Neural correlates of reciprocity bias: social debts modulate unfairness perception of violation during third-party observation

The phenomenon of beneficiaries ignoring benefactors' violations, ranging from everyday favors to bribes, is widespread yet lacks targeted theoretical and empirical attention. We propose a conceptual framework that includes "social debt" and "reciprocity bias," where "social debt" is defined as information about benefits bestowed by benefactors and "reciprocity bias" as the influence of social debt on beneficiaries' perceptions and decisions in situations involving the benefactor. To investigate this bias in moral perception and its cognitive-neural mechanisms, we manipulated three levels of social debt (none, less, more) by varying the amount of unasked benefits that benefactors bestowed upon participants. Participants then observed the distributor's fair or unfair allocation of resources to another person, while their electroencephalography (EEG) was recorded. Results indicate that more (vs. none/less) social debt reduces perceptions of unfairness toward benefactors' violations and enhances fairness perceptions of their norm adherence. This was, accompanied by the diminished fairness effect on fronto-centered P2 and a reversal fairness effect on the power of theta oscillations (4 to 7 Hz). These findings support a multilevel reciprocity bias in fairness perception, suggesting that strong social debt may heighten concern for benefactor's interests and increase the adaptive value of their violations at the cognitive-neural level.