Revenge, which refers to taking actions of harming someone in retaliation for an injury (Elshout et al., 2015; Jackson et al., 2019), is a global phenomenon and a causal factor in many homicides and transgenerational conflicts (Kopsaj, 2016; Jackson et al., 2019). Although revenge is an aggressive act, not all aggressive acts represent vengeance. For example, unsolicited acts of aggression, like deviance, incivility, and bullying, would not count as revenge (Raver and Barling, 2008; Jackson et al., 2019). Revenge often occurs between families or clans when an outgroup member brings harm to an ingroup member which, in turn, induces retaliation upon outgroup members (Ericksen and Horton, 1992). According to early social psychological theories (Allport et al., 1954; Brewer, 1999), a desire to help the ingroup (‘ingroup love’) and/or an aggressive motivation to hurt the outgroup (‘outgroup hate’) may drive participation in intergroup conflict by taking revenge. Recent behavioral research using economic games suggests that ingroup love plays a key role in driving economic punishment towards the outgroup (Halevy et al., 2008; De Dreu, 2010; De Dreu et al., 2010; Halevy et al., 2012). Nevertheless, despite the severe social consequences of revenge, its neurobiological underpinnings remain unclear. Building upon previous findings (Halevy et al., 2008; De Dreu, 2010; De Dreu et al., 2010; Halevy et al., 2012), we suggest that there may be a neurobiological mechanism that links perceived ingroup pain caused by an outgroup and the propensity to seek revenge upon an outgroup during intergroup conflict. The present work specifically examined the hormonal (i.e., oxytocin) and neural responses to ingroup suffering caused by an outgroup that predict revenge propensity against outgroups.

Previous brain imaging research has revealed neural responses to ingroup/outgroup members' suffering, yet they have been done in contexts that lack the key character of real-life intergroup conflict, that is ingroup and outgroup members causing each other's pain. Functional magnetic resonance imaging (fMRI) studies have identified increased activity in both the empathy network (e.g., the anterior midcingulate [aMCC] and anterior insula [AI]) and the theory-of-mind network (e.g., the medial prefrontal cortex [mPFC] and the temporoparietal junction [TPJ]) in response to ingroup pain (Hein et al., 2010; Cikara et al., 2011; Han, 2018). Outgroup pain, on the other hand, is related to enhanced activity in the reward system (e.g., the ventral striatum and nucleus accumbens [Hein et al., 2010; Cikara et al., 2011; Luo et al., 2015]). In addition, the mPFC activity in response to perceived pain is associated with decisions to help ingroup members (Hein et al., 2010; Mathur et al., 2010), and the activity in the nucleus accumbens predicts decisions not to help outgroup members (Hein et al., 2010; Luo et al., 2015). These findings highlight ingroup favoritism in brain responses to others' pain as neural underpinnings of ingroup love but leave open a critical question: are brain responses to ingroup pain inflicted by outgroup members during intergroup conflict associated with subsequent revenge? Specifically, it is unclear whether activities in the empathy and/or theory-of-mind networks in response to perceived ingroup pain are associated with revenge motives during intergroup conflict. If revenge aims to bring suffering to an outgroup in order to get reward during intergroup conflict, one may expect the involvement of the reward system in decision making related to outgroup punishment (Hein et al., 2010; Cikara et al., 2011; Han, 2018). However, if the goal of revenge is to help ingroup members who suffer from physical harm caused by an outgroup (Lickel et al., 2006), the mPFC, which responds to ingroup pain and is associated with ingroup help (Hein et al., 2010; Mathur et al., 2010), may be associated with tendencies to punish the outgroup.

Previous fMRI research has also examined the neurobiological correlates of punishment decision-making pertaining to those who have violated social norms in economic games (Seymour et al., 2007; Krueger and Hoffman, 2016). Punishment decisions to prevent social norm violations have been associated with increased activities in both the empathy and theory-of-mind networks, including the aMCC, AI, and mPFC (Krueger and Hoffman, 2016). Yet these studies focused on brain activities related to punishment decisions rather than neurobiological mechanisms that link perceived ingroup suffering to propensity to seek revenge upon outgroups. During previously studied economic games, punishment decisions were likely motivated by prevention of social norm violations rather than by perceived physical harm to ingroup members caused by outgroup members, which characterizes most revenge behavior in real-life situations.

