People Dunbar’s number

Dunbar's number

Dunbar’s number is a theoretical cognitive limit to the number of people with whom one can maintain stable social relationships. These are relationships in which an individual knows who each person is, and how each person relates to every other person. Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group. No precise value has been proposed for Dunbar’s number. It lies between 100 and 230, but a commonly used value is 150.

Dunbar’s number was first proposed by British anthropologist Robin Dunbar, who theorized that “this limit is a direct function of relative neocortex size, and that this in turn limits group size … the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained.” On the periphery, the number also includes past colleagues such as high school friends with whom a person would want to reacquaint themselves if they met again.

Dunbar
Robin Dunbar

Research background

Primatologists have noted that, due to their highly social nature, non-human primates have to maintain personal contact with the other members of their social group, usually through social grooming. Such social groups function as protective cliques within the physical groups in which the primates live. The number of social group members a primate can track appears to be limited by the volume of the neocortex region of their brain. This suggests that there is a species-specific index of the social group size, computable from the species’ mean neocortex volume.

In a 1992 article, Dunbar used the correlation observed for non-human primates to predict a social group size for humans. Using a regression equation on data for 38 primate genera, Dunbar predicted a human “mean group size” of 148 (casually rounded to 150), a result he considered exploratory due to the large error measure (a 95% confidence interval of 100 to 230).

Dunbar then compared this prediction with observable group sizes for humans. Beginning with the assumption that the current mean size of the human neocortex had developed about 250,000 years ago, during the Pleistocene, Dunbar searched the anthropological and ethnographical literature for census-like group size information for various hunter-gatherer societies, the closest existing approximations to how anthropology reconstructs the Pleistocene societies. Dunbar noted that the groups fell into three categories — small, medium and large, equivalent to bands, cultural lineage groups and tribes — with respective size ranges of 30-50, 100-200 and 500-2500 members each.

Dunbar’s surveys of village and tribe sizes also appeared to approximate this predicted value, including 150 as the estimated size of a neolithic farming village; 150 as the splitting point of Hutterite settlements; 200 as the upper bound on the number of academics in a discipline’s sub-specialization; 150 as the basic unit size of professional armies in Roman antiquity and in modern times since the sixteenth century; and notions of appropriate company size.

Dunbar has argued that 150 would be the mean group size only for communities with a very high incentive to remain together. For a group of this size to remain cohesive, Dunbar speculated that as much as 42% of the group’s time would have to be devoted to social grooming. Correspondingly, only groups under intense survival pressure, such as subsistence villages, nomadic tribes, and historical military groupings, have, on average, achieved the 150-member mark. Moreover, Dunbar noted that such groups are almost always physically close: “… we might expect the upper limit on group size to depend on the degree of social dispersal. In dispersed societies, individuals will meet less often and will thus be less familiar with each, so group sizes should be smaller in consequence.” Thus, the 150-member group would occur only because of absolute necessity — due to intense environmental and economic pressures.

Dunbar, in Grooming, Gossip, and the Evolution of Language, proposes furthermore that language may have arisen as a “cheap” means of social grooming, allowing early humans to efficiently maintain social cohesion. Without language, Dunbar speculates, humans would have to expend nearly half their time on social grooming, which would have made productive, cooperative effort nearly impossible. Language may have allowed societies to remain cohesive, while reducing the need for physical and social intimacy.

Dunbar’s number has since become of interest in anthropology, evolutionary psychology, statistics, and business management. For example, developers of social software are interested in it, as they need to know the size of social networks their software needs to take into account; and in the modern military, operational psychologists seek such data to support or refute policies related to maintaining or improving unit cohesion and morale.

Alternative numbers

Dunbar’s number is not derived from systematic observation of the number of relationships that people living in the contemporary world have. As noted above, it comes from extrapolation from non-human primates and from inspection of selected documents showing network sizes in selected pre-industrial villages and settlements in less developed countries.

Anthropologist H. Russell Bernard and Peter Killworth and associates have done a variety of field studies in the United States that came up with an estimated mean number of ties – 290 – that is roughly double Dunbar’s estimate. The Bernard-Killworth median of 231 is lower, due to upward straggle in the distribution: this is still appreciably larger than Dunbar’s estimate. The Bernard-Killworth estimate of the maximum likelihood of the size of a person’s social network is based on a number of field studies using different methods in various populations. It is not an average of study averages but a repeated finding.

Popularization

  • Dunbar’s number has been most popularized by Malcolm Gladwell’s The Tipping Point, where it plays a central role in Gladwell’s arguments about the dynamics of social groups.
  • In a 1985 paper titled “Psychology, Ideology, Utopia, & the Commons,” psychologist Dennis Fox proposed the same concept as it is applied to anarchy, politics, and the tragedy of the commons.
  • Recently, the number has been used in the study of Internet communities, especially MMORPGs such as Ultima Online, and social networking websites such as Facebook (Dunbar himself is doing a study on Facebook to be released in 2010) and MySpace.
  • Freshmen orientation groups at the University of Texas at Austin, and also orientations for the Law School and Business School, are now being organized around the Dunbar number.
  • The Swedish tax authority planned to reorganize its functions in 2007. The number 150 was set as the maximum number of people in an office, referring to Dunbar’s research.
  • It has also been popularized as the “monkeysphere”, a neologism coined by David Wong in an article, What is the Monkeysphere?, which introduces this concept in a humorous manner.
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