Across cultures, enduring conflict remains an integral part of human societies with often devastating consequences. In the twentieth century alone, almost 135 million individuals died from conflict, including 22 million murdered and 112 million killed in war (https://informationisbeautiful.net/visualizations/senseless-conflict-deaths-per-hour/). In their superb article, De Dreu and Gross focus on conflict by decomposing its underlying behaviors into “attack” and “defense.” However, because humans are notable for the scope and variety of their prosocial behavior, which find no equal among any other species (Silk & House Reference Silk and House2011), the nature of conflict may not be defined exclusively by antisocial behaviors. In this commentary, we therefore extend the view of De Dreu and Gross by highlighting the role of pathological altruism and its associated neuroendocrine substrates in the origin of conflict.

Altruistic and hostile behaviors are often viewed as diametrically opposed. However, given that both behaviors can take on different facets owing to variability in individual, motivational, and situational factors, attack and defense may not be uniformly linked to antisocial acts in the same way as altruism may not be universally prosocial (Marsh Reference Marsh2018). One such example is pathological altruism. In contrast to the positive outcomes associated with philanthropic altruism, in general, pathological altruism is thought to reflect extreme acts of selfless behavior resulting in negative consequences to the self and even to innocent others (e.g., family members), ranging from co-dependency to death (Oakley Reference Oakley2013). Prototypical examples include Christian martyrdom or burnout in health care professionals (Hurlemann & Marsh Reference Hurlemann and Marsh2016); even suicide bombings might be caused by pathological altruism when perpetrators of these acts believe they are behaving altruistically toward those who share their ideology (Habash Reference Habash2011). Pathological altruism is deeply rooted in the evolution of cooperation and parochialism, characterized by in-group favoritism on the one hand and hostile and derogatory behaviors toward out-groups on the other (Choi & Bowles Reference Choi and Bowles2007). The formation and maintenance of in-group alliances are often enforced by internalized social norms (Bernhard et al. Reference Bernhard, Fischbacher and Fehr2006; Gavrilets & Richerson Reference Gavrilets and Richerson2017) and personally costly sanctions against defectors of these norms (Izuma & Adolphs Reference Izuma and Adolphs2013) – an inclination defined as altruistic punishment, possibly evolving to protect in-group alliances from erosion through selfish motives (Fehr & Gächter Reference Fehr and Gächter2002). Thus, pathological forms of altruism may result from unconditional in-group commitment, potentially translating into extreme behaviors in a limited number of individuals who are willing to make sacrifices for their groups no matter the personal costs and consequences for others.

Current concepts of the neurobiology of altruism hold that it is anchored in the motivational architecture of the social brain. Over the past decade, research from various fields has documented the effects of the evolutionarily conserved hypothalamic peptide hormone oxytocin, in modulating a diverse repertoire of human (pro)social behaviors (Donaldson & Young Reference Donaldson and Young2008), in addition to its well-established role in reproduction and the formation and maintenance of pair-bonding (Hurlemann & Scheele Reference Hurlemann and Scheele2016; Insel & Young Reference Insel and Young2001; Young & Wang Reference Young and Wang2004). The peptide's effects have been substantiated by a plethora of studies showing that the exogenous delivery of oxytocin as a nasal spray alters outcome measures of behavioral and neural response (Meyer-Lindenberg et al. Reference Meyer-Lindenberg, Domes, Kirsch and Heinrichs2011; Striepens et al. Reference Striepens, Kendrick, Maier and Hurlemann2011; Yamasue et al. Reference Yamasue, Yee, Hurlemann, Rilling, Chen, Meyer-Lindenberg and Tost2012). Studies have consistently shown that nasally administered oxytocin dampens amygdala reactivity toward social fear signals in both humans (Eckstein et al. Reference Eckstein, Becker, Scheele, Scholz, Preckel, Schlaepfer, Grinevich, Kendrick, Maier and Hurlemann2015; Kanat et al. Reference Kanat, Heinrichs, Mader, van Elst and Domes2015) and macaques (Liu et al. Reference Liu, Hadj-Bouziane, Jones, Turchi, Averbeck and Ungerleider2015). Specifically, its potential to attenuate hypothalamic-pituitary-adrenal axis activity in response to social stressors, at least under conditions of social support (Heinrichs et al. Reference Heinrichs, Baumgartner, Kirschbaum and Ehlert2003), has substantiated oxytocin's anxiolytic effect profile, which facilitates interpersonal trust (Baumgartner et al. Reference Baumgartner, Heinrichs, Vonlanthen, Fischbacher and Fehr2008; Kosfeld et al. Reference Kosfeld, Heinrichs, Zak, Fischbacher and Fehr2005) and the formation and maintenance of interpersonal bonds (Insel Reference Insel1997).

