Retaliation or selfishness? An rTMS investigation of the role of the dorsolateral prefrontal cortex in prosocial motives.

Equity, fairness and cooperative behavior are crucial for everyday social interactions. Recent neuroimaging studies suggest that the dorsolateral prefrontal cortex (DLPFC) is involved in the evaluation of violations of fairness rules, though difficulties remain to determine its role in implementing retaliating or forgiving responses to unfairness. Accordingly, we applied repetitive transcranial magnetic stimulation (rTMS) to the left and right DLPFC and investigated the impact of the DLPFC on retaliation and selfishness using a sequential neuroeconomic task establishing a role reversal. That is, participants first played an Ultimatum Game (in the role of a recipient) against fair or unfair proposers, followed by a Dictator Game in the role of a proposer. Following inhibition of the right DLPFC, subjects showed an increased punishment rate regarding previously unfair opponents. Surprisingly, previously fair opponents were also treated less fairly after rTMS to the right DLPFC, but not after left or sham rTMS. Previous work suggests that the right DLPFC provides "top-down" cognitive control over prepotent emotional responses to unfairness. Our results indicate, however, that the right DLPFC may be involved in controlling selfish behavior and that its suppression leads to maximization of one's own benefit, regardless of another's fairness or unfairness in previous encounters.