Heritability of cooperative behavior in the trust game

*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)

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Fig. 1.
Distribution of trust and trustworthiness responses, by zygosity. Blue bars indicate frequencies for MZ twins, and yellow bars indicate frequencies for DZ twins. Upper shows trust, which is the percentage of endowment sent by investor to trustee. Lower shows trustworthiness, which is the percentage returned by the trustee to the investor. Left shows results for Sweden, and Right shows results for the U.S. For amount sent, the Swedish experiment permitted only six choices, whereas the U.S. experiment permitted 11.
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Fig. 2.
Heritable traits generate a significant fraction of the variation in trust and trustworthiness. Each triangle is a “ternary plot” that represents the proportions of three variance components (genetic, shared environment, and unshared environment) that sum up to one. A point in the center indicates all three components contribute equally, whereas a point at a vertex indicates that a single component fully explains the variance (i.e., a point at the top vertex indicates all variance is explained by heritability). Each ternary plot shows the posterior Bayesian distribution of estimated components of total variance in ACE models from independent experiments in Sweden and the U.S. Colors indicate credible areas calculated using 10,000 posterior draws to estimate a 2D kernel density of the most likely proportions given the data. The blue areas indicate the highest-density regions with the most credible estimates. The colored areas contain 95% of the draws (i.e., the probability that the true coefficients lie outside the colored regions is P = 0.05). Heritability (h ²) of trust is estimated to be (a) 20% in Sweden and (b) 10% in the U.S. Heritability of trustworthiness is estimated to be (a) 18% in Sweden and (b) 17% in the U.S.