Finally, at the hormone level, recent research reported increased levels of urinary oxytocin (OT) — a nine-amino-acid peptide synthesized in hypothalamic cells — in chimpanzees immediately before and during border patrols and intergroup encounters (Samuni et al., 2017). Likewise, intranasal administration of OT (vs. placebo) in humans enhanced both empathic neural responses to ingroup pain (Sheng et al., 2013) and individuals’ contributions to ingroup payoffs (De Dreu et al., 2010; De Dreu, 2010). OT administration also promotes motivation to sacrifice outgroup targets (De Dreu et al., 2011) and facilitates within-group coordination for successful outgroup attack during economic games (Zhang et al., 2019). These findings shed light on a functional role of the oxytocinergic system in decision making related to outgroup punishment. However, there has been little direct evidence to show that endogenous OT in humans is modulated during intergroup conflict (but see Levy et al., 2016). In addition, neural architectures that mediate endogenous OT and revenge propensity during intergroup conflict have been largely unexplored. Among the brain regions in which activities are sensitive to ingroup pain, the mPFC contains OT-sensitive neurons (Ninan, 2011). The mPFC, cingulate, and insula express OT receptors (Gimpl and Fahrenholz, 2001; Macdonald and Macdonald, 2010; Boccia et al., 2013) and mPFC/aMCC activities are modulated by administered OT (Sabihi et al., 2014; Eckstein et al., 2015; Liu et al., 2017; Wang et al., 2017). However, to date, whether the neural systems that are involved in empathy or theory-of-mind link endogenous OT to revenge propensity during intergroup conflict has yet to be examined.

A key challenge to our ability to address these issues empirically is the need for an experimental paradigm of intergroup conflict that can be used in a neuroimaging laboratory setting to measure: (i) neurobiological responses to perceived ingroup physical pain caused by an outgroup and (ii) revenge propensity to bring physical harm to the outgroup. Another challenge for empirical research on the neurobiological association of revenge propensity during intergroup conflict is the need to disentangle the effect of the key component of revenge (i.e., the punishment of outgroup members for harm that they have caused to one's ingroup) from other concomitant but nonessential factors, including perceived group identity (Kahn et al., 2017), negative evaluation of the outgroup (Schiller et al., 2014), and decreased empathy for outgroup pain (Hein et al., 2010; Cikara et al., 2011; Han, 2018). These factors themselves may lead to negative treatment of outgroup members on the basis of ingroup biases in social behavior that occur even in the absence of intergroup conflict. It is therefore necessary to examine neurobiological responses in two conditions in which ingroup biases in emotions, attitudes, and behavior are matched, but the motive to punish the outgroup is different. That is, in the revenge condition, individuals punish outgroup members because they bring physical harm to the ingroup, whereas in the control condition, all else being equal, individuals punish outgroup members to show their ingroup favoritism in the absence of perceived intergroup conflict.

Toward these ends, we developed a new neural-behavioral paradigm that simulates real-life revenge during intergroup conflict. In this paradigm, participants viewed an ingroup and an outgroup member who gave each other painful electric shocks (Revenge group) or received electric shocks given by a computer (Control group) during a competitive game. The key difference between the two conditions is whether outgroup members brought physical harm to ingroup members during an intergroup conflict while all other aspects of the experimental manipulations were the same for the two groups. We measured participants' salivary levels of OT, brain responses to perceived ingroup pain, and revenge propensity to bring physical harm to the outgroup. These measures allowed us to investigate the neurobiological correlates of how harm to an ingroup member caused by an outgroup member inspires an uninvolved ingroup member to punish outgroup members.

