Introduction
In earlier work I proposed the solitary forager hypothesis of autism, which suggests that some of the cognitive and behavioral characteristics associated with autism reflect adaptations that were advantageous in contexts where individuals spent extended periods foraging or working alone. Under such conditions, reduced social monitoring, sustained attention to environmental detail, heightened sensory acuity, and persistent focus on non-social problems could have been beneficial. These traits may have increased the efficiency of solitary resource acquisition, tool use, tracking, and ecological observation.

Although the solitary forager framework remains useful, it may describe an ecological extreme rather than the most common selective context. Complete solitary living was probably rare during human evolution. However, fluctuations in social group size were likely ubiquitous. Hunter-gatherer societies commonly exhibit fission–fusion dynamics in which large communities temporarily divide into smaller bands, family groups, or individual foraging parties. Seasonal dispersal, resource scarcity, migration, and conflict can all produce periods where individuals operate within substantially reduced social groups.
The present article proposes that autism may partly reflect evolutionary calibration to smaller social group environments rather than strictly solitary conditions. In this view, autism-related traits represent one end of a continuum of social cognitive strategies that evolved in response to variation in group size. Individuals operating within smaller groups may benefit from cognitive architectures that allocate fewer resources to large-scale social monitoring and more resources to environmental analysis, pattern detection, and sustained task engagement.
Importantly, this proposal does not contradict the solitary forager hypothesis. Instead it extends it by suggesting that the relevant evolutionary pressure may have been reduced social complexity rather than complete social isolation. Many of the same behavioral, neural, and genetic signatures predicted by the solitary forager model would also be expected in populations adapting to smaller group environments.
Group Size as a Major Driver of Mammalian Brain Organization
Across mammals, social group size is one of the strongest predictors of brain organization. Comparative studies have repeatedly found that species living in larger and more complex social groups tend to possess larger neocortices relative to body size. This relationship is often interpreted through the social brain hypothesis, which proposes that the computational demands of tracking social relationships, alliances, hierarchies, and reputations place substantial demands on neural processing capacity.
Large social groups require individuals to monitor numerous conspecifics simultaneously. This involves facial recognition, emotional interpretation, memory for social interactions, deception detection, and prediction of other individuals’ behavior. Brain regions frequently implicated in these tasks include the orbitofrontal cortex, anterior cingulate cortex, superior temporal regions, amygdala, and temporoparietal areas involved in social cognition.
In contrast, mammals living in smaller or less socially complex groups often show reduced investment in some of these social processing systems. Solitary carnivores, nocturnal prosimians, and certain small primates frequently display lower neocortical ratios and less elaborate neural specialization for social monitoring. Their cognitive resources may instead be directed toward ecological navigation, sensory processing, spatial memory, and resource detection.
These findings suggest that mammalian brains are not optimized for a single social environment but are instead shaped by the expected scale of social interaction.
Neurobiological Systems That Track Social Environment
Several neuromodulatory systems appear particularly sensitive to social structure.
The vasopressin and oxytocin systems play central roles in regulating social bonding, territoriality, and social recognition. Variation in genes such as AVPR1A and OXTR is associated with differences in social behavior across many mammalian species. In humans, polymorphisms in these genes have also been repeatedly associated with autism and variation in social cognition.
The endogenous opioid system is another key regulator of social reward. Social interaction activates opioid signaling pathways that reinforce bonding and group cohesion. Some autism theories propose reduced sensitivity of this reward system to social stimuli.
Stress regulation through the hypothalamic–pituitary–adrenal axis is also strongly influenced by social environment. Animals living in dense or competitive social hierarchies often show different patterns of cortisol regulation compared with those living in small or loosely structured groups.
Taken together, these systems appear to function as biological mechanisms that help calibrate the brain to the expected level of social engagement.
Autism and Reduced Social Monitoring
Many of the neural and cognitive characteristics associated with autism can be interpreted through this framework of social calibration.
Individuals with autism often show differences in brain regions associated with social cognition, including the orbitofrontal cortex, amygdala, anterior cingulate cortex, and superior temporal regions. These areas play central roles in evaluating social signals, tracking reputation, and maintaining models of other individuals’ mental states.
At the same time, autistic individuals frequently demonstrate strengths in domains that are less dependent on complex social processing. These include sustained attention, pattern detection, rule learning, perceptual discrimination, and detailed analysis of environmental information.
