Neural Mechanisms of Cooperation and Fairness in Iterative Prisoner's Dilemma

Cooperation is a universal human principle reflecting working with others to achieve common goals. The rational decision-making model contends that cooperation is the best strategy for maximizing benefits in an iterative prisoner's dilemma. However, the motivations for cooperation (or betrayal) are complex and diverse, and often include fairness reflections. In this study, we used functional magnetic resonance imaging to study underlying neural differences in brain regions related to fairness when people interact with an opponent prone to cooperate or betray, at different decision-making stages. Results based on 40 university students (25 women) indicate that experiences of cooperation or betrayal affect people's fairness perception. Distinct neural activities occur in expectation, decision, and outcome phases of decisions. In the expectation phase, those in the cooperative condition exhibited increased activation in the anterior cingulate gyrus, medial superior frontal gyrus, and caudate nucleus compared to those in the uncooperative condition. During the decision phase, those in the cooperative condition showed greater activation in the middle frontal gyrus, caudate nucleus/frontal insula, inferior frontal gyrus, and cingulate gyrus compared to those in the uncooperative condition. In the outcome feedback phase, the caudate nucleus, insula, cingulate gyrus, and inferior frontal gyrus of the orbit were more active in the uncooperative condition than in the cooperative condition. Results also showed a significant correlation between caudate activity and the perception of fairness when anticipating uncooperative conditions.