On the basis of the findings of increased endogenous OT immediately before and during border patrols and intergroup encounters in chimpanzees (Samuni et al., 2017), we hypothesized that endogenous OT in humans is also sensitive to intergroup conflict and that salivary levels of OT would increase in the Revenge compared to Control groups. As brain regions in both the empathy network and the theory-of-mind network express OT receptors (Gimpl and Fahrenholz, 2001; Macdonald and Macdonald, 2010; Boccia et al., 2013), and as activities in both networks are modulated by administered OT (Sabihi et al., 2014; Eckstein et al., 2015; Liu et al., 2017; Wang et al., 2017), activities in both networks in response to ingroup members' pain may be associated with endogenous OT in the context of intergroup conflict that involves physical harm. We tested this hypothesis by conducting whole-brain analyses of neural responses to perceived ingroup pain. Specifically, we searched for brain regions in which salivary levels of OT predicted activity in response to ingroup pain caused by a member of the outgroup. Our whole-brain analyses revealed that salivary levels of OT were associated with mPFC activity in the Revenge group. Accordingly, we further tested whether the mPFC activity predicted propensity to punish outgroup members and mediated the association between endogenous OT and revenge propensity. The results of these analyses allowed us to test the association between endogenous OT and mPFC responses to ingroup pain as a neurobiological correlate of revenge propensity during intergroup conflict. To provide a broad test of the neurobiological underpinnings of revenge propensity, we also examined whether the tendency to retaliate against outgroup members who are not directly involved in the conflict, which has been termed ‘vicarious retribution’ (Lickel et al., 2006; Gelfand et al., 2012; Lee et al., 2013), has the same neurobiological association.

In the text below, after describing our experimental design, we first present behavioral results that show comparable ingroup biases in emotions and attitudes in the Revenge and Control groups. We then examine whether the Revenge group, when compared to the Control group, showed higher endogenous OT levels after witnessing ingroup members' pain caused by outgroup members. Thereafter, we report the results of whole-brain analyses that identified neural responses to outgroup-inflicted ingroup pain, which were predicted by salivary levels of OT. Finally, we report evidence for the association between revenge propensity and mPFC activity in response to ingroup pain caused by the outgroup, as well as evidence that the mPFC activity mediates the association between endogenous OT and revenge propensity. These results together suggest an association between endogenous OT and mPFC activity in response to ingroup pain as a neurobiological correlate of revenge propensity during intergroup conflict.

Revenge behavior is universal and highly costly. Accordingly, understanding its neurobiological bases is of critical theoretical and practical importance. Revenge behavior during intergroup conflict engages multiple psychological processes from perceiving ingroup suffering to making aggressive decisions toward outgroups. Our research focused on how neurobiological responses to ingroup pain are related to revenge decisions during intergroup conflict. Although multiple motives are involved in vengeful behaviors during intergroup conflict, we compared salivary OT and brain activity in the Revenge and Control groups. This design allowed us to isolate the neurobiological responses to perceived ingroup suffering resulting from physical harm caused by an outgroup member and its association with seeking revenge through physical harm upon outgroup members.

Previous research has reported endogenous OT reactivity during intergroup conflict in wild chimpanzees (Samuni et al., 2017), but we showed the first evidence of increased levels of endogenous OT in humans during intergroup conflict that involves physical harm to ingroups caused by an outgroup. The salivary OT level in humans is affected by affiliative contact and is interrelated with the plasma OT level (Feldman et al., 2011). Our results complement previous research on the effect of intranasal OT administration on ingroup cooperation and outgroup defensive competition in economic games (De Dreu et al., 2010; De Dreu et al., 2011). Importantly, our findings suggest that endogenous OT is an influential physiological mechanism in humans that is activated in response to intergroup conflicts that involve physical harm between ingroup and outgroup members. It is worth noting that, similar to the finding in chimpanzees (Samuni et al., 2017), our results suggest that endogenous OT increases after initially witnessing intergroup conflict (e.g., at Time-1). It appears that, in both humans and chimpanzees, the oxytocinergic system quickly responds to intergroup conflict. In addition, the endogenous OT increased more after further witnessing intergroup conflict (e.g., at Time-2) and dropped when intergroup conflict had ended (e.g., at Time-3). These results illustrate dynamic changes of endogenous OT that occur throughout the entire duration of an intergroup conflict.

