British anthropologist Robin Dunbar has proposed that humans can only maintain around 150 meaningful relationships at any given time, a concept known as Dunbar's number. This limit, derived from studies of primate brain size and social grooming, suggests that the neocortex—the part of the brain linked to cognition and language—constrains the size of cohesive social groups.
Dunbar and his colleagues found consistent evidence for this number across historical and modern contexts, including hunter-gatherer societies, military organisations, and even Christmas card lists. Exceeding 150 connections, they argue, makes a network less stable and cohesive. However, the number is not absolute; it represents a range, with extroverts tending to have larger networks and introverts smaller ones.
Within the 150 limit, Dunbar identifies nested layers: five loved ones, 15 good friends, 50 friends, 500 acquaintances, and 1,500 people you can recognise. These layers are determined by the frequency of face-to-face interaction, which is constrained by time. Women, he notes, often have slightly more contacts in the closest layers.
Not all researchers agree with the social brain hypothesis. Some question whether 150 is the correct number, while others doubt that a single magic number can capture the complexity of social relationships. Nonetheless, the concept has influenced organisations like the Swedish Tax Authority, which restructured offices to stay within the 150-person threshold.
