Dunbar's number

Dunbar’s numbersolving social problems100 to 150 peopleDunbar's theoryDunbars numbermonkeyspheremore than 100–200 people
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.wikipedia
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Social group

groupsocial groupsgroups
Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group.
According to Dunbar's number, on average, people cannot maintain stable social relationships with more than 150 individuals.

Robin Dunbar

R. DunbarR. I. M. DunbarR.I.M Dunbar
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.
He is best known for formulating Dunbar's number, a measurement of the "cognitive limit to the number of individuals with whom any one person can maintain stable relationships".

Group cohesiveness

social cohesiongroup cohesioncohesion
Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group.
In primatology and anthropology, the limits to group size are theorized to accord with Dunbar's number.

Company (military unit)

companycompaniesrifle companies
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-specialisation; 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.
Furthermore, recent studies have indicated that humans are best able to maintain stable relationships in a cohesive group numbering between 100 and 250 members, with 150 members being the common number (see Dunbar’s number).

Primatology

primatologistprimatologistsprimate
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.
This number is referred to as the monkeysphere.

Neocortex

neocorticalisocortexneopallium
Dunbar theorised 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".
(See Dunbar's number) Humans have a large neocortex as a percentage of total brain matter when compared with other mammals.

Clique

cliquescoteriepopular clique
Such social groups function as protective cliques within the physical groups in which the primates live.

Peter Killworth

Killworth, PeterPeter D. Killworth
Anthropologist H. Russell Bernard, 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.
One academic outcome from this work was a challenge to Dunbar's number theorem.

The Tipping Point

The Tipping Point: How Little Things Can Make a Big DifferenceConnectorsbook of the same name
Gladwell describes the bystander effect, and explains how Dunbar's number plays into the tipping point, using Rebecca Wells' novel Divine Secrets of the Ya-Ya Sisterhood, evangelist John Wesley, and the high-tech firm W. L. Gore and Associates.

Virtual community

virtual communitiesInternet communityonline community

David Wong (writer)

David WongJason Pargin
In a popular article published at Cracked.com, Wong coined the neologism "monkeysphere" which introduces the concept of Dunbar's number in a humorous manner.

Individual

individualityindividualshuman identity
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.

Long-term memory

long term memorylong-term memorieslong-term
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.

Friendship

friendfriendsbest friend
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.

Primate

primatesnon-human primatesnon-human primate
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.

Social grooming

groomingallogroomingmutual grooming
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.

Genus

generageneric namegeneric
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).

Pleistocene

Pleistocene epochLate PleistocenePleistocene era
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.

Anthropology

anthropologistanthropologicalanthropologists
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. 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.

Ethnography

ethnographicethnographerethnographical
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.

Hunter-gatherer

hunter-gatherershunting and gatheringhunter gatherer
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.

Band society

hordeshordeband
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.

Kinship

kinship systemsciondescendants
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.

Tribe

tribaltribestribals
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.

Neolithic

Neolithic periodNeolithic AgeNew Stone Age
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-specialisation; 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.