CURRENT DIRECTIONS IN PSYCHOLOGICAL S CIENCE

Where Do Social Norms ComeFrom?
The Example of Communal Sharing
Tatsuya Kameda,1 Masanori Takezawa,2 and Reid Hastie3
Department of Behavioral Sciences, Hokkaido University, Sapporo, Japan; 2Max Planck Institute for Human
Development, Berlin, Germany; and 3Center for Decision Research, Graduate School of Business, University of Chicago
1OF

sponse that achieves high levels of individual welfare (or an
equilibrium accruing from self-interested individual strategic
choices). This theme of adaptive functionality comes with acollection of theoretical methods—most notably, in the present
application, game theory. The remainder of this essay describes
an illustrative application of evolutionary game theory to explain
the development of a social norm for communal sharing. This is anorm about social exchange, designating uncertain resources ascommon goods to be shared with other members of a social group.
We conclude with some surprising manifestations of this norm inmodern social life.

PR

answer must surely lie in such norms’ ability to support
individual adaptive success in local ecologies. This theme isdominant in analyses of social behavior by economic gametheorists and behavioral-ecology researchers, but it has
been neglected by psychologists. An illustration of themethods and advantages of the adaptationist approach tounderstanding the emergence of social norms is provided.
Some surprising behavioral results from modern industrial
societies that reflect social-sharing norms of modern
hunter-gatherer societies are consistent with our adaptive
analysis.

O

ABSTRACT—Where do social norms come from? Part of theD

KEYWORDS—social norm; adaptation; evolutionary game;TE

local ecologies

R

O

C

N

U

(BWUS CDIR 392.PDF 03-Jan-06 16:50 81052 Bytes 4 PAGES n operator=M.V.Anantha)

R

EC

The concept of social norms is one of the most central theoreticalconstructs in the social sciences—including sociology, law,political science, anthropology, and increasingly economics(e.g., Axelrod, 1986). But, its status in modern psychology has
been less firm, and the notion of social norms has been criticizedfor not being able to predict behavior in some real, complex
social contexts. We find the peripheral status allotted to social
norms in psychology to be conceptually inappropriate. We want
to return this venerable, fundamental construct to its proper
central role in social-psychological theories.
We propose a general unifying theme for research on social
behavior, one that is familiar in the other behavioral sciences(see Cosmides & Tooby, 1992, and Gigerenzer, Todd, & the ABC
Research Group, 1999, for similar proposals in evolutionary
psychology and in judgment and decision making): Most important social behavior can be understood as an adaptive reAddress correspondence to Tatsuya Kameda, Department of Behavioral Science, Hokkaido University, Sapporo, 060–0810, Japan;
e-mail: tkameda@let.hokudai.ac.jp.

336

THE COMMUNAL-SHARING NORM IN HUNTERGATHERER SOCIETIESSharing important resources, such as food, with non-kin associates is a general practice in human societies. Although aprimitive form of food sharing is known in some primates (deWaal, 1996), no primates other than humans have broad socialsharing habits. Anthropologists have studied social exchange
and sharing in various hunter-gatherer societies to explore its
origins and early forms. Kaplan and Hill (1985) observed thatfood transfers among the Ache foragers, who live in subtropicaleastern Paraguay, show markedly different patterns between
hunted meat (e.g., peccary) and collected resources (e.g., vegetables). While some collected resources are shared with nonfamily members, hunted meat is much more likely to be thetarget of communal sharing (see Gurven, 2004, for worldwide
ethnographic examples including forager-agriculturalists aswell as hunter-gatherers). Because kin-sharing is a universal
practice across many species (Hamilton, 1963), the central
question here is why hunted meat is shared communally beyond
the acquirer’s direct kin and why different sharing norms apply
to different resources within the same culture.
Kaplan and Hill (1985) explained these differences in terms ofthe degree of uncertainty involved in resource acquisition.

Copyright r 2005 American Psychological Society

C D I R3 9 2

Journal Name

Manuscript No.