Our results also revealed intermediate brain mechanisms linking endogenous OT reactivity to perceived intergroup conflict and revenge propensity during intergroup conflict. Unlike previous research that focused on increased neural activity following aggressive decisions (Seymour et al., 2007; Krämer et al., 2007; Krueger and Hoffman, 2016; Chester and DeWall, 2016), our work showed that the mPFC activity in response to ingroup pain predicted the propensity for subsequent revenge behavior during intergroup conflict. The mPFC is well-known for its functional role in representing mental states (Amodio and Frith, 2006), social emotion (Harris and Fiske, 2007; Mathur et al., 2010), and group identity (Volz et al., 2009; Molenberghs and Morrison, 2014). Our results further revealed that the neurobiological association between endogenous OT and mPFC activity in response to Involved_Ingroup targets' suffering is related to propensity to punish outgroup members, regardless of whether the outgroup members were directly involved in the conflict. This cross-group brain-propensity association is different from previous findings of a within-group brain-propensity association in an intergroup context without direct conflict. That is, neural responses to ingroup pain predict tendencies to help ingroup members, or neural responses to outgroup pain predict tendencies not to help outgroup members (Hein et al., 2010; Cikara et al., 2011; Mathur et al., 2010). Our results cast a new perspective on the neural underpinnings that drive decisions to apply physical harm toward outgroups during intergroup conflict. More generally, the cross-group brain-propensity association suggests a potential neural mechanism underlying the contagion of revenge behavior, and may help us to understand why disputes between two individuals can escalate across groups and across time (Gelfand et al., 2012).

Our findings make fundamental contributions to the intergroup conflict literature and, in particular, towards understanding the neurobiological associations of ingroup love and outgroup hate. Intergroup conflict plays a substantial role in the evolution of both aggressiveness against outgroup and cooperativeness towards ingroups (Rusch, 2014). Although previous studies have demonstrated the role of OT and mPFC activity in altruistic decisions favoring the ingroup (De Dreu et al., 2010; Mathur et al., 2010; De Dreu et al., 2011), our results suggest that the context of intergroup conflict may shift the key function of the oxytocinergic system from mediating ingroup love (a desire to help the ingroup) to facilitating outgroup hate (an aggressive motivation to hurt the outgroup). Such variation in the social function of OT may assist individuals to adapt to changing social contexts (Shamay-Tsoory and Abu-Akel, 2016; Ma et al., 2016). Unlike previous research that focused on increased activity in the reward system as a consequence of aggressive decisions (Krämer et al., 2007; Chester and DeWall, 2016), our findings highlight a neurobiological association between endogenous OT and the mPFC that occurs prior to but is linked to revenge propensity during intergroup conflicts. Our results open a new avenue toward understanding the neurobiological mechanisms that mediate aggression-related hormones and social decisions related to intergroup hostility, and provide a neuroscientific account of revenge motives during intergroup conflict.

The revenge propensity shown in the Revenge group cannot simply be explained by OT-induced negative emotions. Increasing evidence suggests that the oxytocinergic system is involved in modulating multiple social emotions that are either positively (e.g., empathy, Sheng et al., 2013) or negatively (e.g., schadenfreude, Shamay-Tsoory et al., 2009) related to social behaviors. Our participants from both the Revenge and Control groups reported greater schadenfreude when viewing outgroup suffering rather than ingroup suffering. Schadenfreude has been linked to striatum activation induced by misfortunes happening to envied persons (Takahashi et al., 2009). Although previous research has shown that viewing outgroup pain can activate the nucleus accumbens (NAcc) and greater NAcc activity predicted less motivation to help outgroup members (Hein et al., 2010; Luo et al., 2015), neither the Revenge group nor the Control group in our work showed activations in the reward system when viewing outgroup members' suffering. Thus, negative emotion such as schadenfreude may play a minimal role in modulating revenge propensity in our experimental settings.

