If only the fittest members of a species survive, then cooperative behavior doesn’t make sense. It is costly to individuals and benefits others. Over the past few years, however, many researchers have shown how cooperative behavior plays nicely with evolutionary theory: from yeast that “share” food to people acting selflessly, cooperative behavior is advantageous for a group’s evolutionary fitness compared to an uncooperative individual’s fitness. Now, in an Early Edition PNAS paper, Schroeder et. al. have shown that cooperative behavior’s enforcement mechanism—punishment—also appears to have been shaped by natural selection. The researchers showed how variants of two genes can predict our sensitivity to punishment, in both giving and receiving it.
Previous research already had linked our sensitivity to punishment and other social decision-making behaviors to serotonin, a neurotransmitter associated with well-being. Taking the next logical step, Schroeder et al. hypothesized that cooperative
behavior could be predicted based on variants of serotonin transporter and receptor genes, which give rise to the molecular machinery that responds to serotonin.
To test study participants’ cooperative behavior and willingness to punish, the researchers used a standard of experimental economics called the Public Goods Game, in which participants individually decide in each round how much of his or her own fortune to share with the group. Then, the total contributed by all members is multiplied by some number greater than one and distributed evenly among all players. A player who does not contribute to the group maximizes her payoff unless other players can punish such uncooperative behavior by imposing fines. Study participants played both a No Punishment and Punishment version of the game via computer and were instructed not to communicate with one another in any other way.
Comparisons between each participant’s DNA, behavior, cortisol levels as a measure of stress, and self-reported mood and background information led researchers to conclude that “participants with a variant at the serotonin transporter gene contribute more” than those without it in the No Punishment version of the game. In the Punishment version, participants
with a particular variant of the serotonin receptor gene contribute more and also have “a more stressful experience of the games” than those without it.
Previous studies have shown these variants in serotonin transporter and receptor genes confer susceptibility to psychopathology. In this study, the apparent genetic effect on cooperative behavior depended on whether punishment was a part of the game. Because of this, the authors conclude that these genetic variants—and their being turned on or off epigenetically—may also confer an increased adaptability to social contexts, including sanctioning institutions.
Variants at serotonin transporter and 2A receptor genes predict cooperative behavior differentially according to presence of punishment. Kari B. Schroeder, Richard McElreath, and Daniel Nettle. PNAS. Published online before print February 19, 2013, doi: 10.1073/pnas.1216841110