorsers’’ or ‘‘equity rule endorsers’’ based on their choices about desirable
distributions. A 2 (Distributive Ideology)  2 (Resource Acquisition Mode) hierarchical loglinear analysis of the amount of donation (treated as a binary variable; see footnote 3)
revealed only a main effect of the acquisition mode [c2(1, n = 59) = 3.97, P < .05]. Theinteraction effect was not significant [c2(1, n = 59) = 0.06, ns], implying that the sharing
pattern in response to uncertainty was not qualified by participant’s distributive ideology, as
predicted by Hypothesis 3.
4.2.2.2. Labor values. Similarly, a 2 (Labor Value Ideology)  2 (Resource Acquisition
Mode) hierarchical log-linear analysis revealed only a main effect of the acquisition mode[c2(1, n = 65) = 6.41, P < .05]. The interaction effect was once again not significant [c2(1,n = 65) = 1.32, ns]. Taken together, these patterns support our Hypothesis 3 that the windfall
effect is not qualified by modern sharing ideologies.

5. General discussion
This paper has explored a computational algorithm (Cosmides & Tooby, 1992) that
underlies the psychology of social sharing by empirically focusing on the ‘‘windfall effect’’3

Since the amounts of donation were distributed in a skewed manner (see Fig. 5), we transformed the amounts
into upper or lower 50 percentile, and treated them as binary in the following analyses.

T. Kameda et al. / Evolution and Human Behavior 23 (2002) 11–33

29(e.g., Arkes et al., 1994). Based on a risk-reduction notion (Kaplan & Hill, 1985; Kaplan et al.,1990), we hypothesized that a key factor for triggering the psychology of social sharing is thedegree of uncertainty associated with resource acquisition. It was predicted that, independentof the amount of effort actually invested, high-variance information (i.e., high uncertainty)
should facilitate the sharing mode for both acquirers and nonacquirers of the resource, andthat such a windfall psychology should operate independently of individual preferences forvarious modern distributive ideologies. Four multisample/multimethod studies, using Japanese and American participants and laboratory as well as vignette experiments, under theidentical conceptual framework consistently supported these predictions. These results, ofcourse, do not imply that other factors such as invested efforts, modern distributive
ideologies, different cultural/societal values, etc., are irrelevant to social sharing. (Indeed,in our experiments, these variables did affect people’s actual sharing behaviors.) Nor do theseresults mean that high-variance information is the strongest predictor of sharing breadth anddepth (cf. Gurven, in preparation). What we have demonstrated is that, even controlling forthese factors, our minds are still sensitive to variance information associated with resource
acquisition; high-variance information is an essential ingredient of a computational algorithm
that underlies social sharing, as envisioned by Cosmides and Tooby (1992).
Given the successful demonstration of such a windfall algorithm, we now turn to its
broader theoretical implications for human resource sharing. Specifically, we speculate about
potential relevance that the windfall effect may have for the egalitarian ideology. The
windfall effect we have examined in this paper was people’s increased WTS and increased
demand to be shared with, as a function of uncertainty involved in resource acquisition. It isthus important to remember that this paper did not demonstrate an increased tendency
towards egalitarian sharing (‘‘divide by n’’ rule) per se. Nevertheless, we believe that thetheme of uncertainty may also have some relevance to ‘‘egalitarianism’’ observed not only inhunter–gatherer bands (cf. Boehm, 1993; Cashdan, 1989) but also in highly industrializedsocieties broadly.
5.1. Egalitarian resource sharing and windfall psychology
5.1.1. Equality as a social decision heuristic
Resource sharing involving two or more individuals has been a major research concern not
only in anthropology but in social psychology as well. Recent social psychological researchon group decision-making suggests that, when people have to decide on which sharing rules
are appropriate, one decision rule, egalitarian division, often emerges as a prominent solution(e.g., Allison, McQueen, & Schaerfl, 1992; De Vries & Wilke, 1992; Ohtsubo & Kameda,
1998). To summarize these empirical findings, Messick (1993, 1995) argued that ‘‘shareequally’’ serves as a social decision heuristic in many complex decision-making situations.
Messick proposed that equality is qualified as a social decision heuristic due to the following
three characteristics. First, it is simple and easily understood. Second, equality is effective inthe sense that its use usually leads to an unambiguous choice or allocation. Third, the use ofequality is relatively easy to justify to those people affected by the social decision.

