Pain modulates brain potentials and behavioral responses to unfairness.

Pain may initially contribute to the evolution of moral decision-making as it elicits avoidance behavior. The current study aims to support this perspective by conducting a behavioral study to investigate whether pain leads to a self-oriented tendency and an exploratory electroencephalogram (EEG) study to examine how pain affects moral decision-making. In Experiment 1, 34 participants were recruited and treated with both capsaicin (pain condition) and hand cream (control condition) in separate days. After treatment, they were asked to complete a third-party punishment task. Results showed that pain increased punishment and decreased compensation towards unfair allocations in the task. In Experiment 2, 68 participants with either pain or control treatment participated in an EEG experiment. It revealed that pain enlarged the disparity of late positive potential (LPP) between fair and unfair situations, suggesting that individuals in pain may exert more cognitive effort when facing unfair allocations. Meanwhile, pain did not affect the early components P2 and the medial frontal negativity, indicating unaffected attentional or anticipatory responses toward unfairness. It demonstrates that pain can effectively modulate responses to unfairness, manifesting as a self-oriented approach with negative consequences for others. It suggests a potential evolutionary impact of pain on moral decisions.