From an evolutionary perspective, these traits may represent a cognitive strategy that prioritizes ecological and analytical processing over large-scale social monitoring.
Smaller Groups and the Distribution of Cognitive Strategies
If social brain systems evolved in response to group size, it is plausible that natural selection maintained variation in social cognitive calibration within human populations.
In large groups, individuals who excel at tracking many social relationships may have an advantage. However, in smaller groups the benefits of extensive social monitoring may decline while the value of environmental specialization increases.
Small hunting parties, dispersed foraging units, or frontier populations may benefit from individuals who devote more attention to tracking animals, detecting environmental patterns, constructing tools, or solving technical problems. In these contexts, the cognitive style associated with autism could be advantageous.
This does not imply that autism evolved solely because of small groups. Instead, the hypothesis suggests that variation in social cognitive architecture may have been maintained because human populations historically experienced frequent shifts in group size.
Small Group Ecology in Human Evolution
Human social organization has rarely been static. Ethnographic studies of hunter–gatherer societies consistently show that human populations operate within flexible fission–fusion systems in which larger communities periodically divide into smaller subgroups. These subgroups may consist of nuclear families, temporary hunting parties, or small foraging units that travel and work together for extended periods before rejoining the larger community.
In many hunter–gatherer societies the effective daily social group is considerably smaller than the total population of the band. Foraging tasks frequently require individuals to disperse across the landscape in small units in order to track animals, collect plant resources, or explore new territory. Seasonal migration, resource fluctuations, and environmental pressures can further fragment social groups. As a result, individuals often spend substantial portions of their lives interacting primarily with a limited set of social partners.
Anthropological observations also suggest that group size can fluctuate dramatically depending on ecological conditions. During periods of resource abundance, communities may aggregate into larger camps that facilitate cooperation, information exchange, and mate selection. During periods of scarcity, however, groups may divide into smaller and more mobile units in order to reduce competition for resources. Such dynamics would have repeatedly exposed human populations to environments in which the cognitive demands of managing large social networks were temporarily reduced.
These fluctuations in social scale may have created opportunities for natural selection to maintain diversity in social cognitive strategies. Individuals who were particularly skilled at navigating large and complex social networks may have been advantaged in densely populated camps or cooperative hunting groups. Conversely, individuals who devoted more cognitive resources to environmental monitoring, tool use, and independent problem solving may have been well suited to smaller foraging parties or dispersed family groups.
Importantly, many of the cognitive traits associated with autism appear consistent with functioning in such smaller social environments. These include reduced reliance on constant social feedback, heightened attention to environmental detail, strong persistence in problem solving, and a preference for predictable routines. In a small group context, these traits might not represent disadvantages but rather alternative strategies for interacting with the environment.
This interpretation does not imply that autism evolved specifically for small group living. Rather, it suggests that human populations may have maintained variation in social cognitive calibration because the environments experienced by our ancestors fluctuated between larger and smaller social networks. The brain systems that regulate social attention, social reward, and social communication may therefore be capable of tuning themselves along a continuum that reflects expected levels of social complexity.
Within this framework, autism may represent an extreme expression of a cognitive orientation that is less dependent on large-scale social monitoring and more focused on ecological and analytical processing. Because early humans frequently operated within small foraging groups, such cognitive variation may have persisted within human populations without being strongly eliminated by natural selection.
Comparative Evidence From Mammalian Social Brain Scaling
A large body of comparative research indicates that social group size is one of the strongest predictors of brain organization across mammals and especially across primates. Species that live in larger and more socially complex groups tend to exhibit expansion of brain regions involved in social cognition, communication, and behavioral flexibility. This relationship is often described through the social brain hypothesis, which proposes that the cognitive demands of managing social relationships played a central role in the evolution of mammalian and primate brains.
One of the most consistent findings in this literature is the relationship between group size and neocortex volume. Comparative analyses across primate species show that the ratio of neocortex size to total brain size increases with the number of individuals typically encountered within a social group. Larger groups require individuals to track multiple relationships simultaneously, remember past interactions, and predict the behavior of many different partners and rivals. These demands place heavy computational burdens on neural systems responsible for social memory and behavioral prediction.
