Dunbar's number is the proposal that humans can maintain roughly 150 stable social relationships, and that the limit is set by the size of the brain. It is one of the most widely repeated findings in popular social science, and one of the most contested within the field.

Robin Dunbar observed in the early 1990s that among primates, the size of the neocortex correlates with the typical size of the social group. His explanation is the social brain hypothesis: tracking who is allied with whom, who owes what, and who can be trusted is cognitively expensive, so a bigger group requires more processing. Extrapolating the primate line to human brain size produced a predicted group of about 150.

Robin Dunbar, who extrapolated a primate relationship between neocortex size and social group size to humans and arrived at a figure of roughly 150.
Robin Dunbar, who extrapolated a primate relationship between neocortex size and social group size to humans and arrived at a figure of roughly 150.Credit: Festival della Scienza from Genova (CC BY-SA 2.0).

He then assembled supporting cases: the size of hunter-gatherer bands and Neolithic villages, the strength of Roman and modern military units, the membership of Hutterite communities, and the size of address books and Christmas card lists. Later work proposed nested layers around the figure, roughly five intimates, fifteen close friends, fifty friends, and 150 acquaintances, each about three times the last.

The neocortex. The social brain hypothesis holds that maintaining relationships is cognitively expensive, so the size of this structure constrains how many can be sustained.
The neocortex. The social brain hypothesis holds that maintaining relationships is cognitively expensive, so the size of this structure constrains how many can be sustained.Credit: Henry Vandyke Carter (Public domain).

The statistics do not support a precise number. A 2021 Stockholm University reanalysis found that extending the primate regression to humans yields enormous confidence intervals, consistent with anything from about four to five hundred. The authors concluded that a specific figure cannot be derived this way, and that it is not possible to make an estimate with any precision using this method.

The confirming examples are selected. Critics note the supporting cases span a very wide range and that 150 sits comfortably in the middle of almost any collection of group sizes one might assemble. Village and unit sizes have plenty of non-cognitive explanations, including logistics, land, and command structure.

Cognition may not be the binding constraint. Time, opportunity, and social technology plausibly limit relationship count more than brain capacity does. Writing, telephones, and social platforms change the cost of maintaining a tie, which a purely neural limit would not predict.

Cultural variation. Societies differ enormously in how relationships are counted and what counts as knowing someone, which makes a single cross-cultural number difficult to define, let alone measure.

Dunbar has responded that the criticism misreads the claim as a hard cap rather than a central tendency with wide variation, and that the layered structure has held up in independent datasets, including studies of mobile phone call records.

The number's popularity outside the academy far exceeds its standing within it. It has been used to justify office sizes, company structures, and the design of social products, and it appears constantly in business writing. That gap between public confidence and scientific dispute is itself the most interesting thing about it, and a reasonable reader should treat 150 as a suggestive order of magnitude rather than a measured constant.