Researchers from the Massachusetts Institute of Technology, Stockholm School of Economics and Karolinska Institute (also in Stockholm) discovered this confluence of nature and nurture by having 324 pairs of identical twins, who share the same genetic makeup, and fraternal twins, who do not, participate in an exercise known as "the ultimatum game." Each game featured two players, one designated a proposer and the other a responder. The proposer was given a sum of money (in this experiment, roughly $15) to split however he or she chose with the responder, who could accept or decline the offer. If the responder nixed the deal, then neither player received a cut of the cash. Common sense dictates that the responder would accept anything over zero given the alternative, but previous research shows that responders typically reject offers they deemed insultingly low to punish their stingy counterparts.
Researchers determined that the average minimum the group as a whole would accept was around five bucks, or a third of the pot. They then plotted the threshold of each twin in pairs of identical and fraternal twins. Their findings: an identical twin's behavior could help predict that of his or her sibling, but not so with fraternal pairs. Researchers say the results indicate that at least 40 percent of the variation in perceiving fairness may stem from genetics.
"If you're a fraternal twin, your brother or sister's behavior does not predict your behavior at all," says study co-author David Cesarini, a doctoral student in economics at M.I.T. The results "invite speculation that economic behaviors and strategies are under considerable genetic influence."
Ernst Fehr, an economics professor at the University of Zurich, says this study is the first to show the heritability of altruistic punishment and notes that it could serve to "speed up efforts to find concrete genes and the mechanisms the genes are involved in that code for altruistic behaviors."
Cesarini says the research team is now probing possible genetic influences on cooperation and risk preferences.