Drawing on our previous work on human trust networks, we provide further evidence of how group structure can foster group cohesion. But this work also raises doubts about two central tenets of the target paper: (1) the role assigned to cognitive abilities in group cohesion and stabilization; and (2) the emphasis on group size as the critical variable.

Dunbar suggests structural, behavioral, and cognitive mechanisms to mitigate the costs of living in large groups. While we generally concur with the notion of group size effects on female productivity, we call for a more explicit treatment of how functional support alleviates social costs and disagree with the outright dismissal of ecological drivers and phylogenetic inertia.

Meta-cognition enhances the social bonding hypothesis for musicality, integrating imagination, episodic simulation, causal inference, and inhibition. Music fosters group cohesion by engaging the endogenous opioid system, supporting intergroup understanding through vivid mental imagery, and facilitating socio-affective fiction. Additionally, causal inference enables contextual interpretation of music, while inhibition refines musical coordination and executive function, reinforcing cognitive flexibility for cooperative social behavior.

Dunbar proposes strategies to solve the fragmentation problem experienced by group-living animals. We highlight that bondedness not only mitigates stress but also provides structural scaffolding for group stability. Furthermore, we posit tolerance as a complementary mechanism smoothing social interactions and argue that variation in cohesion-promoting traits reflects context-dependent socio-ecological pressures, challenging static models linking sociality to cognition. Finally, we propose two further mechanisms-cultural transmission and dominance dynamics-that can enhance social cohesion by aligning behaviour and reducing uncertainty.

Grooming and cognition support primate group cohesion but are insufficient for maintaining stability in large groups. We propose tolerance, the capacity to accommodate social stress, as an additional mechanism. Tolerance fosters flexible social skills and cooperation beyond small cliques. Shaped by hormonal adaptation and development, tolerance plays a foundational role in overcoming group size limits by sustaining complex social networks.

While grooming and other forms of physical bonding are crucial for stress management, social play and laughter deserve equal recognition as tools for both stress relief and the reinforcement of social relationships. They play a pivotal role in the development of motor and social skills and serves as a foundational behavior in species that rely on cooperation and alliance-building.

Dunbar presents an intriguing analysis of variance in primate group sizes, and social glue's (grooming) relationship to cognitive evolution. This focus on primates with consistent and stable grouping excludes perspectives on the evolution of grouping beyond predation and competition. The analysis raises important questions about variation, dynamic sizes, and the conservation implications of variance for primate population extinction vulnerabilities.

Grooming is one strong mechanism allowing primate groups to grow larger and more cohesive, but a reduction in reactive aggression responses can be expected to have contributed to this trend too. There is indeed a partial overlap between the neurobiology of grooming and the neurobiology of reactive aggression.

We propose that the emergence of relationship-based social expectations and their evolution into fairness expectations played a key role in the size and cohesion of hominin societies. One of the central challenges of group living is the need to create and sustain stable and mutually beneficial patterns of cooperation. By regulating collaborative interactions, social expectations make group living less stressful.

To explain human social sophistication, and proximal phylogenetic steps leading to it, Dunbar claims that mentalising expands to increasingly high levels of recursion. However, the evidential basis for this claim is weak, exposing both a limitation in Dunbar's account and in the field's current understanding of social sophistication.

The paper of Dunbar (2025) on social stress is a strong demonstration that stress in itself can have a purely cognitive origin. The paper shows that the cognitive system can have profound impacts on the hypothalamus. As detailed in my commentary, this opens up new avenues of how to interpret psychiatric conditions, placebo, and other associations between perceptions and vegetative functions in the brain.

The current manuscript rightly points out that non-human primates evolved complex social cognitive skills to maintain weaker social ties. However, these capacities are likely more expansive than currently proposed: research shows that apes behave more socially to those with whom they experience similar things, suggesting that they possess some precursor of humans' capacity to bond through shared experiences.

Challenges of group-living include foundational problems of cooperation and coordination that extend beyond anthropoid primates and may potentially be managed through evolved group-mindedness rather than expanded neocortical size and enhanced capacities for executive functions.

The Social Brain Hypothesis (SBH) connects primate brain size to social complexity but faces empirical limitations. We propose expanding the SBH by incorporating hippocampal functions across species, demonstrating how cognition emerges from both social and ecological pressures. This extended framework moves beyond cortical-centric models, providing a comprehensive understanding of brain evolution and the origins of human cognitive abilities, including language.

Primate species deploy a suite of behavioural and cognitive adaptations to offset the costs of group-living. Dunbar uses species-level comparisons to posit a series of cumulative steps that describe large-scale phylogenetic patterns in the evolution of sociality. Here, we highlight the value of population-level variation within species for empirically testing the predicted socio-ecological correlations that underpin Dunbar's hypothesis.

Life history strategies adaptively calibrated to levels of environmental harshness and unpredictability shape not only the fundamental issue of fertility but also whether and to what extent people engage in the structural, behavioral, and cognitive solutions proposed by Dunbar. Considering behavioral ecology can, therefore, add nuance to Dunbar's novel and important theory.

Dunbar exclusively sees groups as arising through the aggregate relationships between individuals and thereby makes the serious omission of not considering the capacity of those individuals to categorize one another as ingroup versus outgroup members.

Dunbar's emphasis on dyadic relationships in group formation overlooks the roles of interdependence and joint commitment in social cohesion. We challenge his premise by highlighting the importance of group-level processes, particularly where top-down group pressures like cooperative breeding and out-group threat can induce joint commitment as an alternate means to sustain group cohesion.

Dunbar suggests that social stressors set "glass ceilings" on the evolution of mammalian group size and cohesion. We argue that this glass ceiling narrative conceals three contentious anthropocentric assumptions. First, large stable groups would always be beneficial. Second, grooming is an indicator for maintaining group cohesion. Third, group size is primarily limited by cognitive or behavioral incapacity. We challenge all three assumptions.