Serotonin and reaction to unfairness.

How should one deal with line cutters? Or, more generally, what would you do if you faced unfair or wrong behavior? Studies have shown that machiavellian individuals accept unfair offers more often in ultimatum games (UG), using realism and opportunism to maximize their self-interest. 5-HT (serotonin) transmission, for which the dorsal raphe nucleus (DRN) is a major source, is important in brain regions such as the dorsal lateral prefrontal cortex that are recruited for this kind of cognitive control. Honest and trustful persons who cannot easily separate themselves from moral precepts tend to adhere to a norm of fairness and thus show lower tolerance of unfairness. Takahashi et al. have now used positron emission tomography to directly measure 5-HT transporters (5-HTT) and 5-HT1A receptors and find that low 5-HTT in the DRN is associated with straightforwardness and trust personality traits and predicts higher rejection rates of unfair offers in the ultimatum game. Here is their abstract:

How does one deal with unfair behaviors? This subject has long been investigated by various disciplines including philosophy, psychology, economics, and biology. However, our reactions to unfairness differ from one individual to another. Experimental economics studies using the ultimatum game (UG), in which players must decide whether to accept or reject fair or unfair offers, have also shown that there are substantial individual differences in reaction to unfairness. However, little is known about psychological as well as neurobiological mechanisms of this observation. We combined a molecular imaging technique, an economics game, and a personality inventory to elucidate the neurobiological mechanism of heterogeneous reactions to unfairness. Contrary to the common belief that aggressive personalities (impulsivity or hostility) are related to the high rejection rate of unfair offers in UG, we found that individuals with apparently peaceful personalities (straightforwardness and trust) rejected more often and were engaged in personally costly forms of retaliation. Furthermore, individuals with a low level of serotonin transporters in the dorsal raphe nucleus (DRN) are honest and trustful, and thus cannot tolerate unfairness, being candid in expressing their frustrations. In other words, higher central serotonin transmission might allow us to behave adroitly and opportunistically, being good at playing games while pursuing self-interest. We provide unique neurobiological evidence to account for individual differences of reaction to unfairness.

Correlation between rejection rate of unfair offers in UG and 5-HTT binding in DRN. (A) SPM image showing regions of negative correlation between rejection rate of unfair offers and 5-HTT binding in DRN. (B) Plots and regression line of correlation between rejection rate of unfair offers and 5-HTT binding in DRN (R = −0.50, P = 0.026). Dashed lines are 95% confidence interval boundaries.