March 05, 2008
Genetic Component Detected For Trust

Researchers at MIT, UCSD, and the Karolinska Institutet in Stockholm Sweden conducted two sets of studies on twins and found that part of human trust and trustworthiness seem due to genetic influences (PDF at PNAS site). (and thanks to one of the researchers, MIT's David Cesarini, for the heads up)

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 independently conceived and executed a very similar experiment on twins [see supporting information (SI) for experimental procedures]. These teams became aware of each other for the first time 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 administered 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 partners.

They looked at both how much trust people had in strangers and also how much people lived up to the expectations of those who trusted them.

The results of our mixed-effects Bayesian ACE analysis suggest that variation in how subjects play the trust game is partially accounted for by genetic differences (Tables 2 and 3 and Fig. 2). In the ACE model of trust, the heritability estimate is 20% (C.I. 338%) in the Swedish experiment and 10% (C.I. 421%) in the U.S. experiment. The ACE model of trust also demonstrates that environmental variation plays a role. In particular, unshared environmental variation is a much more significant source of phenotypic variation than genetic variation (e2 = 68% vs. c2 = 12% in Sweden and e2 = 82% vs. c2 = 8% in the U.S.; P < 0.0001 in both samples). In the ACE model of trustworthiness, heritability (h2) generates 18% (C.I. 830%) of the variance in the Swedish experiment and 17% (C.I. 532%) in the U.S. experiment. Once again, environmental differences play a role (e2 = 66% vs. c2 = 17% in Sweden and e2 = 71% vs. c2 = 12% in the U.S.; P < 0.0001 in both samples).

The researchers found more trust in their Swedish participants (which is what I'd expect from such a high trust society). They also found a higher heritability of trust in their Swedish participants. See figure 2.

Heritability (h2) 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.

That difference in trust between US and Swedish participants might be due to genetic differences.

They suspect their results understate extent of heritability for a couple of reasons including the very plausible idea that people are more likely to mate with people who have similar levels of trust. So the DZ twins and MZ twins are not as different in their genetic sequences which influence trust as would be the case if matings were more random.

Moreover, we believe that the reported estimates indicate a lower bound on heritability and shared environment for two reasons. First, the estimate of the variance explained by the unshared environmental differences includes all idiosyncratic error, including measurement error. If our subjects had each played several rounds of the trust game with different individuals, our measures of trust and trustworthiness may have been more precise, which would have yielded higher estimates of heritability and common environmental influences. Second, one assumption of the ACE model is that there is no assortative mating with respect to the trait of interest. If preferences for cooperation are indeed heritable, and if people who cooperate tend to mate with other cooperative individuals, then this will increase the similarity in cooperative behavior in their children. This inflates the correlation of the genotypes of DZ siblings, making it harder to detect differences in MZ and DZ twins. As a result, the more assortativity, the more it biases downward the estimate of heritability.

The problem is that it is easier to compare twins than to compare any two random individuals to tease out genetic influences. We don't know how much two random individuals differ in their genetic sequences. Using twins studies researchers can detect the presence of genetic influences on behavior. That's important because that detection is the first step toward finding the actual genetic variations that cause people to behave differently from each other.

In the last couple of years the rate of detection of the meaning of genetic differences greatly sped up. The rapid decline in DNA testing costs and the development of more extensive maps of genetic differences (e.g. as done in the International Haplotype Map project) has made this task much easier. Within 10 years at most studies such as this one will routinely include genetic testing information on all test subjects.

Cheap DNA sequencing will probably increase the optimal size of trust studies and other behavioral studies of twins. Much larger groups of participants are needed to better control for all the genetic variations in order to identify which genetic variations contribute to behavioral differences.

Studies of this sort demonstrate that natural selection played a major role in shaping human behavior. Economic behavior is not simply the result of rational calculations by humans. The extent of willingness to engage in exchanges and enter into business deals is influenced by our evolutionary past. That notion doesn't sit well with people who imagine they have total free will and control over their decisions.

The results of this study understate the extent to which genetic sequences control our economic behavior. The study tried to measure differences in behavior. Many genetic sequences identical in all study participants didn't cause differences and yet did influence behavior of all the study participants.

Cesarini and Swedish researchers have previously published work in this area. See my previous post Large Genetic Component To How People Play Economic Game.

What I most want to know: once all the genetic alleles which influence trust and trustworthiness are identified and offspring genetic engineering becomes possible will people choose genes that make their offspring more or less trusting and more or less trustworthy?

Share |      Randall Parker, 2008 March 05 11:46 PM  Brain Economics

Wolf-Dog said at March 6, 2008 10:01 AM:

But does this mean that children of criminals will face discrimination in the future?

Also, some of the most dangerous thieves in the corporate world, have worked very hard for many years in order to build an impeccable history of very trustworthy behavior, only to misuse their good reputation when the opportunity presents itself.

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