Glowacki provides an interesting account regarding the conditions necessary for peaceful intergroup relations. On a behavioral level, the account makes sense, but it falls short when describing the underlying psychological processes. Glowacki submits that the key to peace is the ability to predict the behaviors of both in-group and out-group members, a process that is facilitated by the presence of group-level norms. However, this explanation omits processes critical to understanding the origin of peaceful intergroup relations.

One account for the development of human sociality is that positive intergroup relations developed from the processes that governed intragroup processes. Indeed, numerous theorists, beginning with Darwin (Reference Darwin1859, Reference Darwin1871), have proposed that the challenges associated with regulating interactions with other persons were preeminent in guiding the development of the hominid psyche (e.g., Alexander & Noonan, Reference Alexander, Noonan, Chagnon and Irons1979; Bigelow, Reference Bigelow1969; Hamilton, Reference Hamilton and Fox1975; Humphrey, Reference Humphrey, Bareson and Hinde1976; Wilson, Reference Wilson1973). The origins of the psychology of intragroup relations likely had their roots in simpler interindividual relationships. Starting with the first facilitative bipeds and the ability to gather, transport, and accrue resources (e.g., last common ancestor; Ardipithecus ramidus; 5.8–5.2 million years ago; WoldeGabriel et al., Reference WoldeGabriel, Ambrose, Barboni, Bonnefille, Bremond, Currie and White2009), the social structure changed to include sharing and cofeeding (Belisle & Chapais, Reference Belisle and Chapais2001). These changes fueled the growth of males' monopolization of access to females, and the physical proximity led males to spend more time with them (and her offspring) to adopt a stabilizing reproductive strategy (Grueter, Chapais, & Zinner, Reference Grueter, Chapais and Zinner2012). Parent–offspring and offspring–parent recognition and investment stemmed from this intimate and sustained parental care (Chapais, Reference Chapais2008; Lieberman, Tooby, & Cosmides, Reference Lieberman, Tooby and Cosmides2007). The psychology that governs intragroup relations – which at its most basic form suggests that kin should consider first the interests of fellow kin – was borne from the relatively basic processes related to kin-formation, including motherhood, fatherhood, siblingship, incest avoidance, and in-law recognition (e.g., Hill & Hurtado, Reference Hill and Hurtado2009; Korchmaros & Kenny, Reference Korchmaros and Kenny2001; Lieberman, Reference Lieberman2009).

Early hominids extended these pair-bond-like relationships beyond breeding couples to develop relationships in wider social networks (Aureli et al., Reference Aureli, Schaffner, Boesch, Bearder, Call, Chapman and van Schaik2008). Thus, the intergroup norms that Glowacki describes as critical are important, but they are of secondary importance relative to the underlying psychological process bearing on intragroup processes.

A critical implication of an intergroup psychology derived from intragroup processes is that the “default” intergroup dynamic is uncertainty rather than the aggression/conflict. Laboratory research into intergroup relations reveals that conflict is strongly rooted in uncertainty or fear of the out-group (Wildschut, Pinter, Vevea, Insko, & Schopler, Reference Wildschut, Pinter, Vevea, Insko and Schopler2003). In the context of mixed motive game (i.e., the prisoner's dilemma; Luce & Raiffa, Reference Luce and Raiffa1957), interactions that include a safe, “withdrawal” option – in addition to the usual cooperative and competitive choices – groups prefer to withdraw to competing (Schopler et al., Reference Schopler, Insko, Graetz, Drigotas, Smith and Dahl1993, Reference Schopler, Insko, Drigotas, Wieselquist, Pemberton and Cox1995). Indeed, when there is uncertainty about what the out-group will do, in-groups cooperate less with them (Insko, Kirchner, Pinter, Efaw, & Wildschut, Reference Insko, Kirchner, Pinter, Efaw and Wildschut2005).

Groups avoid conflict – and even seek peace – when given the opportunity. Laboratory variations using “minimal” groups (Tajfel, Billig, Bundy, & Flament, Reference Tajfel, Billig, Bundy and Flament1971) show that when group members allocate resources to the in-group and out-group, they divide money equally between them (Bornstein et al., Reference Bornstein, Crum, Wittenbraker, Harring, Insko and Thibaut1983; Gaertner & Insko, Reference Gaertner and Insko2001). And when in-group and out-group evaluations are made separately, participants prefer bolstering the in-group over harming the out-group (e.g., Brewer, Reference Brewer1999; Brewer & Campbell, Reference Brewer and Campbell1976). When participants are tasked with allocating painful noise, they favor equal distributions to ones that predominantly hurt the out-group (e.g., Mummendey et al., Reference Mummendey, Simon, Dietze, Grünert, Haeger, Kessler and Schäferhoff1992).

Such intergroup interactions reflect motivations grounded in an intragroup psychology. Many theorists, including those focusing on social identity (Tajfel, Reference Tajfel1970; Tajfel & Turner, Reference Tajfel, Turner, Worchel and Austin1986) and on in-group favoring norms (Montoya & Pinter, Reference Montoya and Pinter2016; Montoya & Pittinsky, Reference Montoya and Pittinsky2013) emphasize that intergroup motivations are fueled by normative pressure to favor the in-group. For instance, group members behave cooperatively or competitively with an out-group according to the group norm that was emphasized to them (Jetten, Spears, & Manstead, Reference Jetten, Spears and Manstead1996; Montoya & Pittinsky, Reference Montoya and Pittinsky2013). As noted by Glowacki, there are a number of pressures that can push intergroup relations to be hostile. For instance, conflict is more likely when in-group members overtly support hostilities (Wildschut, Insko, & Gaertner, Reference Wildschut, Insko and Gaertner2002) and when individual group member's behaviors are concealed from the out-group (Schopler et al., Reference Schopler, Insko, Drigotas, Wieselquist, Pemberton and Cox1995), to name but two.

Uncertainty permits a range of potential intergroup interactions, including ones that are positive. But as we have noted, the potential for intergroup peace is dependent on the concerns of the in-group.

The possibility of peace – and the advantages of positive intergroup relations – likely began early in human evolutionary history. The growing complexity of human's social relationships is reflected in a dramatic increase in encephalization that began approximately 4–6 million years ago (Grabowski, Reference Grabowski2016; Van Schaik, Triki, Bshary, & Heldstab, Reference Van Schaik, Triki, Bshary and Heldstab2021). The primary factor for the growth in brain size in hominids (controlling for body mass) has been attributed to the problems associated with the complexity of social relationships (including alloparenting, coalition formation, tactical deception; Dunbar & Shultz, Reference Dunbar and Shultz2007; Isler & Van Schaik, Reference Isler and Van Schaik2006). Indeed, cooperative intergroup relations were essential for survival, and their benefits that started earlier than 300,000 years ago. The degree of outbreeding identified by studies of genetic diversity (e.g., Workman & Niswander, Reference Workman and Niswander1970) cannot be explained solely by competitive strategies (e.g., taking slaves after victory). Cooperative intergroup relations (which include group merging or emigration) were necessary not only to avoid inbreeding, but also to create groups that served needs relative to the food supply and mating (Knauft et al., Reference Knauft, Abler, Betzig, Boehm, Dentan, Kiefer and Rodseth1991; Loehle, Reference Loehle1995).