Although our measures of endogenous OT and brain activity suggest that the association between endogenous OT and mPFC activity in response to perceived ingroup suffering is related to revenge tendencies, we noted that the latter were estimated by self-reports. It is unclear whether such measures were actually correlated with revengeful behavior. To test this, we conducted an independent behavioral experiment in a new sample (see Supplementary files 10 and 11). The experimental procedures were the same as those in our fMRI experiment except that Phase three was modified in the following way. While viewing Involved_Ingroup and Involved_Outgroup targets who played the competitive game with each other (Revenge group) or with a computer (Control group), participants were occasionally (in four trials, two on Involved_Ingroup targets and two on Involved_Outgroup targets) asked to make punishment decisions by deciding the intensity on a Likert Scale (1 = not painful at all, 9 = extremely painful) of electric shocks that were believed to be inflicted on the targets. To enhance participants' beliefs about the experimental setting, they only viewed Involved_Ingroup and Involved_Outgroup targets during the game and they made punishment decisions simultaneously but in different rooms. Moreover, after each punishment decision, a feedback face with either painful or neutral expression, depending on a participant's decision, was presented to inform the participant of the consequence of his decision. The results showed evidence that the measures of punishment tendencies were positively correlated with the measures of actual punishment decisions toward Involved_Outgroup target in both Revenge and Control groups (r = 0.75 and 0.70, FDR corrected ps <0.001). These results indicate that our measures of punishment tendencies can, to a certain degree, reflect individuals' punishment decisions with real consequences.

Our findings also raise new questions about the role of other brain regions in the process of revenge. For example, recent research has shown that, in a scenario in which an observer punishes transgressors due to social norm violation (i.e., third-party punishment), the willingness to punish severely was associated with increased amygdala activity (Stallen et al., 2018), possibly reflecting the encoding of affective arousal associated with harm done to someone else (Buckholtz and Marois, 2012; Krueger and Hoffman, 2016). The current work, however, did not find evidence of an association between amygdala activity and punishment tendencies during intergroup conflict. It is possible that punishment decisions toward outgroup in the context of intergroup conflict are justified as revenge that reduces ingroup suffering and thus bring less negative arousal. Future research is needed to support this speculation.

Our work also expands the literature to examine neural responses that are implicated in vicarious revenge, which occurs when a person punishes an outgroup member who is not one of the direct causal agents in the original attack against an ingroup member (Lickel et al., 2006; Gelfand et al., 2012; Lee et al., 2013). Neither the agent of retaliation nor the target of retribution is directly involved in the original conflict during vicarious retribution, similar to the punishment of Uninvolved-Outgroup targets in our work. We showed that the mPFC activity in response to ingroup pain similarly predicted punishment tendencies toward Involved-Outgroup targets and Uninvolved-Outgroup targets. The mPFC activity also mediated the relationship between endogenous OT and tendency to punish Involved_Outgroup targets, as well as the relationship between endogenous OT and tendency to punish Uninvolved_Outgroup targets. Thus, our findings suggest a neurobiological correlate of punishment tendency during intergroup conflict that does not differentiate between direct and indirect vicarious retribution. This is a possible result of the fact that outgroup members are perceived as a unified and coherent entity and share the same blameworthy qualities during intergroup conflict (McConnell et al., 1997; Crawford et al., 2002; Lee et al., 2013).

In conclusion, by integrating a neural-behavioral paradigm with fMRI, we provided evidence that intergroup conflict is associated with increased salivary levels of OT in humans, which further predicted stronger mPFC activity in response to ingroup suffering caused by an outgroup member. Moreover, the mPFC activity mediates the association between endogenous OT and propensity to seek revenge by giving painful electric shocks to outgroup members. Our findings highlight the coupling of the OT system and the mPFC as a neurobiological correlate of revenge propensity during intergroup conflict. Our paradigm can be applied to other samples (e.g., females) and cultures (e.g., where individualism is more dominant) to advance our understanding of the neurobiological underpinnings of revenge propensity and behavior during intergroup conflict.

Finally, because other motivations also drive revenge behavior in intergroup contexts, including feeling threat to group pride (Turner and Tajfel, 1986), empathy for the harmed ingroup members (Smith et al., 1999; Davis, 2018), and normative pressure to avenge the ingroup (Deutsch and Gerard, 1955), future research should examine different motivations driving revenge and concomitant emotions that become activated in a host of revenge situations.

