Natural Rationality | decision-making in the economy of nature


Of Genes and Trust

In a post last October, I presented a study on the heritability of fairness preference in the Ultimatum game. It was found that genetics account for 42% of the variation in responder behavior: i.e., identical twins are more likely to behave similarly in their reaction to Ultimatum proposition.

Now the same team studied the genetic influence of choice behavior in the Trust game. In this game, Alice (player 1) has an initial amount of money that she could either keep or transfer to Bob (player 2). Is she transfers it to Bob, the amount is tripled. Bob could keep this amount, or transfer it (partially or totally) to Alice. Game theory predicts that Alice should keep everything, or if she transfers any amount, Bob should keep all of it. Experimental studies have shown that players in Alice’s position invest about 50% of their money and get more or less what they invest (Camerer, 2003). Cesarini et al. (2008) found that genetics can account for 15% to 30% of the variations (they used two samples of American and Swedish identical and non-twins). Thus, there is no "trust gene", but an important influence of genetics. They conclude by urging social scientist to "take seriously the idea that peer and parental socialization are not the only forces that explain differences in cooperative attitudes".

Knobe and Leiter (2006) illustrate vividly why genetics should be given a more important place in the conceptualization of cognitive functions (in their cases, moral psychology):

The most important evidence here comes from studies in behavioral genetics. Typically, these studies are conducted either by looking at twins (comparing monozygotic to dizygotic) or by looking at adopted children. The results of such studies are as consistent as they are shocking. Almost every personality trait that has been studied by behavioral geneticists has turned out to be heritable to a surprising degree. So, for example, a recent review of five studies in five different countries (comprising a total 15 sample size of 24,000 twins) estimates that genetic factors explain 60% of the variance inextraversion and 50% of the variance in neuroticism (Loehlin 1992).
It is difficult to convey just how astoundingly high these numbers are, but perhapsone can get a better sense for the issue by considering the effect sizes obtained in someclassic social psychology experiments. The Festinger and Carlsmith (1959) study of cognitive dissonance found an effect that explained 13% of the behavioral variance; the Darley and Batson (1968) study of bystander intervention and the diffusion of responsibility found an effect that explained 14% of the behavioral variance; the Milgram (1975) study of obedience and proximity showed an effect that explained 13% of the behavioral variance. These are among the most influential and important experiments inall of social psychology. In each case, the fact that researchers were able to explain 13-14% of the variance led to a veritable revolution in our understanding of the relevant phenomena. Now consider, by contrast, the fact that behavioral geneticists routinely find effects that explain fifty percent of the variance in trait measures.

  • Camerer, C. F. (2003). Psychology and Economics. Strategizing in the Brain. Science, 300(5626), 1673-1675.
  • Cesarini, D., Dawes, C.T., Fowler, J.H., Johannesson, M., Lichtenstein, P., Wallace, B. (2008). Heritability of cooperative behavior in the trust game. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0710069105
  • Knobe, Joshua and Leiter, Brian R., "The Case for Nietzschean Moral Psychology" (April 28, 2006). Available at SSRN:


michael said...

My wife and I have twins, who are four at the moment.

But when they grow up, I will teach them how to play the game this way: