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Dunbar's number

Dunbar's number

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Dunbar's number is suggested to be a theoretical cognitive limit to the number of people with whom one can maintain stable social relationships
Interpersonal relationship
An interpersonal relationship is an association between two or more people that may range from fleeting to enduring. This association may be based on limerence, love, solidarity, regular business interactions, or some other type of social commitment. Interpersonal relationships are formed in the...

. 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 has been proposed to lie between 100 and 230, 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
Long-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...


Dunbar's number was first proposed by British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 anthropologist Robin Dunbar
Robin Dunbar
Robin Ian MacDonald Dunbar is a British anthropologist and evolutionary psychologist and a specialist in primate behaviour. He is currently Professor of Evolutionary Anthropology and the Director of the Institute of Cognitive and Evolutionary Anthropology of the University of Oxford and the...

, who theorized that "this limit is a direct function of relative neocortex
The neocortex , also called the neopallium and isocortex , is a part of the brain of mammals. It is the outer layer of the cerebral hemispheres, and made up of six layers, labelled I to VI...

 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 oneself if they met again.

Research background

Primatologists have noted that, due to their highly social nature, non-human primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

s must maintain personal contact with the other members of their social group, usually through social grooming
Social grooming
In social animals, including humans, social grooming or allogrooming is an activity in which individuals in a group clean or maintain one another's body or appearance. It is a major social activity, and a means by which animals who live in proximity can bond and reinforce social structures, family...

. 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 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
Genera is a commercial operating system and development environment for Lisp machines developed by Symbolics. It is essentially a fork of an earlier operating system originating on the MIT AI Lab's Lisp machines which Symbolics had used in common with LMI and Texas Instruments...

, 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
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

, 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
Band society
A band society is the simplest form of human society. A band generally consists of a small kin group, no larger than an extended family or clan; it has been defined as consisting of no more than 30 to 50 individuals.Bands have a loose organization...

, cultural lineage groups and tribe
A tribe, viewed historically or developmentally, consists of a social group existing before the development of, or outside of, states.Many anthropologists use the term tribal society to refer to societies organized largely on the basis of kinship, especially corporate descent groups .Some theorists...

s — with respective size ranges of 30–50, 100–200 and 500–2500 members each.

Dunbar's surveys of village
A village is a clustered human settlement or community, larger than a hamlet with the population ranging from a few hundred to a few thousand , Though often located in rural areas, the term urban village is also applied to certain urban neighbourhoods, such as the West Village in Manhattan, New...

 and tribe sizes also appeared to approximate this predicted value, including 150 as the estimated size of a Neolithic
The Neolithic Age, Era, or Period, or New Stone Age, was a period in the development of human technology, beginning about 9500 BC in some parts of the Middle East, and later in other parts of the world. It is traditionally considered as the last part of the Stone Age...

 farming village; 150 as the splitting point of Hutterite
Hutterites are a communal branch of Anabaptists who, like the Amish and Mennonites, trace their roots to the Radical Reformation of the 16th century. Since the death of their founder Jakob Hutter in 1536, the beliefs of the Hutterites, especially living in a community of goods and absolute...

 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
Ancient Rome
Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 and in modern times
Modern World
Modern World or The Modern World may refer to:*Modernity, a popular academic term.*The modern era, the age in which people today now live.*Modern World, a song by Wolf Parade from their 2005 album Apologies to the Queen Mary....

 since the 16th century; and notions of appropriate company
Company (military unit)
A company is a military unit, typically consisting of 80–225 soldiers and usually commanded by a Captain, Major or Commandant. Most companies are formed of three to five platoons although the exact number may vary by country, unit type, and structure...


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, nomad
Nomadic people , commonly known as itinerants in modern-day contexts, are communities of people who move from one place to another, rather than settling permanently in one location. There are an estimated 30-40 million nomads in the world. Many cultures have traditionally been nomadic, but...

ic tribes, and historical military
War is a state of organized, armed, and often prolonged conflict carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political...

 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
Language may refer either to the specifically human capacity for acquiring and using complex systems of communication, or to a specific instance of such a system of complex communication...

