Dunbar Number

Dunbar Number = the cognitive limit to the number of individuals with whom any one person can maintain stable relationships, and therefore, a limit to viable groups.

Background

From the Life with Alacrity blog:

"Dunbar is an anthropologist at the University College of London, who wrote a paper on Co-Evolution Of Neocortex Size, Group Size And Language In Humans where he hypothesizes:

... there is a cognitive limit to the number of individuals with whom any one person can maintain stable relationships, 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.

Dunbar supports this hypothesis through studies by a number of field anthropologists. These studies measure the group size of a variety of different primates; Dunbar then correlate those group sizes to the brain sizes of the primates to produce a mathematical formula for how the two correspond. Using his formula, which is based on 36 primates, he predicts that 147.8 is the "mean group size" for humans, which matches census data on various village and tribe sizes in many cultures." (http://www.lifewithalacrity.com/2004/03/the_dunbar_numb.html)

Specifications

See Christopher Allen on the Dunbar Number for more details.

John Robb, citing Christopher Allen:

""...according to Chris Allen's online group analysis, can be seen at two levels: both small and medium sized. Small, viable (in that they can be effective at tasks) groups (or cells) are optimized at 7-8 members. A lower boundary can be seen at 5 (with groups less than 5 not having sufficient resources to be effective) and an upper boundary at 9. Medium sized groups are optimal at 45-50 members, with a lower limit of 25 and an upper limit of 80. Between these levels is a chasm that must be surmounted with significant peril to the group. This is due to the need for groups above 9-10 members to have some level of specialization by function. This specialization requires too much management oversight to be effective given the limited number of participants in each function. At 25 members, the group gains positive returns on specialization given the management effort applied (a break- even point)." (http://globalguerrillas.typepad.com/globalguerrillas/2004/03/what_is_the_opt.html)

More Information

Dunbar's original essay is at http://www.bbsonline.org/documents/a/00/00/05/65/bbs00000565-00/bbs.dunbar.html

See the related entry on the Power Law.

Clay Shirky's commentary on Communities vs Audiences are very relevant.

Christopher Allen, from the Life with Alacrity blog, lists his different postings on the topic here at http://www.lifewithalacrity.com/2006/08/dunbar_number_p.html

Also listen to a podcast by Christopher Allen on the Dunbar Number