The time course of neural responses to social versus non-social unfairness in the ultimatum game.

The unfairness effects are always the hotspot within social psychology and cognitive neuroscience. However, people's neural responses to social versus non-social unfairness remain under-researched, especially about temporal features. We engaged participants in the Ultimatum Game to respond to human and computer proposers (representing social and non-social contexts respectively) and recorded their event-related potentials. The interactions elicited three components of interest: medial frontal negativity (MFN), late positive potential (LPP) and response related negativity (RRN). First, unfair human offers elicited larger MFN than unfair computer offers did, suggesting a greater perception of unfairness in the social context. Second, rejected unfair human offers elicited smaller LPP than rejected unfair computer offers did. This finding implies that the rejection to social unfairness could down-regulate the unfairness-aversive emotions. These two mechanisms explained the stronger resistance to social unfairness in behavioural results. Last, the RRN for unfairness rejections were larger than for fairness acceptances, but showed no variance to the two types of proposers, signifying a similar degree of response conflict behind rejections to social and non-social unfairness. These results of our exploratory study will be helpful in revealing the sociality effect on the perceptual, emotional and reappraisal processing during the unfairness response.