B

Volume 14—Number 6

Dispatch: 3.1.06

Journal: CDIR

CE: Blackwell

Author Received:

No. of pages: 4

PE: Saravan/Suresh

Tatsuya Kameda, Masanori Takezawa, and Reid Hastie

TABLE 1
Four Behavioral Strategies in the Evolutionary-Game Model When Resource Acquisition (Hunted Meat) is Uncertain
When one is an unsuccessful hunter:

Saint

EgoistBourgeois

modeling such adaptive dynamics (Maynard Smith, 1982; Gintis, 2000). Evolutionary game theory is different from classicalgame theory in that it does not assume that players possess
perfect information; instead, it represents various behavioral
tendencies as strategies in a game and examines how eachstrategy performs against other strategies in terms of net profit.
Even though a given strategy may be limited by players’ information-processing capacity, it proliferates gradually in the
population if it can perform better than the other strategies.
We (Kameda, Takezawa, & Hastie, 2003) developed an evolutionary-game-theory model for the emergence of communalsharing norm when foraging under conditions of uncertainty. Ourmodel assumed that, due to the highly uncertain nature of meatacquisition, an individual hunter constantly faces two kinds ofdecision problems: How to behave when successful and how
to behave when unsuccessful. This analysis yields four mutually-exclusive and exhaustive behavioral strategies depicted inTable 1; each individual in a group is assumed to behave according to one of these strategies. The model also posited that,
due to the highly uncertain nature of hunting, acquisition of meatby some members yields a large asymmetry in resource level
between haves and have-nots (cf., the ‘‘twists of fate’’ situation asconceived of by Kelley et al., 2003). A hunter’s attempt to monopolize the meat under such situations can lead to fights withother community members who demand communal sharing,incurring a cost to each loser. The theoretical question thenbecomes whether the ‘‘communal sharers,’’ the purest supportersof the sharing ideology (see Table 1), outperform other types ofmembers (‘‘egoists’’ in particular) in fitness. If communal sharers
perform well, the evolutionary logic implies that they will proliferate and dominate in the group, resulting in the establishmentof a communal-sharing system.

O

While provision of vegetables and fruits is relatively stable anddependable, acquisition of meat is a highly variable, uncertain
prospect. On average, there is a 40% chance that an Ache hunter
will come back from a hunt empty-handed. It is essential for the
Ache to manage the variance associated with meat acquisition,
securing a stable supply of the precious resource. Storage by
freezing or other preservation methods is not efficient in ahunter-gatherer situation. Kaplan and Hill (1985) argued that,instead, the sharing system functions as a collective risk-reduction device. By including many individuals in the riskpooling group, the variance in meat supply decreases exponentially. Once established and maintained, the communalsharing system can buffer the variance in the meat supply.

Communal sharer

F

Distributes provisions
as a common property
Claims private ownership of meat

Grants successful hunter’s
private ownershipO

When one is a successful
hunter:Demands share of meatas a common property

PR

EVOLUTIONARY COMPUTER SIMULATIONS

R

O

C

N

U

(BWUS CDIR 392.PDF 03-Jan-06 16:50 81052 Bytes 4 PAGES n operator=M.V.Anantha)

R

EC

TE

D

Problem of Egoism
The risk-reduction hypothesis is a functional explanation; thecommunal-sharing system serves the survival of the wholegroup. Yet, from the adaptationist perspective that focuses on
fitness outcomes to each individual, this explanation leaves one
critical question unanswered: the problem of egoism in social
dilemmas (Dawes, 1980). Hunted meat, especially when a large
portion is acquired, is regarded as a common property in mosthunter-gatherer societies; the process of meat distribution istreated as appropriation from the public domain. Then, what if
some individuals behave as egoists who share in other people’s
acquisitions but are unwilling to share their own acquisitions
with others? Such egoists might be better off in terms of individual fitness than are those who are loyal to the communalsharing norm. If so, the Darwinian logic implies that such egoists
would proliferate in a group, eventually dominating the group.
The risk-reduction explanation is incomplete, because it is silent about how the proliferation of such egoists is suppressed.

