Dunbar's number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships—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 comfortably maintain only 150 stable relationships. Dunbar explained it informally as "the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar".
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 himself or herself 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.  This result is confirmed by the mathematical formulation of the social brain hypothesis, that showed that it is unlikely that increased brain size would have led to large groups without the kind of complex communication that only language allows.
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). Participants of the European career-oriented online social network XING who have about 157 contacts reported the highest level of job offer success, which also confirms Dunbar’s number of about 150.
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.
Philip Lieberman argues that since band societies of approximately 30–50 people are bounded by nutritional limitations to what group sizes can be fed without at least rudimentary agriculture, big human brains consuming more nutrients than ape brains, group sizes of approximately 150 cannot have been selected for in paleolithic humans.[dubious ] Brains much smaller than human or even mammalian brains are also known to be able to support social relationships, including social insects with hierarchies where each individual knows its place (such as the paper wasp with its societies of approximately 80 individuals ) and computer-simulated virtual autonomous agents with simple reaction programming emulating what is referred to in primatology as "ape politics".
- 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 reorganise its functions in 2007 with a maximum 150 employees per office, referring to Dunbar's research.
- In 2007, Cracked.com editor David Wong wrote a humour piece titled "What is the Monkeysphere?" explaining Dunbar's number.
- In the 2012 novel This Book Is Full of Spiders, also by David Wong, the character Marconi explains to David the impact Dunbar's number has on human society. In Marconi's explanation, the limit Dunbar's number imposes on the individual explains phenomena such as racism and xenophobia, as well as apathy towards the suffering of peoples outside of an individual's community.
- In Rikki de la Vega's "Free Spirits" series of novels (published by Sizzler Editions), the fictional Free Spirit Connection places limits on the size of local and regional groups, citing Dunbar's number, thus leading to the "birthing" of new groups and growth of the movement.
- 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.
- 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.
- Dunbar, Robin (1998). Grooming, gossip, and the evolution of language (1st Harvard University Press paperback ed.). Cambridge, Mass.: Harvard University Press. p. 77. ISBN 0674363361. Retrieved 17 December 2016.
- 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: .
- "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.
- Dunbar, Robin (2004), "Gossip in Evolutionary Perspective" (PDF), Review of General Psychology, 8 (2): 100–110, retrieved 2013-01-24
- Dávid-Barrett, T.; Dunbar, R. I. M. (2013-08-22). "Processing power limits social group size: computational evidence for the cognitive costs of sociality". Proc. R. Soc. B. 280 (1765): 20131151. doi:10.1098/rspb.2013.1151. ISSN 0962-8452. PMC . PMID 23804623.
- Nuno Themudo (23 March 2007). "Virtual Resistance: Internet-mediated Networks (Dotcauses) and Collective Action Against Neoliberalism" (PDF). University of Pittsburgh, University Center for International Studies. Archived from the original (pg. 36) on 9 July 2009. Retrieved 2 December 2007.
- 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 (2013). "Time as a limited resource: Communication strategy in mobile phone networks". Social Networks. 35: 89–95. doi:10.1016/j.socnet.2013.01.003.
- Buettner, Ricardo (2017). "Getting a job via career-oriented social networking markets: The weakness of too many ties". Electronic Markets: The International Journal on Networked Business. Springer. 27 (4): 371–385. doi:10.1007/s12525-017-0248-3.
- 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. doi:10.17730/humo.60.1.efx5t9gjtgmga73y. Archived from the original on 28 January 2013.
- 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/
- The Unpredictable Species: What Makes Humans Unique, Philip Lieberman
- David Attenborough: Micro Monsters 3D
- How the Body Shapes the Way We Think :A New View of Intelligence, Rolf Pfeifer, Josh Bongard
- "Primates on Facebook". The Economist. 26 February 2009.
- One example is Christopher Allen, "Dunbar, Altruistic Punishment, and Meta-Moderation".
- "Swedish tax collectors organized by apes". The Local – Sweden's news in English. 2007-07-23. Archived from the original on 2007-08-16.
- "What is the Monkeysphere?". Retrieved 23 November 2015.
- Wong, David (2012). This Book is Full of Spiders. NY: St. Martin's Press. pp. 295–296.
- 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. doi:10.1098/rspb.2006.3748.
- 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. doi:10.1017/s0140525x00032325.
- Edney, J.J. (1981a). "Paradoxes on the commons: Scarcity and the problem of equality". Journal of Community Psychology. 9: 3–34. doi:10.1002/1520-6629(198101)9:1<3::aid-jcop2290090102>3.0.co;2-b.
- Sawaguchi, T.; Kudo, H. (1990). "Neocortical development and social structure in primates". Primates. 31: 283–290. doi:10.1007/bf02380949.
- 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
- 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