Anger expression facilitates social punishment and neural synchrony in collective unfairness

When confronted with unfair treatment, individuals often forgo personal interests to punish those who act unjustly, a behavior known as social punishment. Prior research suggests that the experience of anger can unite individuals in executing social punishment. However, how an individual's angry expression following unfairness influences others' social punishment behaviors, as well as the underlying interpersonal neural mechanisms, remains insufficiently explored. To address these questions, we conducted a functional near-infrared spectroscopy-based hyperscanning study using an adapted three-person ultimatum game involving one proposer and two responders. By manipulating both the emotional expression and the visibility of facial expressions between responders, we found that clear angry expressions significantly increased social punishment and enhanced interpersonal neural synchrony (INS) in the right temporoparietal junction (r-TPJ). Moreover, INS emerged as a strong predictor of collective rejection rates. In contrast, when angry expressions were masked, collective rejection rates decreased, and INS did not significantly increase. These findings provide evidence that angry expressions in unfair situations amplify social punishment directed toward advantaged parties and are associated with increased INS, underscoring the role of angry expression in coordinated group responses.