Evolutionary Games
Social dynamics as illustrated above are analogous to biologicalcompetition for an ecological niche in that a behavioral trait that
produces the highest fitness outcomes spreads and eventually
dominates in a population. Biologists and economists have developed a mathematical tool, evolutionary game theory, forVolume 14—Number 6

Computer Simulations
A series of evolutionary simulations in which model parameters(group size, resource value, fighting cost) were varied systematically revealed the following results. First, even when communal sharers were introduced as a rare ‘‘mutant’’ strategy into
an egoist-dominant group, they overcame the initial handicap infrequency and dominated the group rather quickly, within a few
hundred iterations (‘‘generations’’). Second, once dominant, thecommunal sharers continued to outperform any other mutant

337

Communal-Sharing Norm

F

O

PR

SOME REMARKABLE REFLECTIONS OF THE
COMMUNAL-SHARING NORM IN INDUSTRIALIZEDSOCIETIES

uncertainty factor was manipulated independently of the effort
factor. Japanese and American participants were provided with aseries of hypothetical scenarios in which they (or a friend) obtained some money, either (a) contingent on investing substantial
effort, (b) unexpectedly but after investing substantial effort (i.e.,
low contingency between effort and outcome; chance was another key factor for success, yielding high outcome variance inthe situation), or (c) unexpectedly with almost no effort. Participants were then asked to rate their willingness to share themoney with a friend (or the extent to which they would demandsome share from a friend). Both Japanese and American participants were more willing to share (and demand more sharingfor) the unexpected money, even when the amount of effort invested was identical for expected and unexpected gains. More
importantly, these differences were significant, even when personal ideologies about desirable distribution were controlled for.
Endorsers of merit-based ideology and of egalitarian ideology
were both affected by the uncertainty factor. This was also
confirmed by a laboratory experiment (Study 4 in Kameda et al.,2002). After being paid for their work during the experiment,
participants were solicited to donate some money to help participants in another, unrelated experiment. Even though they
had received the identical amount of money for the identicalamount of work, participants whose final rewards were determined in a random manner by using a roulette wheel of fortune
made a greater donation than those who were rewarded in a
deterministic manner. Notice that the modern notion of property
rights makes no distinction between the legitimacy of entitledownership between these two conditions.

Ostrategies (egoists, saints, bourgeois; see Table 1) in fitness, thus
blocking their intrusions into the group. In all simulation runs,
the dominance of communal sharers continued over thousands ofgenerations.
In terms of evolutionary game theory, these results imply that
communal sharing is an evolutionarily stable strategy. The egoist
strategy does not qualify as such, because egoists’ attempts to
defend their own acquisitions against many ‘‘have-nots’’ (including other egoists who were unsuccessful) tax them heavily infighting costs. But how sensitive is this result to model parameters such as group size, resource value, and fighting cost? A
sensitivity analysis, whereby we varied the parameters systematically, revealed that the communal-sharing strategy was indeed robust. For instance, except in rather unrealistic conditions
in which the cost of potential injury accruing from a fight wasessentially nonexistent (i.e., smaller than 0.3% of the resource
value), the communal-sharing strategy always qualified as anevolutionarily stable strategy. In other words, the communalsharing norm emerged and was sustained under a broad range ofparametric conditions as a result of individual fitness maximization, while overcoming the problems of egoism and free-riding in norm enforcement (Axelrod, 1986; Yamagishi, 1986).

R

O

C

N

U

(BWUS CDIR 392.PDF 03-Jan-06 16:50 81052 Bytes 4 PAGES n operator=M.V.Anantha)

