'Rationality' enhancement: The effect of anodal tDCS over the right ventrolateral prefrontal cortex among ultimatum game responders

Contrary to classical economic theories, experimental findings show that people are not exclusively self-interested, rather, they have other-regarding preferences, such as fairness and reciprocity. Further, these social preferences are emotionally driven, and deliberative processes are required to implement 'rational' self-interested motives. Here, we aimed to enhance 'rational' self-interested behavior by enhancing the neuronal activity of the right ventrolateral prefrontal cortex (rVLPFC), a brain area associated with emotion regulation processes, using anodal transcranial-direct-current-stimulation (tDCS), among ultimatum game responders. We found that stimulated, compared to control (sham stimulation), participants accepted unfair offers significantly more often. Interestingly, this effect was not moderated by whether the unfair offers were made intentionally by the participants' partners or through a fair mechanism (i.e., randomly made by the computer), suggesting that inequality per se is emotionally aversive even when it was not determined deliberately. In contrast, the effect was absent when playing on behalf of another random participant, suggesting that when the self is not involved, decisions are less emotional. These findings reveal the causal and vital role of the rVLPFC in promoting self-interested behavior in social exchange situations. We discuss theoretical implications for dual-system models and specifically in the context of social exchange situations.