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

Experimental findings challenge classical economic theories by demonstrating that people are not solely self-interested but also have social preferences, such as fairness and reciprocity, that often conflict with self-interest motives. An ongoing debate examines whether social preferences arise automatically (requiring deliberate actions to ensure economic self-interest) or require cognitive control (implying that self-interest is the default motive and must be inhibited). Two distinct cognitive-control brain regions, each with potentially unique inhibitory functions, have been implicated in managing this conflict: the right dorsolateral prefrontal cortex (rDLPFC) and the right ventrolateral prefrontal cortex (rVLPFC). Despite extensive neuromodulation research on the rDLPFC, the role of the rVLPFC remains unexplored. To address this gap, we examined the effect of anodal transcranial direct-current stimulation (tDCS) over the rVLPFC on responders' behavior in the ultimatum game. Stimulated participants accepted unfair offers significantly more often than those in the control (sham) group. This effect was not influenced by whether offers were made intentionally by a human partner or randomly by a computer but was absent when participants played on behalf of another random individual. These findings establish the causal role of the rVLPFC in promoting "rational" self-interested behavior in social exchange situations. We discuss theoretical implications.