In the target article, Stanford argues that moral norms serve as fundamental pillars of human interaction, which regulate cooperation, fairness, and altruism within social groups. In turn, severe punishment and social exclusion of in-group wrongdoers might serve the particular purpose to promote stable in-group cooperation. As such, maintaining a shared in-group morality reduces the necessity of constantly tracking the potential risk of exploitation in in-group interactions, thus providing the basis for spontaneous in-group cooperation. Although we concur with the author's reasoning and conclusion in principal, it is unfortunately only a selective excerpt of the full picture. Specifically, the target article overlooks the dramatic negative consequences of (opposing) moral convictions for human interactions. While morality may indeed foster cooperation and harmony within groups, it may also fuel aggression and conflict between groups. We therefore argue that morality represents an important cue through which both in-group cooperation and intergroup conflict are channeled.

Indeed, the presence of a morally opposing out-group is a central criterion of in-group identification (Parker & Janoff-Bulman Reference Parker and Janoff-Bulman2013). A prominent example is the widespread political separation and radicalization based on opposing moral ideologies regarding issues such as refugees, abortion, or socialized healthcare. In such group constellations, moral disagreement becomes the defining aspect of social identity, uniting a joint moral position in opposition to others. Because immorality is associated with harmful acts (Gray et al. Reference Gray, Young and Waytz2012), morality-based out-groups are perceived as a severe threat to the in-group. Specifically, morality-based out-groups (i.e., pro-life vs. pro-choice) have been shown to be perceived with more negative emotions and seen as a greater threat to the in-group than non–morality-based out-groups (i.e., fans of Boston Red Sox vs. New York Yankees; Parker & Janoff-Bulman Reference Parker and Janoff-Bulman2013). Likewise, actions of morality-based out-groups are likely perceived in terms of offensive aggression toward the in-group (Waytz et al. Reference Waytz, Young and Ginges2014), which may, in turn, spark intentions to defend and protect the in-group, even by means of actively harming the out-group (Böhm et al. Reference Böhm, Rusch and Gürerk2016).

This perspective is supported by research comparing intergroup conflict between morality-based and non–morality-based opponent groups: In morality-based intergroup conflict, the motivation to absolutely benefit the in-group (i.e., “in-group love”) is closely tied to the motivation to aggressively harm or competitively outperform the out-group (i.e., “out-group hate”; Brewer Reference Brewer1999). Interactions between non-morality-based groups have been shown to be primarily guided by in-group love (e.g., Halevy et al. Reference Halevy, Bornstein and Sagiv2008; Thielmann & Böhm Reference Thielmann and Böhm2016), whereas out-group hate increases substantially in interactions between morality-based out-groups (Parker & Janoff-Bulman Reference Parker and Janoff-Bulman2013; Weisel & Böhm Reference Weisel and Böhm2015). This suggests that the nature of interactions between morality-based out-groups is crucially different from interactions between individuals with opposing moral demands within the same social group. For example, Weisel and Böhm (Reference Weisel and Böhm2015) measured out-group hate as the willingness to actively diminish out-group members’ resources at personal cost in an intergroup social dilemma game. Despite the availability of an outside option that had the same benefit for the in-group without necessarily harming the out-group, findings revealed a clear motivation to harm the out-group. Importantly, out-group hate increased substantially only in interaction with members of a morality-based out-group but not in interaction with members of a non–morality-based, yet high-enmity out-group. This shows that the incongruence between groups’ moral values may not only hinder intergroup cooperation, but also will even foster harmful intergroup conflict. By implication, introducing morality into intergroup conflict has the destructive potential to override the human aversion of doing harm to others (e.g., Buhl Reference Buhl1999).

Moreover, there are several psychological factors that may further promote and justify aggression toward morality-based out-groups. Most importantly, members of morality-based out-groups are easily dehumanized, denying them the essential rights of humans and therefore providing the psychological basis of harsh and even immoral treatment (Haslam Reference Haslam2006; Struch & Schwartz Reference Struch and Schwartz1989). Serving the same purpose, the immoral behaviors of the in-group toward morality-based out-groups (e.g., torture) are reframed (i.e., morality shifting; Leidner & Castano Reference Leidner and Castano2012), such that the mistreatment of (alleged) moral opponents becomes psychologically more acceptable.

In addition to the rise of out-group hate, moral opposition in intergroup conflict may also hinder peacemaking and reconciliation in ongoing conflicts (Halperin et al. Reference Halperin, Russell, Dweck and Gross2011; Waytz et al. Reference Waytz, Young and Ginges2014). For instance, group-based anger has been shown to increase the support for compromises in intergroup negotiations when there is low hatred between groups (i.e., low levels of out-group hate), whereas it leads to increased aggression when there is high hatred between groups (Halperin et al. Reference Halperin, Russell, Dweck and Gross2011).

Overall, the available evidence clearly demonstrates that there are two sides to how morality shapes human interaction. Whereas morality may indeed foster harmony within groups as reasoned by Stanford, it also fuels conflict between groups: As soon as several individuals with shared moral convictions form opposing groups, the moral foundation is also a basis for long-term intergroup conflict and violence, which aims at actively harming out-group members rather than at restoring cooperative social interactions. Understanding the role of morality for human interactions on a more global level therefore also requires awareness of its downside: Whereas it has the potential to unite, it will concurrently divide. As a consequence, the positive effects of morality for the functioning of interactions within groups are intrinsically tied to the negative and destructive effects for interactions between groups. This inherent link might even have co-evolved as suggested by the theory of parochial altruism (e.g., Choi & Bowles Reference Choi and Bowles2007). We thus conclude that morality – though it may well drive cooperation and harmony within groups – also fuels aggression and conflict on a larger scale.