Dunbar's number is a suggested 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. This number was first proposed in the 1990s by British anthropologist Robin Dunbar, who found a correlation between primate brain size and average social group size. By using the average human brain size and extrapolating from the results of primates, he proposed that humans can only comfortably maintain 150 stable relationships. Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group. It has been proposed to lie between 100 and 250, with a commonly used value of 150. Dunbar's number states the number of people one knows and keeps social contact with, and it does not include the number of people known personally with a ceased social relationship, nor people just generally known with a lack of persistent social relationship, a number which might be much higher and likely depends on long-term memory size.
Dunbar 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 themself if they met again.
Primatologists have noted that, due to their highly social nature, primates must 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. This suggests that there is a species-specific index of the social group size, computable from the species' mean neocortical volume.
In 1992, 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 16th 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 other, 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 maintain social cohesion efficiently. 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. A recent study has suggested that Dunbar's number is applicable to online social networks  and communication networks (mobile phone) 
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, which is roughly double Dunbar's estimate. The Bernard–Killworth median of 231 is lower, due to upward straggle in the distribution, but 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. Nevertheless, the Bernard–Killworth number has not been popularized as widely as Dunbar's.
- 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.
- Malcolm Gladwell discusses the Dunbar number in his popular 2000 book The Tipping Point. Gladwell describes the company W. L. Gore and Associates, now known for the Gore-Tex brand. By trial and error, the leadership in the company discovered that if more than 150 employees were working together in one building, different social problems could occur. The company started building company buildings with a limit of 150 employees and only 150 parking spaces. When the parking spaces were filled, the company would build another 150 employee building. Sometimes these buildings would be placed only short distances apart. The company is also known for the open allocation company structure.
- The number has been used in the study of virtual communities, especially MMORPGs, such as Ultima Online, and social networking websites, such as Facebook (Dunbar himself did a study on Facebook in 2010) and MySpace.
- The Swedish tax authority planned to reorganize its functions in 2007 with a maximum 150 employees per 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. Wong referred to Dunbar's number again in his novel This Book Is Full of Spiders.
- Dunbar, R. I. M. (1992). "Neocortex size as a constraint on group size in primates". Journal of Human Evolution 22 (6): 469–493. doi:10.1016/0047-2484(92)90081-J.
- Brashears, M. E. (2013). "Humans use Compression Heuristics to Improve the Recall of Social Networks". Scientific Reports 3: 1513–0151. doi:10.1038/srep01513. PMC 3604710. PMID 23515066.
- Wellman, B. (2012). "Is Dunbar's number up?". British Journal of Psychology 103 (2): 174–176; discussion 176–2. doi:10.1111/j.2044-8295.2011.02075.x. PMID 22506743.
- De Ruiter, J.; Weston, G.; Lyon, S. M. (2011). "Dunbar's number: Group size and brain physiology in humans reexamined". American anthropologist 113 (4): 557–568. doi:10.1111/j.1548-1433.2011.01369.x. PMID 22216422.
- Gonçalves, B.; Perra, N.; Vespignani, A. (2011). Perc, Matjaz, ed. "Modeling Users' Activity on Twitter Networks: Validation of Dunbar's Number". PLoS ONE 6 (8): e22656. doi:10.1371/journal.pone.0022656. PMC 3149601. PMID 21826200.
- Gladwell, Malcolm (2000). The Tipping Point – How Little Things Make a Big Difference. Little, Brown and Company. pp. 177–181, 185–186. ISBN 0-316-34662-4.
- Purves, D. (2008). Principles of cognitive neuroscience. Sinauer Associates Inc.
- Hernando, A.; Villuendas, D.; Vesperinas, C.; Abad, M.; Plastino, A. (2009). "Unravelling the size distribution of social groups with information theory on complex networks". Preprint. arXiv:0905.3704.
- "Don't Believe Facebook; You Only Have 150 Friends". NPR. 4 June 2011.
- Carl Bialik (16 November 2007). "Sorry, You May Have Gone Over Your Limit Of Network Friends". The Wall Street Journal Online. Retrieved 2007-12-02.
- Dunbar, Robin (1998). Grooming, Gossip, and the Evolution of Language. Harvard University Press. ISBN 0-674-36336-1.
- Nuno Themudo (23 March 2007). "Virtual Resistance: Internet-mediated Networks (Dotcauses) and Collective Action Against Neoliberalism" (pg. 36). University of Pittsburgh, University Center for International Studies. Retrieved 2007-12-02.
- Goncalves, B., Perra, N., Vespignani, A. (28 May 2011). "Modeling Users' Activity on Twitter Networks: Validation of Dunbar's Number".
- Validation of Dunbar's number in Twitter conversations, Bruno Goncalves, Nicola Perra, Alessandro Vespignani
- Giovanna Miritello, Esteban Moro, Rubén Lara, Rocío Martínez-López, John Belchamber, Sam G.B. Roberts, Robin I.M. Dunbar. "Time as a limited resource: Communication strategy in mobile phone networks".
- McCarty, C.; Killworth, P. D.; Bernard, H. R.; Johnsen, E.; Shelley, G. (2000). "Comparing Two Methods for Estimating Network Size". Human Organization 60 (1): 28–39.
- Bernard, H. R.; Shelley, G. A.; Killworth, P. (1987). "How much of a network does the GSS and RSW dredge up?". Social Networks 9: 49. doi:10.1016/0378-8733(87)90017-7.
- H. Russell Bernard. "Honoring Peter Killworth's contribution to social network theory." Paper presented to the University of Southampton, 28 September 2006. http://nersp.osg.ufl.edu/~ufruss/
- "Primates on Facebook". The Economist. 26 February 2009.
- One example is Christopher Allen, "Dunbar, Altruistic Punishment, and Meta-Moderation".
- The Local – Sweden's news in English, 23 July 2007. "Swedish tax collectors organized by apes".
- David Wong. "Inside the Monkeysphere". Retrieved 2007-12-25.
- Healy, S. D., & Rowe, C. (2007). A critique of comparative studies of brain size. Proceedings of the Royal Society B: Biological Sciences, 274(1609), 453-464.
- Dunbar, R.I.M. (June 1992). "Neocortex size as a constraint on group size in primates". Journal of Human Evolution 22 (6): 469–493. doi:10.1016/0047-2484(92)90081-J.
- Dunbar, R.I.M. (1993), Coevolution of neocortical size, group size and language in humans, Behavioral and Brain Sciences 16 (4): 681–735.
- Edney, J.J. (1981a). Paradoxes on the commons: Scarcity and the problem of equality. Journal of Community Psychology, 9, 3–34.
- Sawaguchi, T., & Kudo, H. (1990), Neocortical development and social structure in primates, Primates 31: 283–290.
- Wong, David (2005) Inside the Monkeysphere, , a semi-satirical introduction to Dunbar's Number for the average internet user.
- "The ultimate brain teaser" – an article on Dunbar's research at University of Liverpool Research Intelligence
- Some speculations about a correlation between the monkeysphere and guild size in online multiplayer role-playing games
- Mospos blog entry – Communities of practice and Dunbar's number
- The Dunbar Number as a Limit to Group Sizes by Christopher Allen – applying Dunbar's number to on-line gaming, social software, collaboration, trust, security, privacy, and internet tools, by Christopher Allen
- Robin Dunbar: How Many Friends Does One Person Need? Fora.TV talk at the RSA
- What is the Monkeysphere? By David Wong at Cracked