Heritability of ultimatum game responder behavior

doi:10.1073/pnas.0706642104

Heritability of ultimatum game responder behavior

*Department of Economics, Stockholm School of Economics, Box 6501, SE-113 83 Stockholm, Sweden;
^†Department of Economics, Massachusetts Institute of Technology, 50 Memorial Drive, Cambridge, MA 02142; and
^§Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, SE-171 77 Stockholm, Sweden

Edited by Henry C. Harpending, University of Utah, Salt Lake City, UT, and approved August 24, 2007 (received for review July 16, 2007)

Abstract

Experimental evidence suggests that many people are willing to deviate from materially maximizing strategies to punish unfair behavior. Even though little is known about the origins of such fairness preferences, it has been suggested that they have deep evolutionary roots and that they are crucial for maintaining and understanding cooperation among non-kin. Here we report the results of an ultimatum game, played for real monetary stakes, using twins recruited from the population-based Swedish Twin Registry as our subject pool. Employing standard structural equation modeling techniques, we estimate that >40% of the variation in subjects' rejection behavior is explained by additive genetic effects. Our estimates also suggest a very modest role for common environment as a source of phenotypic variation. Based on these findings, we argue that any attempt to explain observed ultimatum bargaining game behavior that ignores this genetic influence is incomplete.

Footnotes

^‡To whom correspondence should be addressed. E-mail: cesarini{at}mit.edu
Author contributions: B.W., D.C., P.L., and M.J. designed research; B.W., D.C., and M.J. performed research; B.W., D.C., P.L., and M.J. analyzed data; and B.W., D.C., P.L., and M.J. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
↵ ¶ There is too little variation in the proposal stage to estimate model parameters with reasonable precision, and hence we focus on responder behavior. The equality of distributions of acceptance thresholds by zygosity was tested with a design-based independence test, which takes the correlation between twins into account. The χ² statistic is adjusted by using the second-order correction of Rao and Scott (11).
↵ ‖ To estimate the mean acceptance threshold, the acceptance threshold is set at the middle value of the acceptance threshold intervals.
↵ ** The maximum likelihood estimation is implemented in Mx, a numerical optimizer for behavior genetics (12). We estimate a threshold model based on the response categories for the acceptance threshold in the experiment. The programming code for the threshold model we estimate is adapted from the digital scripts library at the University of Amsterdam (13). The algorithm that estimates confidence intervals for the parameters is explained in some detail by Neale and Miller (14).
↵ †† One interpretation, suggested to us by a referee, is that the high heritability casts doubt on accounts of ultimatum rejections relying on notions of evolutionary disequilibrium. In light of our findings, it seems plausible that the trait has been under stabilizing selection in the post-Pleistocene era.
↵ ‡‡ Previous experimental research has shown that it is fairly common to observe some subjects rejecting both low (<50%) and high (>50%) offers (37). In the parlance of experimental economics, these subjects are “hyperfair.” In our data, slightly fewer than one-third of the subjects exhibit such behavior.
↵ §§ For example, a response in which an individual accepts an offer of 10% but then rejects an offer of 20% is considered inconsistent because an acceptance threshold is not uniquely defined on the interval from 0 to 50%.
↵ ¶¶ Furthermore, our results are robust to the manner in which we deal with these inconsistent responses. Dropping the five inconsistent responses clarified by e-mail has no discernible effect on the heritability estimates or their significance levels.
Abbreviations:

DZ,

dizygotic;

MZ,

monozygotic.
Freely available online through the PNAS open access option.