The Social Brain Hypothesis (SBH) is a theory in evolutionary psychology explaining the unusually large brain size observed in primates, particularly humans. It suggests that the primary selective pressure driving brain expansion was not environmental complexity or finding food. Instead, the intellectual demands of managing a complex social life were the main evolutionary engine. Successfully living in a coordinated, stable group requires sophisticated cognitive skills, such as tracking, predicting, and maintaining relationships.
The Link Between Brain Size and Social Groups
The foundational work of the SBH established a quantitative relationship across primate species between group size and specific aspects of brain anatomy. Researchers determined that the size of a species’ typical stable social group correlates strongly with the relative size of a particular brain region. This correlation relies not on the overall weight or volume of the brain, but on the proportion dedicated to higher-order functions.
The neocortex, the outer layer of the cerebrum involved in conscious thought and complex planning, shows this specific scaling relationship. Across primate species, a larger ratio of the neocortex to the rest of the brain correlates with a larger average social group size. This suggests the neocortex places a cognitive limit on the number of relationships an individual can effectively process. The hypothesis proposes that the ultimate size of a social group is constrained by the information-processing capacity of this brain region.
Defining Human Social Network Limits
Applying this primate-wide correlation to the human brain yielded a prediction for the maximum size of a coherent human social group. This number, often referred to by the lead researcher’s name, is approximately 150 individuals. This figure represents the number of people with whom a person can maintain a genuine, stable relationship based on personal knowledge and mutual understanding.
This maximum number of meaningful contacts is not a fixed boundary but rather the limit of a layered structure of relationships. Empirical studies have consistently identified this nested organization, where emotional intensity decreases as the group size increases. The innermost layer consists of around five closest supports, followed by a layer of approximately 15 good friends. These are nested within a layer of about 50 friends, with the total network stabilizing at the 150 figure. This number has been corroborated across various contexts, including Neolithic villages, military company units, and modern personal address books.
Applying the Hypothesis to Modern Society
The predictive power of the SBH extends beyond evolutionary history to offer insights into contemporary organizational structures and digital life. The 150-person limit appears in modern settings, where organizational efficiency often declines when units exceed this size without adopting formal hierarchies. The hypothesis suggests that beyond this number, maintaining social cohesion and mutual trust becomes increasingly difficult, necessitating formal rules and external enforcement rather than relying on personal knowledge.
This cognitive constraint also helps explain the dynamics of online interaction, where people often connect with thousands of individuals on social media platforms. While digital tools remove physical limitations, the hypothesis suggests that the cognitive capacity for meaningful relationships remains bound by the 150-person limit. Research confirms that even with vast online networks, the number of individuals with whom a person actively engages or relies on for support remains within the predicted layers.