Dunbar's Number
Dunbar's Number is the cognitive limit on the number of stable social relationships a human brain can maintain simultaneously — approximately 150, with nested layers of 5, 15, 50, and 150. Proposed by British anthropologist Robin Dunbar in 1992 based on primate neocortex studies, it explains why human organisations above ~150 people require formal management structures, why small teams are more cohesive, and why network effects in personal relationships saturate.