All data generated or analysed for figures of this study are included in the manuscript and supporting files. Source data files have been provided for Figures 2–6.

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Author details

1. Xiaochun Han

   School of Psychological and Cognitive Sciences, PKU-IDG/McGovern Institute for Brain Research, Beijing Key Laboratory of Behavior and Mental Health, Peking University, Beijing, China

   Contribution

   Conceptualization, Data curation, Formal analysis, Validation, Investigation, Visualization, Methodology, Writing - original draft, Writing - review and editing

   Competing interests

   No competing interests declared

2. Michele J Gelfand

   Department of Psychology, University of Maryland, College Park, United States

   Contribution

   Conceptualization, Methodology, Writing - review and editing

   Competing interests

   No competing interests declared

3. Bing Wu

   Department of Radiology, The 7th Medical Center of PLA General Hospital, Beijing, China

   Contribution

   Data curation, Methodology, Writing - review and editing

   Competing interests

   No competing interests declared

4. Ting Zhang

   School of Psychological and Cognitive Sciences, PKU-IDG/McGovern Institute for Brain Research, Beijing Key Laboratory of Behavior and Mental Health, Peking University, Beijing, China

   Contribution

   Data curation, Methodology, Writing - review and editing

   Competing interests

   No competing interests declared

5. Wenxin Li

   Peking-Tsinghua Center for Life Sciences, Peking University, Beijing, China

   Contribution

   Data curation, Methodology, Writing - review and editing

   Competing interests

   No competing interests declared

6. Tianyu Gao

   School of Psychological and Cognitive Sciences, PKU-IDG/McGovern Institute for Brain Research, Beijing Key Laboratory of Behavior and Mental Health, Peking University, Beijing, China

   Contribution

   Data curation, Methodology, Writing - review and editing

   Competing interests

   No competing interests declared

7. Chenyu Pang

   School of Psychological and Cognitive Sciences, PKU-IDG/McGovern Institute for Brain Research, Beijing Key Laboratory of Behavior and Mental Health, Peking University, Beijing, China

   Contribution

   Data curation, Methodology, Writing - review and editing

   Competing interests

   No competing interests declared

8. Taoyu Wu

   School of Psychological and Cognitive Sciences, PKU-IDG/McGovern Institute for Brain Research, Beijing Key Laboratory of Behavior and Mental Health, Peking University, Beijing, China

   Contribution

   Data curation, Methodology, Writing - review and editing

   Competing interests

   No competing interests declared

9. Yuqing Zhou

   School of Psychological and Cognitive Sciences, PKU-IDG/McGovern Institute for Brain Research, Beijing Key Laboratory of Behavior and Mental Health, Peking University, Beijing, China

   Contribution

   Data curation, Methodology, Writing - review and editing

   Competing interests

   No competing interests declared

10. Shuai Zhou

    Department of Radiology, The 7th Medical Center of PLA General Hospital, Beijing, China

    Contribution

    Data curation, Methodology, Writing - review and editing

    Competing interests

    No competing interests declared

11. Xinhuai Wu

    Department of Radiology, The 7th Medical Center of PLA General Hospital, Beijing, China

    Contribution

    Data curation, Supervision, Methodology, Writing - review and editing

    For correspondence

    bei925@sina.com

    Competing interests

    No competing interests declared

12. Shihui Han

    School of Psychological and Cognitive Sciences, PKU-IDG/McGovern Institute for Brain Research, Beijing Key Laboratory of Behavior and Mental Health, Peking University, Beijing, China

    Contribution

    Conceptualization, Resources, Data curation, Formal analysis, Supervision, Funding acquisition, Validation, Investigation, Visualization, Methodology, Writing - original draft, Project administration, Writing - review and editing

    For correspondence

    shan@pku.edu.cn

    Competing interests

    No competing interests declared

    "This ORCID iD identifies the author of this article:" 0000-0003-3350-5104

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