In recent years, studies in the field of social neuroscience have provided new insights into the psychobiological mechanisms of human altruism (Hurlemann & Marsh Reference Hurlemann and Marsh2016; Reference Hurlemann and Marsh2017). In particular, oxytocin has been found to modulate philanthropic altruism (Israel et al. Reference Israel, Weisel, Ebstein and Bornstein2012; Marsh et al. Reference Marsh, Scheele, Gerhardt, Strang, Enax, Weber, Maier and Hurlemann2015), empathy (Hurlemann et al. Reference Hurlemann, Patin, Onur, Cohen, Baumgartner, Metzler, Dziobek, Gallinat, Wagner, Maier and Kendrick2010; Strang et al. Reference Strang, Gerhardt, Marsh, Oroz Artigas, Hu, Hurlemann and Park2017), and parochialism and fairness (De Dreu et al. Reference De Dreu, Greer, Handgraaf, Shalvi, Van Kleef, Baas, Ten Velden, Van Dijk and Feith2010; Stallen et al. Reference Stallen, Rossi, Heijne, Smidts, De Dreu and Sanfey2018) along with social norm compliance (Stallen et al. Reference Stallen, De Dreu, Shalvi, Smidts and Sanfey2012). The existing evidence indicates that the peptide does not invariably promote positive social behaviors but may also evoke protective-aggressive responses (Striepens et al. Reference Striepens, Scheele, Kendrick, Becker, Schäfer, Schwalba, Reul, Maier and Hurlemann2012). These effects may result from changes in perceptions of and attitudes toward the environment, including increased attention to socially relevant cues (Guastella et al. Reference Guastella, Mitchell and Dadds2008) and altered sensing of and responding to emotional stimuli (Spengler et al. Reference Spengler, Scheele, Marsh, Kofferath, Flach, Schwarz, Stoffel-Wagner, Maier and Hurlemann2017). This resonates with evidence suggesting that the subtle, modulatory effects of oxytocin are highly susceptible to individual personality and contextual variables (Hurlemann Reference Hurlemann2017), such as gender (Scheele et al. Reference Scheele, Striepens, Kendrick, Schwering, Noelle, Wille, Schläpfer, Maier and Hurlemann2014), the presence of social cues (Marsh et al. Reference Marsh, Scheele, Gerhardt, Strang, Enax, Weber, Maier and Hurlemann2015), or group affiliation (De Dreu et al. Reference De Dreu, Greer, Handgraaf, Shalvi, Van Kleef, Baas, Ten Velden, Van Dijk and Feith2010). In contrast to previous experimental evidence, which emphasized either the efficacy of social norms as a potential means of stabilizing altruistic cooperation (Fehr & Fischbacher Reference Fehr and Fischbacher2004) or the influence of oxytocin signaling on social conformity (Huang et al. Reference Huang, Kendrick, Zheng and Yu2015; Stallen et al. Reference Stallen, De Dreu, Shalvi, Smidts and Sanfey2012), a recent study combined both interventions. Therein, in a series of experiments, it was shown that oxytocin-enforced norm compliance promotes an altruistic response bias toward outsiders, even in those individuals who made more selfish decisions in the absence of such exogenous triggers (Marsh et al. Reference Marsh, Scheele, Feinstein, Gerhardt, Strang, Maier and Hurlemann2017).

Collectively, this evidence suggests that oxytocin is centrally involved in modulating altruistic behavior, with recent studies stressing the substantial influence of message frames (Marsh et al. Reference Marsh, Scheele, Gerhardt, Strang, Enax, Weber, Maier and Hurlemann2015; Reference Marsh, Scheele, Feinstein, Gerhardt, Strang, Maier and Hurlemann2017) in determining the direction and magnitude of oxytocin effects on behavioral readouts in humans. Decomposing the underlying causes of conflict requires consideration of the evolutionary, developmental, and neurobiological origins of altruistic motivation and how individual variation with respect to this motivation may result in extreme social behaviors and actions that are dissociated from reasonable risks and rewards. Given this empirical background, we propose that pathological altruism and its determining factors may spiral into conflict, long-term hostilities or even warfare between groups, and that this behavior likely depends on and is influenced by oxytocin signaling in the brain.