Dunbar identifies cohesion as an important adaptive challenge for group living species and outlines structural, behavioral, and cognitive strategies for maintaining the stability of ever larger communities. Dunbar’s discussion of cognitive strategies focuses on behavioral inhibition and mentalizing. While both capacities are undoubtedly crucial for group cohesion, this insightful analysis neglects a key stressor on group living: the need to regulate cooperative relationships within groups. From friendships to alliances, social bonds in primates serve not just to defang social tensions; they function as mutually beneficial collaborative relationships. Here, we argue that fairness functions as a crucial cognitive-emotional strategy for maintaining group cohesion. We describe how dyadic expectations that emerged in the context of nonhuman primate relationships evolved into humans’ sense of fairness, allowing for larger and more cohesive groups in the process.

Dunbar is right to highlight friendship as a foundational mechanism underlying primate group cohesion. Friendships are dyadic cooperative relationships, which structure primate sociality. For example, chimpanzees trust their friends (Engelmann & Herrmann, Reference Engelmann and Herrmann2016), help them more (Engelmann, Haux, & Herrmann, Reference Engelmann, Haux and Herrmann2019), and console them more after conflict (Fraser, Stahl, & Aureli, Reference Fraser, Stahl and Aureli2008) relative to neutral partners. Friendship even predicts chimpanzees’ willingness to join a territorial patrol—a high-risk behavior crucial for group cohesion (Samuni et al., Reference Samuni, Preis, Mielke, Deschner, Wittig and Crockford2018).

As Dunbar points out, building and maintaining friendships is a costly process, requiring both time spent grooming and a suite of cognitive and affective adaptations. For this investment to be worthwhile, the friendship needs to confer more benefits than the costs it incurs; staying attached to a free-rider will improve the fitness of no one. Stable friendships thus require regulatory mechanisms that ensure that both individuals remain satisfied with the relationship.

Primates expect certain manners of treatment from their collaborative partners and react with negative emotions when these expectations are violated (Engelmann et al., Reference Engelmann, Clift, Herrmann and Tomasello2017; Ritov et al., Reference Ritov, Völter, Raihani and Engelmann2024; Titchener et al., Reference Titchener, Thiriau, Hüser, Scherberger, Fischer and Keupp2023). For example, chimpanzees express their dissatisfaction when a collaborative partner fails to live up to their expectations (Engelmann et al., Reference Engelmann, Clift, Herrmann and Tomasello2017; Ritov et al., Reference Ritov, Völter, Raihani and Engelmann2024). Such displays of negative emotion can act as a corrective force, influencing the partner to act as they expect them to and thereby stabilizing the relationship (Sell et al., Reference Sell, Sznycer, Al-Shawaf, Lim, Krauss, Feldman, Rascanu, Sugiyama, Cosmides and Tooby2017). In other words, social expectations can help individuals implement a strategy of “partner control” (Schino & Aureli, Reference Schino and Aureli2017). Chimpanzees’ expectations are flexible, responding not just to outcomes but to their partners’ intentions (Engelmann et al., Reference Engelmann, Herrmann, Proft, Keupp, Dunham and Rakoczy2022). This indicates that they rely on complex cognitive representations in addition to their strong affective component.

Nonhuman primates form these expectations based on prior interactions with the individual. Being the recipient of a cooperative act triggers a positive emotional response (e.g., gratitude); being the target of an uncooperative act triggers a negative emotional response (e.g., anger; Gervais & Fessler, Reference Gervais and Fessler2017). These momentary emotional responses are linked to the identity of the actor causing them, updating an attitudinal representation of the actor and contributing to the formation (and dissolution) of an affective bond between both parties (Gervais & Fessler, Reference Gervais and Fessler2017; Schino & Aureli, Reference Schino and Aureli2021). Hence, these expectations are relationship-specific in nature. Indeed, in a recent experiment, we compared food theft between friends and non-friends. Supporting this account, we recently found that friendship not only inhibited antisocial behavior but shaped chimpanzees’ emotional reactions to it (Ritov et al., Reference Ritov, Lewis, Völter, Samuni, Keltner, Herrmann and Engelmann2025).

Relationship-based social expectations can only scaffold cooperation with a limited number of partners. Given the high cost of close relationships, there are natural limits to the number of individuals one can form such bonds with. Therefore, even larger groups require a different type of mechanism to regulate collaborative relationships with distant group members (Dunbar’s “weak ties”).

The relationship-based expectations we see in other species evolved into fairness expectations in humans. In fairness, the protest is grounded not in an expectation for how a specific relationship-partner should treat us, but expectations for how one should be treated - i.e., how costs and benefits should be distributed between collaborative partners in general. While it has been argued that an aversion to unequal treatment is an ancestral trait, our recent meta-analysis has shown that this does not appear to be the case. Nonhuman animals respond to violations of social expectations, but they do not seem to care whether a conspecific is treated better than they are (Ritov et al., Reference Ritov, Völter, Raihani and Engelmann2024). Fairness therefore appears to be a human-unique regulatory strategy.

The emergence of generalized fairness expectations allowed for the increase in group size and cohesion we see in humans. A sense of fairness guides how we treat others even when a prior relationship is lacking, allowing individuals with otherwise weak ties to coordinate their expectations. Furthermore, fairness expectations can manifest in a direct comparison of immediate costs and rewards, allowing even one-off interactions to be mutually beneficial—unlike relationship-based expectations, which depend on long periods spent grooming and in close proximity. As such, fairness expectations—and other normative standards—allow the sphere of people we collaborate with to be much larger than the number of people we are closely bonded with (Henrich et al., Reference Henrich, Ensminger, McElreath, Barr, Barrett, Bolyanatz, Cardenas, Gurven, Gwako, Henrich, Lesorogol, Marlowe, Tracer and Ziker2010).

To sum up, we propose that social expectations have played a central role in the size and cohesion of primate social groups. Nonhuman primates form social expectations within the context of close relationships, which allow for these relationships to be mutually beneficial and thus more stable over time. This relationship stability reduces the tensions inherent to group living. In humans, these expectations generalized to fairness expectations, which allow for mutually beneficial collaborations with a larger number of individuals. Dunbar’s “cognitive solutions” can be thought of as a suite of adaptations enhancing group cohesion, within which social expectations deserve particular attention.