Right DLPFC stimulation reveals context-dependent regulation of competing motives in third-party fairness decisions

The right dorsolateral prefrontal cortex (rDLPFC) plays a crucial role in fairness-related decision-making, yet it remains unclear whether its role reflects a fairness-promoting mechanism or a broader control process that regulates competing motivations. Here, we combined noninvasive brain stimulation with a third-party ultimatum game to examine how modulating rDLPFC excitability influences fairness decisions in which fairness-related considerations and concern for others differ in relative salience. Ninety healthy female university students (mean age 20.92 ± 2.35) decided whether to accept or reject offers on behalf of university students or charity recipients across three inequality conditions (advantageous, disadvantageous, and equal). Behavioral analyses revealed context-dependent effects of rDLPFC stimulation. Compared with the sham condition, anodal stimulation increased acceptance of advantageous inequality offers for university students (β = 1.27, P = 0.002) but decreased acceptance for charity recipients (β = -1.77, P = 0.003), with no significant effects in disadvantageous or equality conditions. Subjective emotion and fairness ratings followed similar patterns, and mediation analyses indicated that emotion partially mediated the influence of stimulation on choice. These findings support a domain-general account, indicating that the rDLPFC flexibly regulates dominant motivational tendencies depending on social context. Overall, the study demonstrates the adaptive role of the rDLPFC in integrating fairness norms with competing social motives during decision-making.