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.

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.

Human social networks are far larger than those of nonhuman primates. Maintaining cohesion in large networks requires a robust mechanism that can accommodate the dense webs of connections within communities. A parsimonious account of how humans achieve social cohesion is mental abstraction, which enables individuals to construct fuzzy network representations that facilitate information flow tracking and mitigate conflict.

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.

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.

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.

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.

Sentences written in Chinese are composed of continuous sequences of characters, without spaces or other visual cues to mark word boundaries. While skilled L1 readers can efficiently segment this naturally unspaced text into words, little is known about the word segmentation capabilities of L2 readers, including whether they employ the same strategies to process temporary segmental ambiguities. Accordingly, we report two eye movement experiments that investigated the processing of sentences containing temporarily ambiguous “incremental” three-character words (e.g., “体育馆,” meaning “stadium”) whose first two characters could also form a word (“体育,” meaning “sport”), comparing the performance of 48 skilled L1 Chinese readers and 48 high-proficiency L2 Chinese readers in each experiment. Our findings reveal that both groups can process this ambiguity efficiently, employing similar word segmentations strategies. We discuss our findings in relation to models of eye movement control and word recognition in Chinese reading.

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.

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.

In this commentary, I suggest a complementary view to the target paper’s idea that primate social metacognition evolved as an adaptation to living in large groups. I present metacognition as a necessary step in the development of complex allostatic systems and suggest that intrinsic and social metacognition are dissociable, which can be studied in the mammalian default mode network.

Bilingual speakers have been found to outperform monolingual speakers in tasks which involve taking others’ perspectives. This research examined whether bilingualism improves young adults’ performance on visuospatial perspective-taking (VPT) tasks, independently of culture and executive function (EF). Sixty-three East Asian and 61 European bilingual adults, as well as 60 English monolingual adults took part in level-1 VPT tasks (judging what others can see), level-2 VPT tasks (judging how others can see something) and EF tasks. They also filled in questionnaires about their social and language background, cultural orientation and acculturation. Groups did not differ in terms of VPT, suggesting that adult VPT is not affected by bilingualism or cultural orientation. Hierarchical regression revealed that VPT performance was predicted by EF skills, but not by individual differences in bilingualism or culture.

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.

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.

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.

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.

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.

I first summarise the argument in the target article so as to make the main points clear. I then address a number of major misunderstandings (mainly in relation to the social brain hypothesis), consider some specific issues that require clarification, and finally identify points that would merit more detailed consideration. I conclude with a list of possible future projects.