Several specific brain regions appear to scale with social complexity. The orbitofrontal cortex is involved in evaluating social rewards and outcomes and plays a role in flexible decision making during social interactions. The anterior cingulate cortex participates in conflict monitoring, empathy-related processes, and social learning. The amygdala contributes to the detection and evaluation of emotionally relevant social cues, including facial expressions and threat signals. Across primates and other mammals, variation in the size or connectivity of these regions is associated with differences in social behavior and group structure.
Group size also influences neural systems involved in communication and facial signaling. Comparative primate studies have shown that the size of the facial motor nucleus, which controls muscles used in facial expressions, scales with social group size. Species that rely heavily on facial communication within large social groups tend to possess larger facial motor nuclei relative to brain size. Conversely, species that live in smaller or less socially interactive groups often show reduced investment in this system.
Neuromodulatory systems also appear to track social organization. The vasopressin and oxytocin signaling systems, which regulate social bonding and affiliation, vary across mammalian species with different social structures. Differences in the distribution of vasopressin receptors within reward circuits, particularly in regions such as the ventral pallidum and lateral septum, have been linked to species differences in pair bonding and social attachment. These findings demonstrate that relatively small genetic changes affecting receptor distribution can produce substantial changes in social behavior.
Taken together, these findings suggest that mammalian brains are not optimized for a single level of social interaction. Instead they appear to be calibrated to the expected scale of social environments.
This perspective provides a useful framework for interpreting autism. Many neuroimaging studies report differences in brain regions involved in social evaluation and social reward, including the orbitofrontal cortex, amygdala, and anterior cingulate cortex. At the same time, individuals with autism often show enhanced performance in tasks involving pattern detection, system analysis, and sustained attention to detail.
Within a group size framework, these characteristics could reflect neural calibration toward smaller social networks, in which the computational demands of monitoring many social relationships are reduced and greater attention can be devoted to ecological or analytical information.
It is important to emphasize that this interpretation remains speculative. Autism is a heterogeneous condition influenced by many genetic and developmental factors. However, the existence of strong relationships between group size and brain organization across mammals suggests that variation in social cognitive calibration may represent a biologically plausible dimension of human behavioral diversity.
Relationship to the Solitary Forager Hypothesis
The group size hypothesis can be viewed as a refinement of the solitary forager model.
The solitary forager hypothesis emphasized ecological scenarios in which individuals spent extended periods working alone. The present framework suggests that similar cognitive adaptations could arise whenever individuals operate within reduced social networks, even if they remain embedded within a broader community.
Under this interpretation, solitary foraging represents one extreme on a continuum of decreasing social complexity. Smaller group environments may have been far more common during human evolution and therefore provide a more realistic selective backdrop for autism-related traits.
Both models predict similar neurobiological patterns, including altered investment in social brain networks and variation in neurochemical systems involved in social bonding.
Testable Predictions
The group size hypothesis generates several predictions that can be examined empirically.
First, species or populations that live in smaller social groups should show neural signatures that partially overlap with autism-associated brain patterns. These may include differences in orbitofrontal and amygdala structure or reduced specialization for large-scale social cognition.
Second, genes involved in social bonding and social reward systems, including AVPR1A, OXTR, and CD38, may show evolutionary variation associated with group size across mammals.
Third, human populations historically living in small, dispersed bands may show higher frequencies of genetic variants associated with reduced social monitoring.
Finally, comparative studies of solitary and social mammals may reveal convergent neural signatures related to attention allocation, sensory processing, and ecological cognition.
Conclusion
Autism is unlikely to reflect a single evolutionary adaptation. Instead it may represent one end of a spectrum of cognitive strategies shaped by the variable social environments that humans experienced throughout evolutionary history.
The solitary forager hypothesis highlighted the possibility that some autism traits were advantageous in contexts of individual ecological specialization. The group size hypothesis proposed here extends that framework by suggesting that similar traits may have evolved in response to periods of reduced social group size.
Because human societies regularly shifted between larger and smaller social units, natural selection may have preserved diversity in social cognitive calibration. Autism may therefore represent an extreme expression of a strategy that once provided advantages under certain ecological and social conditions.
Understanding autism through this evolutionary lens may help explain both the challenges and the distinctive cognitive strengths often associated with the condition.















