Punishment cost affects third-parties' behavioral and neural responses to unfairness.

Third-party punishment plays a crucial role in fairness norm enforcement. The present study investigated how punishment cost would affect third-parties' behavioral and neural responses to unfairness using a modified Third-Party Dictator Game and functional magnetic resonance imaging (fMRI). Participants acted as third-parties and decided how many monetary units (MUs) to invest to punish norm violations in two punishment cost contexts. Participants' every MU investment reduced dictators' payoff by 6 MUs in the low punishment cost context and 3 MUs in the high one. Participants' invested MUs reflected the cost they would like to pay to punish dictators while dictators' reduced MUs represented the amount of punishment they received. Behavioral results showed participants' fairness ratings were not affected by punishment cost. However, punishment amount decreased in the high punishment cost context where participants invested more MUs and spent more time for decision-making. Neurally, left anterior insula (AI) and bilateral dorsolateral prefrontal cortex (dlPFC) showed stronger responses to unfair relative to fair allocations in both contexts. Moreover, dorsal anterior cingulate cortex (dACC) was more active during unfair allocations in the high punishment cost context than in the low one and the difference of dACC activity between these two conditions was positively correlated with the difference of reaction times. Overall, the present study demonstrated that punishment cost would not affect people's fairness perception but increase the conflicts between norm enforcement and self-interest. The decision for punishment was the outcome of integrating fairness and economic considerations.