# Dunbar 1992 - Neocortex Size and Group Size > [!summary] > Robin Dunbar's foundational 1992 paper established a correlation between neocortex size and social group size in primates, extrapolating to predict a human group size of approximately 150 people. **Author:** Robin I. M. Dunbar **Year:** 1992 **Type:** Paper **Citation:** Dunbar, R. I. M. (1992). "Neocortex size as a constraint on group size in primates." *Journal of Human Evolution*, 22(6), 469–493. ## The Core Method Dunbar examined the brains of 38 non-human primate species, paying particular attention to the neocortex—the region of the brain relating to cognitive abilities such as communication, planning, and thought. He measured the neocortex ratio (neocortex size relative to the rest of the brain) and correlated it with average social group sizes across these species. ## Key Findings The neocortex ratio gave the best statistical fit, accounting for 76% of the variance in mean group size among 36 genera of prosimian and anthropoid primates. This strong correlation suggested that brain size, specifically neocortex size, constrains the maximum group size a species can maintain. Using a regression equation based on this primate data, Dunbar predicted a human "mean group size" of 148, which he casually rounded to 150. ## Important Caveats Dunbar himself considered this result exploratory due to the large error measure. The 95% confidence interval ranged from 100 to 230, meaning the "true" number for humans could plausibly fall anywhere in that wide range. ## Ethnographic Validation To test his prediction, Dunbar spent weeks examining ethnographic journals and books looking for data on hunter-gatherer group sizes. He found that human communities consistently organized into groupings that matched his prediction. The 150-person unit corresponded to the "clan"—a somewhat shadowy grouping halfway between the more visible band (smaller groups) and tribe (larger aggregations). When tested against nine hunter-gatherer groups, only one did not match his predicted number. ## The Theoretical Foundation The paper established what would later become known as the [[Social Brain Hypothesis]]—that primate intelligence evolved primarily to manage complex social relationships rather than to solve environmental problems. The neocortex, in this view, is fundamentally a social processing organ, and its size determines how many relationships an individual can track and maintain. ## Historical Context This 1992 paper launched three decades of research and debate. It was followed by additional papers in 1993 and 1998 that further developed the social brain hypothesis and explored its implications for human evolution, including the role of language as a form of "vocal grooming" that might have allowed humans to maintain larger groups than other primates. ## Subsequent Impact The paper has become one of the most cited works in anthropology and evolutionary psychology. [[Dunbar's Number]] has been applied to organizational design, military structure, social media analysis, and community planning. However, it has also faced significant challenges—see [[Critiques of Dunbar's Number]] for recent reanalyses that question both the statistical methods and the generalizability of the findings. ## See Also - [[Dunbar's Number]] - [[Social Brain Hypothesis]] - [[Nested Social Layers]] - [[Critiques of Dunbar's Number]]