R

EC

TE

D

Resource-Specific Altruism?
Our arguments so far are ecological: An adaptive strategy (e.g.,communal sharing) should emerge in response to a local ecology(e.g., a hunter-gatherer environment). However, given that uncertainty in resource supply was a recurrent adaptive problem inthe environments in which the ancestors of modern humans
evolved (Cosmides & Tooby, 1992), it is likely that human minds
are equipped with evolved algorithms dealing with resource
uncertainty and sharing. People’s reactions to ‘‘windfall profits’’
may provide a case in point: People use windfall money, more
often than money acquired by labor, for altruistic purposes suchas treating friends or donating to charities. Although the fungible
resource under consideration in both cases is the same (money),different habits seem to be triggered depending on how the resource is acquired. A common explanation for this phenomenon
has been provided by a labor theory of value (‘‘money earned
without making effort has little value’’). However, our evolutionary-game-theory analysis suggests the key factor triggering
sharing may be the uncertainty associated with the acquisition of
the resource, rather than the absence of effort. As Cosmides andTooby (1992) noted, it may be the case that ‘‘information aboutvariance in foraging success should activate different modes ofoperation of these algorithms, with high variance due to chancetriggering a psychology of sharing’’ (p. 213).
We (Kameda, Takezawa, Tindale, & Smith 2002) tested thispossibility by conducting vignette experiments in which the338Social-Class Differences
The game-theoretic analysis shows that the communal-sharing
norm is adaptive in an uncertain environment, not only for thegroup but also for the survival of each individual. However, therelevance of such a communal-sharing norm may seem inconsequential to us, living in modern societies in which various buffers
operate to manage uncertainty about resources (e.g., pension
funds, health insurance). Yet, the availability of such buffers may
differ across individuals within the same society, along with theavailability of other defenses against uncertain fate (e.g., personal
wealth, education). Compared to white-collar citizens, blue-collar
citizens have less access to such buffers and are more susceptible
to injury from various life uncertainties. An egalitarian system,
based on a distributive ideology dictating equal allocation of resources regardless of members’ different production levels, could
buffer some of the direct damages that unexpected life events
inflict on individual welfare (as argued by Kaplan and Hill, 1985)
but may be endorsed differently by different groups. Specifically,
preference for such a system should be stronger among bluecollar people than among white-collar people.
We conducted a survey to assess this hypothesis with students
from seven Japanese universities, asking their personal en-

Volume 14—Number 6

Tatsuya Kameda, Masanori Takezawa, and Reid Hastie

REFERENCES

F

Axelrod, R. (1986). An evolutionary approach to norms. American Political Science Review, 80, 1095–1111.
Cialdini, R.B., & Trost, M.R. (1998). Social influence: Social norms,
conformity, and compliance. In D.T. Gilbert, S.T. Fiske, & G.
Lindzey (Eds.), The handbook of social psychology (4th ed., Vol. 2,
pp. 151–192). New York: McGraw-Hill.
Cosmides, L., & Tooby, J. (1992). Cognitive adaptations for social exchange. In J.H. Barkow, L. Cosmides, & J. Tooby (Eds.), The
adapted mind: Evolutionary psychology and the generation ofculture (pp. 163–228). Oxford: Oxford University Press.
Dawes, R.M. (1980). Social dilemmas. Annual Review of Psychology, 31,
169–193.
Gigerenzer, G., & Todd, P.M. the ABC Research Group (1999). Simple
heuristics that make us smart. New York: Oxford University Press.
Gintis, H. (2000). Game theory evolving: A problem-centered introduction to modeling strategic behavior. Princeton: Princeton University Press.
Gurven, M. (2004). To give or not to give: An evolutionary ecology ofhuman food transfers. Behavioral and Brain Sciences, 27, 543–
559.
Hamilton, W.D. (1963). The evolution of altruistic behavior. American
Naturalist, 96, 354–356.
Kameda, T., Takezawa, M., & Hastie, R. (2003). The logic of social
sharing: An evolutionary game analysis of adaptive norm development. Personality and Social Psychology Review, 7, 2–19.
Kameda, T., Takezawa, M., Tindale, R.S., & Smith, C. (2002). Social
sharing and risk reduction: Exploring a computational algorithm
for the psychology of windfall gains. Evolution and Human Behavior, 23, 11–33.
Kaplan, H., & Hill, K. (1985). Food sharing among Ache foragers: Testsof explanatory hypotheses. Current Anthropology, 26, 223–246.
Kelley, H.H., Holmes, J.G., Kerr, N.L., Reis, H.T., Rusbult, C.E., & Van
Lange, P.A.M. (2003). An atlas of interpersonal situations. Cambridge, U.K.: Cambridge University Press.
Maynard, Smith, J. (1982). Evolution and the theory of games. Cambridge: Cambridge University Press.
de Waal, F.B.M. (1996). Good natured: The origins of right and wrong inprimates and other animals. Cambridge, MA: Harvard University
Press.
Yamagishi, T. (1986). The provision of a sanctioning system as a public
good. Journal of Personality and Social Psychology, 50, 110–116.

