Diminishing parochialism in intergroup conflict by disrupting the right temporo-parietal junction

Abstract

Individuals react to violation of social norms by outgroup members differently than to transgressions of those same norms by ingroup members: namely outgroup perpetrators are punished much more harshly than ingroup perpetrators. This parochial punishment pattern has been observed and extensively studied in social psychology and behavioral economics. Despite progress in recent years, however, little is known about the neural underpinnings of this intergroup bias. Here, we demonstrate by means of transcranial magnetic stimulation (TMS) that the transient disruption of the right, but not the left temporo-parietal junction (TPJ), reduces parochial punishment in a third-party punishment paradigm with real social groups. Moreover, we show that this observed TMS effect on parochial punishment is mediated by a classical punishment motive, i.e. retaliation. Finally, our data suggests that a change in perspective-taking might be the underlying mechanism that explains the impact of right TPJ disruption on retaliation motivation and parochial punishment. These findings provide the first causal evidence that the right TPJ plays a pivotal role in the implementation of parochial behaviors.

Keywords: ingroup love; intergroup bias; normative behavior; outgroup hostility; parochialism; temporo-parietal junction; third-party punishment; transcranial magnetic stimulation.

Fig. 1

Schematic representation of the study design. Subjects in the role of Player C received rTMS. After stimulation they were confronted with decisions of Players A and B in a PDG and had the opportunity to assign punishment points to Player A. Players A and B were either ingroup members (IN; depicted in white color) or outgroup members (OUT; depicted in gray color). In order to investigate the impact of rTMS on the two most consistently reported and explored parochial punishment patterns, we calculated the following difference values: OUT/IN vs IN/OUT and IN/IN vs IN/OUT, separately for CC, CD, DC and DD trials.

Fig. 2

Treatment group differences in parochial punishment and retaliation motive. Treatment group differences (means ± s.e.m.) in (A) parochial punishment pattern and (B) retaliation motive in a conflicting intergroup constellation (OUT/IN vs IN/OUT). (A) Depicted is third-parties’ stronger punishment of an outgroup perpertator (OUT/IN) compared with an ingroup perpertator (IN/OUT) and its modulation by rTMS. Subjects whose right TPJ is disrupted exhibit a much lower parochial punishment (smaller difference score) than those in the other two treatment groups (all P < 0.05). This suggests that the right TPJ causally affects third-parties’ parochial punishment in intergroup conflict. (B) Depicted is third-parties’ stronger retaliation motive toward an outgroup perpertator (OUT/IN) compared with an ingroup perpertator (IN/OUT) and its modulation by rTMS. Subjects whose right TPJ is disrupted exhibit a much lower parochial retaliation motivation (smaller difference score) than those in the other two treatment groups (all P < 0.05).

Fig. 3

Mediation analysis. Depicted is the path diagram (including regression coefficients ± s.e.m. and P-values) of the mediation analysis demonstrating that the retaliation motive mediates the impact of disrupting the right TPJ on parochial punishment pattern in a conflicting intergroup constellation. All four requirements for a mediation effect are satisfied: Path a, Path b, and Path c are significant, and Path c′ is significantly smaller than Path c. In detail, Path a represents the effect of TMS (right TPJ vs left TPJ/Sham) on the retaliation motive. Path b represents the impact of the retaliation motive on parochial punishment controlling for the TMS effect. Together, Path a and Path b represent the indirect (mediated) effect of TMS on parochial punishment through the retaliation motive. Path c′ represents the direct effect of TMS on parochial punishment and is calculated controlling for the indirect, mediated effect. Path c represents the total (mediated and direct) effect of TMS on parochial punishment. Finally, the decisive statistical test to examine whether mediation occurs is the statistical test between Path c and Path c′. Bootstrapping statistics (see ‘Materials and Methods’ section for details) revealed that Path c′ is significantly smaller than Path c (P < 0.01), providing evidence that the retaliation motive is indeed a significant mediator.