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The first two characteristics that Messick (1993, 1995) ascribed to equality are essentiallyfeatures often attributed to various cognitive heuristics in the decision-making literature:
cognitive simplicity and frugality along with reliability of outcomes (e.g., Gigerenzer, Todd,& the ABC Research Group, 1999; Kahneman, Slovic, & Tversky, 1982). However, noticethat the third characteristic that he ascribed to equality is not cognitive: ‘‘share equally’’ issocially justifiable. Then, an important question surfaces: why and under what conditions isequal sharing regarded as the just rule? We believe that the windfall psychology may provide
one answer to this question.
An ultimatum bargaining game study by Polzer, Neale, and Glenn (1993) is particularly
informative on this point. Ultimatum bargaining is a two-person game in which Player 1
divides a resource and Player 2 then decides to either reject or accept the division. If Player 2
declines the division, both players receive nothing (Güth, Schmittberger, & Schwarze, 1982).
Polzer et al. (1993) demonstrated that Player 1’s offers were influenced critically byjustifications for being permitted to divide first. When participants earned the Player 1
position by scoring high in a preceding task, the mean offer was 80% of what Player 1
proposed to retain for him or herself. However, when participants were assigned randomly to
the Player 1 position, the mean offer rose to 97% of the sum retained. Samuelson and Allsion(1994), who investigated people’s sequential harvesting behavior from a common resource
pool, reported a parallel finding. For example, when participants were assigned their early
position because ‘‘their birthday was nearest to a date selected at random earlier by theexperimenter,’’ they harvested much less (i.e., requested shares closer to equal division) fromthe common pool, than when they had superior ‘‘justifications.’’
These differences have often been explained by somewhat coarse notions like justifiabilityor entitlement. However, notice that in both studies, uncertainty (i.e., high variance) involvedin the assignment of the advantageous, early positions played a key role in determining why
such an ‘‘entitlement’’ operated strongly in one setting and not in another. Our Study 4,
which manipulated uncertainty in assigning participants to the advantageous rewardcondition, yielded conceptually parallel results. Taken together, these results suggest thathigh-variance information involved in resource or resource status acquisition plays a criticalrole for an egalitarian division to be perceived as the ‘‘just’’ rule in social sharing. In otherwords, adaptive advantage of variance reduction by resource pooling may provide an
ultimate reason why people feel less entitled (i.e., that it is less legitimate) to monopolize
unexpected gains.
5.1.2. Is equal sharing under uncertainty always conducted willingly?
The ‘‘just’’ view of egalitarian sharing may seem to imply that this rule is internalized as abasic moral value that binds us, unconditionally, under uncertainty. However, we feel that thisis perhaps an overstatement. For example, in the aforementioned Polzer et al. (1993) andSamuelson and Allsion (1994) studies, participants’ mean sharing behavior under uncertainty
was close to egalitarian sharing, but the distribution was positively skewed with some deviant
individuals favoring more ‘‘selfish’’ divisions. Similarly, as an acquirer of resources (e.g.,Player 1 in the ultimatum game), participants were highly sensitive to nuances of justifications provided for their early positions, often interpreting the justifications toward their

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31

personal advantage. In other words, it seems that an acquirer of a resource under uncertainty
shows some egalitarian tendency behaviorally, but not always ‘‘willingly.’’ One interesting
study illustrates this point. Eckblad and von der Lippe (1995) investigated 261 lottery winners
of prizes of 1 million Norwegian krone ( = US$150,000). Those winners were asked about
various psychological reactions after winning the prizes. One of the most frequent reactions
among those respondents was a wish for anonymity, together with fear of envy from others!
These observations seem to suggest that social sharing under uncertainty essentially may
be characterized as a ‘‘vigilant sharing’’ (Erdal & Whiten, 1994), in which sharing is
conducted because of vigilant and envious eyes of nonacquirers who are immediate
beneficiaries of sharing. In other words, nonacquirers may play a more active, initiative rolein social sharing, implicitly or explicitly, than the acquirer of the resource (cf. Bliege Bird &Bird, 1997; Blurton Jones, 1987; Hawkes et al., 2001; Peterson, 1993). Notice that this view
is not contradictory to the risk-reduction perspective, since those who face the immediate riskof resource shortage are nonacquirers and not the acquirer of the resource. (See Kameda et al.,submitted, for an evolutionary game model of resource sharing under uncertainty based onthese ‘‘demand sharing’’ notions.) In any event, this reasoning suggests that there may besome asymmetry between acquirers and nonacquirers, with the psychology of windfalls being
more easily and/or more vigorously activated among nonacquirers. Given that the modern
notion of property rights should operate in exactly the opposite way (i.e., sharing is at theacquirer’s discretion), this poses an intriguing possibility awaiting future investigations.
As often pointed out by various theorists, human resource sharing, including food sharing,is a complex phenomenon, and its adaptive origins are most likely manifold. Given thiscomplexity, a multidisciplinary approach seems to be essential to better understand thephenomenon. We followed such a path in this paper, by linking social psychologicalexperimentation to anthropological findings to shed some light on the psychology of social
sharing. Much remains to be done, but we believe that a multidisciplinary approach under theevolutionary perspective will eventually clarify the computational algorithm underlying
social sharing in a rigorous manner.

Acknowledgments
This research was supported by the Grant-in-Aid for Scientific Research 11610096 from
the Ministry of Education, Culture, Sports, Science, and Technology of Japan. We are gratefulto Nobuhito Jin and Motohiko Nagata for their generous help in data collection.

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