Neural mechanisms of third-party punishment: Evidence from transcranial direct current stimulation

Abstract

The social order of human societies is largely maintained by social norms. However, we still know little about the cognitive and emotional foundations that shape social norms, which makes it difficult, if not impossible, to understand how social norms are developed and maintained. Prior neural studies, which mainly perform second-party punishment based on the ultimatum framework, rarely explore the relevant brain areas as well as the neural mechanisms of third-party punishment driven by social norms. In the current study, we provide evidences that support the influences of two types of mechanisms (i.e., negative emotions and self-interest mechanisms) on social norms compliance of third parties at opposite directions. Meanwhile, right dorsolateral prefrontal area (DLPFC) is found to play a crucial role in this process. In this study, we used transcranial direct current stimulation (tDCS) to investigate whether increasing or decreasing right DLPFC excitability influenced third-party punishment in a dictator game. Following an experimental design of “between-subject (tDCS treatments: anodal, cathodal, sham) × within-subject (cost of punishment treatments: without cost, with cost)”, ninety participants were first randomly assigned to receive anodal, cathodal, or sham stimulation in 15 minutes. They then performed two dictator game tasks as third parties. In Task I (without cost) participants did not need to carry any costs for their punishment (none-cost task), while in Task II (with cost) they were required to pay for their punishment actions. The results are given as follows. We first performed repeated measured ANOVA and one-way ANOVA to examine the effect of tDCS treatment (anodal, cathodal and sham) on emotion response. We found a significant main effect of tDCS on emotion response. Meanwhile, post-hoc analysis (SNK) showed that anodal stimulation decreased negative emotions while cathodal stimulation enhanced negative emotions. Second, the results of repeated measured ANOVA and one-way ANOVA showed a significant main effect of tDCS on punishment in the none-cost Task Ⅰ, and post-hoc analysis (SNK) showed that cathodal stimulation significantly increased punishment while the effect of anodal stimulation was insignificant. Third, we also conducted repeated measured ANOVA and one-way ANOVA to test whether the difference of the punishment between the two tasks was affected by tDCS treatments. We found that the main effect of tDCS was significant. Moreover, post-hoc analysis (SNK) showed that the difference of punishment between the two tasks was significantly higher for cathodal stimulation than for sham stimulation, while the difference of punishment between the two tasks for anodal stimulation was insignificant compared to that of sham stimulation. The present study provides one of the first neural evidences for the role of right DLPFC in third-parties’ social norms compliance. The results indicate that DLPFC, by affecting the processes of negative emotions and self-interest, is an important brain area of social norms compliance. When third parties face violations of social norms, their brains first release negative emotions that drive third parties to punish violators. Further, if third parties need to pay for their compliance with social norms, their rational goals about self-interest weaken negative emotional impulses. Finally, the compliance with social norms depends on the trade-offs between negative emotions and self-interest mechanisms.