R

O

C

N

U

(BWUS CDIR 392.PDF 03-Jan-06 16:50 81052 Bytes 4 PAGES n operator=M.V.Anantha)

R

EC

TE

D

PR

Our analysis suggests that norms, such as those controlling social sharing, emerge as a function of individual fitness maximization. Although new to psychology, evolutionary or adaptivemodeling can serve as a powerful theoretical tool to study howsociocognitive systems develop. Along with more traditional
approaches that describe proximate psychological mechanisms
of norm-related behavior (Cialdini & Trost, 1998), game theory
can provide a complementary understanding of social behavior.
For instance, evolutionary game theory should help us understand other social norms including those governing intergrouprelations, marriage, group performance, and group decision
making, just to name a few. Careful analysis of ecological conditions, including individuals’ access to social information (e.g.,
reputation), social mobility, opportunities for sanctioning, and soon, will be important initial steps in such endeavor.
The more general message of this article is that many important social habits support adaptive responses to local ecologies.
Given the fundamental fact that we are a group-living species,
this metatheoretic perspective highlights the importance of examining interdependence structures that exist among people,
including many variants across different task domains andcultures (Kelley et al., 2003). Game-theoretic reasoning, as wehave employed it in this paper, will be an indispensable tool insuch an endeavor, and perhaps these insights will be the basis forfruitful connections between psychology and the other social
sciences.

Acknowledgments—Writing of this paper was supported by the
Grant-in-Aid for Scientific Research 14310048 from the Ministry of Education, Culture, Sports, Science and Technology ofJapan to the first author.

O

CONCLUSION

Kameda, T., Takezawa, M., & Hastie, R. (2003). (See References)
Richerson, P.J., & Boyd, R. (2004). Not by genes alone: How culture
transformed human evolution. Chicago: University of Chicago
Press.
Schaller, M., Simpson, J., & Kenrick, D. (Eds.) (2006). Evolution andsocial psychology. New York: Psychology Press.

O

dorsement of the egalitarian ideology over a merit-based ideology of resource allocation. The proportion of egalitarian-ideology endorsers differed substantially across the schools, ranging
from 63% to 83%. How can we explain these differences? Although these universities differed along many dimensions, including urban versus rural, size of the student body, private
versus public, and so on, only one factor was correlated with thedifferences in the proportion of egalitarians—the social rank of
the university. Students in the less prestigious schools, who
tended to be from the working-class families, endorsed theegalitarian ideology at higher rates. The correlation between thesocial rank of the university and the proportion of egalitarianideology endorsers was substantial (r 5 .85). Although more
rigorous follow-up is certainly needed, the preliminary result
suggests that different distribution ideologies may evolve culturally, depending on availability of personal risk buffers.

Recommended Reading
Camerer, C. (2003). Behavioral game theory: Experiments in strategicinteraction. Princeton: Princeton University Press.
Giraldeau, L.-A., & Caraco, T. (2000). Social foraging theory. Princeton:
Princeton University Press.

Volume 14—Number 6339

Author Query Form_______________________________________________________
Journal
ArticleCDIR392_______________________________________________________
Dear Author,
During the copy-editing of your paper, the following queries arose. Please respond to these by marking up your proofs with the necessarychanges/additions. Please write your answers clearly on the query sheet if there is insufficient space on the page proofs. If returning theproof by fax do not write too close to the paper's edge. Please remember that illegible mark-ups may delay publication.
Query No.

Q1

Description
No query

Author Response