 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
Evolutionary psychology
Evolutionary psychology is an approach in the social and natural sciences that examines psychological traits such as memory, perception, and language from a modern evolutionary perspective. It seeks to identify which human psychological traits are evolved adaptations, that is, the functional...

, statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....

, and business management. For example, developers of social software
Social software
Social software applications include communication tools and interactive tools. Communication tools typically handle the capturing, storing and presentation of communication, usually written but increasingly including audio and video as well. Interactive tools handle mediated interactions between a...

 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
Unit cohesion
Unit cohesion is a military concept, defined by one former United States Chief of staff in the early 1980s as "the bonding together of soldiers in such a way as to sustain their will and commitment to each other, the unit, and mission accomplishment, despite combat or mission stress"...

 and morale.

A recent study has suggested that Dunbar's number is applicable to online social networks as well.

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
Social network
A social network is a social structure made up of individuals called "nodes", which are tied by one or more specific types of interdependency, such as friendship, kinship, common interest, financial exchange, dislike, sexual relationships, or relationships of beliefs, knowledge or prestige.Social...

 in selected pre-industrial villages and settlements in less-developed countries.

Anthropologist H. Russell Bernard and Peter Killworth
Peter Killworth
Professor Peter D. Killworth was an English scientist known for both his work on oceanography and the study of social networks. A prodigious writer, his published output exceeded 160 scientific papers over the course of his career...

 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
In probability theory and statistics, a median is described as the numerical value separating the higher half of a sample, a population, or a probability distribution, from the lower half. The median of a finite list of numbers can be found by arranging all the observations from lowest value to...

 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
Social network
A social network is a social structure made up of individuals called "nodes", which are tied by one or more specific types of interdependency, such as friendship, kinship, common interest, financial exchange, dislike, sexual relationships, or relationships of beliefs, knowledge or prestige.Social...

 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
    Anarchy , has more than one colloquial definition. In the United States, the term "anarchy" typically is meant to refer to a society which lacks publicly recognized government or violently enforced political authority...

    , politics
    Politics is a process by which groups of people make collective decisions. The term is generally applied to the art or science of running governmental or state affairs, including behavior within civil governments, but also applies to institutions, fields, and special interest groups such as the...

    , and the tragedy of the commons
    Tragedy of the commons
    The tragedy of the commons is a dilemma arising from the situation in which multiple individuals, acting independently and rationally consulting their own self-interest, will ultimately deplete a shared limited resource, even when it is clear that it is not in anyone's long-term interest for this...

  • The number has been used in the study of Internet communities, especially MMORPG
    Massively multiplayer online role-playing game is a genre of role-playing video games in which a very large number of players interact with one another within a virtual game world....

    s such as Ultima Online
    Ultima Online
    Ultima Online is a graphical massively multiplayer online role-playing game , released on September 24, 1997, by Origin Systems. It was instrumental to the development of the genre, and is still running today...

    , and social networking
    Social network service
    A social networking service is an online service, platform, or site that focuses on building and reflecting of social networks or social relations among people, who, for example, share interests and/or activities. A social network service consists of a representation of each user , his/her social...

    A website, also written as Web site, web site, or simply site, is a collection of related web pages containing images, videos or other digital assets. A website is hosted on at least one web server, accessible via a network such as the Internet or a private local area network through an Internet...

    s such as Facebook
    Facebook is a social networking service and website launched in February 2004, operated and privately owned by Facebook, Inc. , Facebook has more than 800 million active users. Users must register before using the site, after which they may create a personal profile, add other users as...

     (Dunbar himself is doing a study on Facebook to be released in 2010) and MySpace
    Myspace is a social networking service owned by Specific Media LLC and pop star Justin Timberlake. Myspace launched in August 2003 and is headquartered in Beverly Hills, California. In August 2011, Myspace had 33.1 million unique U.S. visitors....

  • The Swedish
    Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

     tax authority planned to reorganize its functions in 2007 with a maximum 150 people 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.

Further reading

  • 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, http://www.cracked.com/article_14990_what-monkeysphere.html, a semi-satirical introduction to Dunbar's Number for the average internet user.

External links