Heritability of cooperative behavior in the trust game
David Cesarini*, Christopher T. Dawes†, James H. Fowler†‡, Magnus Johannesson§, Paul Lichtenstein¶,
and Bjo ¨rn Wallace§
*Department of Economics, Massachusetts Institute of Technology, 50 Memorial Drive, Cambridge, MA 02142;†Political Science Department, University of
California at San Diego, 9500 Gilman Drive 0521, La Jolla, CA 92093-0521;§Department of Economics, Stockholm School of Economics, Box 6501, SE-113 83
Stockholm, Sweden; and¶Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, SE-171 77 Stockholm, Sweden
Edited by Elinor Ostrom, Indiana University, Bloomington, IN, and approved January 15, 2008 (received for review October 23, 2007)
Although laboratory experiments document cooperative behavior
in humans, little is known about the extent to which individual
differences in cooperativeness result from genetic and environ-
mental variation. In this article, we report the results of two
independently conceived and executed studies of monozygotic
and dizygotic twins, one in Sweden and one in the United States.
The results from these studies suggest that humans are endowed
with genetic variation that influences the decision to invest, and to
reciprocate investment, in the classic trust game. Based on these
findings, we urge social scientists to take seriously the idea that
differences in peer and parental socialization are not the only
forces that influence variation in cooperative behavior.
behavioral genetics ? cooperation ? experimental economics
absence of an expectation that trust will be reciprocated. The
prevalence of trust in a society has therefore been assigned
primacy in a number of domains, for instance empirical (1) and
theoretical (2) studies of economic growth. In recent years, the
trust game (3) has emerged as a favorite instrument to elicit an
individual’s interpersonal trust and willingness to reciprocate
trust. More generally, the game has been widely used to study
cooperative behavior. In a trust game, an individual (the inves-
tor) decides how much money out of an initial endowment to
send to another subject (the trustee). The sent amount is then
multiplied by some factor, usually three, and the trustee decides
how much of the money received to send back to the investor.
The standard game-theoretic prediction for a single anonymous
interaction between two purely self-interested individuals is for
the investor to send nothing, rationally anticipating that the
trustee will not reciprocate. Yet, experiments consistently show
that cooperation flourishes in the trust game; the average
investor sends a significant share of her endowment, and most
trustees reciprocate (4). A voluminous body of theoretical and
experimental work examines the mechanisms through which
natural selection can favor cooperation, and proposed mecha-
nisms include kin selection (5), reciprocity (6), indirect reciproc-
ity (7), and group selection (8). These models offer different
accounts for the ultimate explanation for the existence of
cooperation and also generate different predictions about ge-
notypic variation in equilibrium (9).
To investigate whether humans are endowed with genetic
variation that could help account for individual differences in
trust game behavior, two separate teams of researchers inde-
pendently conceived and executed a very similar experiment on
twins [see supporting information (SI) for experimental proce-
after all data had been collected. One team recruited 658
subjects from the population-based Swedish Twin Registry, and
the other team recruited 706 subjects from the 2006 and 2007
Twins Days Festivals in Twinsburg, OH. Both teams adminis-
tered the trust game to (identical) monozygotic (MZ) and
(nonidentical) dizygotic (DZ) same-sex twin pairs. The game
was played with real monetary payoffs and between anonymous
any mutually beneficial transactions involve an element of
interpersonal trust and may fail to materialize in the
Because MZ twins share the same genes, whereas the genes of
DZ twins are only imperfectly correlated, MZ twins should
exhibit higher correlation in their behavior than DZ twins if
genetic differences help explain heterogeneity in strategies.
Moreover, if we assume that MZ and DZ twins share compa-
rable environments, then we can use these correlations to
estimate explicitly the relative influence of genetic and environ-
mental factors on behavioral variation. Specifically, we conduct
a mixed-effects Bayesian ACE analysis that uses decisions in the
trust game to estimate the degree to which variation in trust and
trustworthiness is influenced by genetic factors (A), environ-
mental factors shared or common to cotwins (C), and unshared
environmental factors (E), which are specific to each twin (see
SI). The analysis provides estimates of proportions of observed
variance in cooperative behavior due to the respective effects of
A, C, and E. This method generates 95% credible intervals (C.I.)
that permit us to test the hypothesis that a factor plays no role
(i.e., explains 0% of the variance; see SI).
Some scholars have objected to the assumption that MZ and
DZ environments are comparable, arguing that MZ twins tend
to be more influenced by one another and are treated more
similarly by their parents than DZ twins (10). If so, then greater
similarity in MZ twins might merely reflect that their shared
environments cause them to become more similar than DZ
twins. However, studies of twins raised together have been
validated by studies of twins reared apart (11), suggesting that
the shared environment does not exert substantially enhanced
influence on MZ twins. Moreover, personality and cognitive
differences between MZ and DZ twins persist even among DZ
twins whose zygosity has been miscategorized by their parents
(12), indicating that being mistakenly treated as an identical twin
by one’s parents is not sufficient to generate observed differ-
ences in similarity. Further, although MZ twins are sometimes in
more frequent contact with each other than DZ twins, it appears
that twin similarity may cause greater contact rather than vice
To evaluate whether the MZ and DZ samples differ, we
performed a series of tests on the mean difference between
MZ and DZ twins for a number of variables (see Table 1). In
the Swedish sample, high P values suggest that differences
between types of twins are not significant for education (P ?
0.95), gender (P ? 0.35), trust (P ? 0.35), trustworthiness
(P ? 0.53), cognitive ability (P ? 0.64), emotional stability
(P ? 0.43), agreeableness (P ? 0.54), extraversion (P ? 0.63),
and conscientiousness (P ? 0.85). In the U.S., differences were
J.H.F., M.J., P.L., and B.W. performed research; C.T.D. and J.H.F. analyzed data; and D.C.,
C.T.D., J.H.F., M.J., and B.W. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
‡To whom correspondence should be addressed. E-mail: firstname.lastname@example.org.
This article contains supporting information online at www.pnas.org/cgi/content/full/
© 2008 by The National Academy of Sciences of the USA
March 11, 2008 ?
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www.pnas.org?cgi?doi?10.1073?pnas.0710069105Cesarini et al.