Dunbar's number

The idea is associated with British anthropologist Robin Dunbar's research on primate sociality. In a 1992 study, Dunbar reported that group size among the primates examined varied with relative neocortical volume. In a 1993 paper, he used that relationship to estimate a human group size of roughly 150. The estimate became widely known as Dunbar's number and has since been used to discuss the scale of communities and personal networks.

The figure is best understood as a proposed approximate average or scale for networks of relatively stable social relationships, not a universal maximum for all people. It does not mean that every person has exactly 150 friends, that all ties are equally close, or that formal organizations must be capped at that size. Human relationships differ in intimacy, frequency of contact and context; the count also depends on how a relationship or group is defined and measured.

Research on human networks has reported patterns consistent with social networks having layered or constrained sizes, and some models have argued for network dynamics near 150. However, the exact value and its interpretation remain debated. A 2021 reanalysis of primate brain and group-size data obtained widely varying estimates, with very broad confidence intervals, and concluded that a specific cognitive limit for humans cannot be derived from that comparative method. Thus, the number is a useful hypothesis and popular shorthand, not a settled biological law.

Dunbar's number is sometimes invoked in discussions of communities, workplaces, military units and online networks. Such applications require care: a group's size is shaped by institutions, communication tools, shared goals and social norms, as well as individual cognition. A numerical guideline should not substitute for direct evidence about how a particular group functions.