Study Shows Genes Exert Behavioural Influence

Roger Highfield, London Telegraph, October 2, 2007

Evidence that some people are born to play fair, while others can’t help being cads, has come from a study of twins.

For the first time it has been shown that genes exert influence on people’s behaviour in the idealised setting of a game, revealing that identical twins are more likely to behave the same way, whether fairly or unfairly, as non-identical siblings.

Traditionally, social scientists have been hesitant to acknowledge a role for genes in explaining economic behaviour. But a study indicates that there is a genetic component to people’s perception of what is fair and what is unfair.

The paper, published today in the Proceedings of the National Academy of Sciences, looked at the ultimatum game, in which a proposer makes an offer to another person on how to divide a sum of money.

The researchers found that twins were more likely to behave in the same way as each other when deciding whether to accept an offer.

“This raises the intriguing possibility that many of our preferences and personal economic choices are subject to substantial genetic influence,” said the study’s lead author, Bjorn Wallace of the Stockholm School of Economics.

[Editor’s Note: “Heritability of Ultimatum Game Responder Behavior,” by Bjorn Wallace et al. can be read on-line or downloaded as a PDF document here. To read the PDF on-line, you will have to increase the size of the image by using the Adobe tool, not your browser tool.

[The complete article can be also read on-line as an HTML document here.

[The abstract appears below.]


*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)

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.

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Freely available online through the PNAS open access option.

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 2 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).

o 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.

oo 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.

ooo 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.

oooo To whom correspondence should be addressed. E-mail: [email protected]

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