Inter-brain synchronization is weakened by the introduction of external punishment.

Punishment is a popular institution to enforce social norms in human society. However, how the punishment institution impacts the inter-brain neural signatures of two-person social interactions is still an open question. By performing electroencephalography recording of brain activity in two interacting parties as they simultaneously played both the revised repeated ultimatum game (rrUG) and the revised repeated dictator game (rrDG), this study focused on exploring how the introduction of external punishment influences inter-brain synchronization between the two parties. The data showed a significant negative effect of external punishment on inter-brain synchronization, with greater inter-brain synchronization observed in the rrDG than in the rrUG. We proposed a possible mechanism underlying this result. In the rrDG, the similar moral motivation of both proposers and responders results in inter-brain synchronization between them. However, in the rrUG, the introduction of external punishment crowds out the intrinsic moral motivation of the proposers, thereby undermining the inter-brain synchronization. Moreover, we found a significant positive correlation between the rejection rate from responders for disadvantageous inequal offer and inter-brain synchronization in the rrDG. These findings contribute to understanding the negative effect of punishment institution and shed light on the inter-brain mechanism underlying social interaction.