Dunbar Number

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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.



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)


Dunbar has developed an equation, which works for most primates, in which he plugs in what he calls the neocortex ratio of a particular species - the size of the neocortex relative to the size of the brain - and the equation gives us the maximum expected group size for each species. For humans, the max group size is 147.8, or about 150. This figure seems to represent the maximum amount of people that we can have a real social relationship with - knowing who another human is and how they relate to us. (http://blog.summation.net/2008/12/the-150-myth.html)


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)


For smaller groups, see the article on Group Tresholds

Dunbar 50

Non Exclusive Networks 50 — "The Non-Exclusive Dunbar Number".

"More properly this group size falls in the range of 25-75 participants, but it seems to feel the most natural in the range of 50-60. Studies of the sizes guilds in online games support this hypothesis. For instance, based on graphs of the guild sizes in Ultima Online, groups have a median of 61 members. Similar numbers hold true in studies of a more recent game, World of Warcraft.

I call this value the "Non-Exclusive Dunbar Number" because it matches the lower end of a threshold that Robin Dunbar set for group sizes. However, at this size it applies to mostly non-exclusive groupings, which includes the above mentioned online guilds, many employee communities, and the majority of social gatherings that manage to rise above the size of a Working Group. Groups of this size can be serious or take up a lot of time, but in general they are not exclusive — they don't tend to be the only group that individual participants are involved in.' (http://www.lifewithalacrity.com/2008/09/group-threshold.html)

Dunbar 90

90—"The Dunbar Valley".

"As Non-Exclusive Dunbar Number communities grow, they reach a point where increased time obligations and the noise of socialization required to keep the group cohesive requires a much more serious commitment from the participants. Like the Judas Number, the Dunbar Valley is a threshold nadir where more energy is required to keep a tightly-knit community together; either the community agrees to a higher level of commitment and grows to the next level, or the community splits apart.

I've found this to be true when growing a small business — where it is too small for any middle-management, but the sub-groups are too large for one person to manage effectively. I've also seen this with more ephemeral groups, such as when a small conference that worked well at 60 participants tries to grow and finds at at 100 participants they can't sustain a high enough intimacy level.

Another illustration of the Dunbar Valley is the history of the ancient Roman "century", a grouping that was originally 100 soldiers. However, as the years went by, centuries tended to decrease in numbers to only include 70 or 80 soldiers. This might well be due to Non-Exclusive Dunbar constraints: even in a very devoted group of military men, there was still the need for relationships with other century groups, with support staff, and with camp followers, ultimately lowering the attention that could be spent on the century itself." (http://www.lifewithalacrity.com/2008/09/group-threshold.html)

Dunbar 150

150 — "The Exclusive Dunbar Number".

"Robin Dunbar got much of the discussion of group thresholds started with his article, "Co-Evolution Of Neocortex Size, Group Size And Language In Humans." However, as I've written previously, and as I've described in this article, Dunbar's group threshold of 150 applies more to groups that are highly incentivized and relatively exclusive and whose goal is survival.

Dunbar makes this obvious by the statement that such a grouping "would require as much as 42% of the total time budget to be devoted to social grooming."

The result of the grooming requirement is that communities bounded by the Exclusive Dunbar Number are relatively few. You will find hunter/gatherer and other subsistence societies where this is a natural tribe size. You'll also find these groups sizes in terrorist and mafia organizations.

As we step up toward higher group thresholds, more and more time is required to simply keep the group going. You see this in depictions of mafia life — in the TV series The Sopranos a lot of time is spent dining, hanging out, and drinking together. That is part of that 42% social grooming time required for that intense of a survival group.

It is possible for a large company to force groups up to this size by expending lots of energy (which is to say money) to keep it healthy. Apple did this during the invention of the Macintosh, the first OS X operating system, and the iPhone, but the intensity required of such large teams is not sustainable for long periods of time.

Without that extra energy, few modern tightly-knit communities can reach this threshold, or else can't hold it for very long. Instead they fracture into groups of individual interest (even if they continue to "meet" in the same real-world or online forum), which are more than more likely to be bounded by the Non-Exclusive Dunbar number.

Given the difficulty in even arriving at the Exclusive Dunbar number, it may well be the highest limit of all for a tightly-knit community. Beyond this limit, communities are less cohesive, less trusted, and less participatory (and the topic of my third article in this series.)" (http://www.lifewithalacrity.com/2008/09/group-threshold.html)


Moving from private to public spaces

Andrew Chen [1]:

" when you move from small private environments where people know each other, or can at least get to know each other over time, and transition to large public spaces, then reputation is drowned out.

All of a sudden, there's zero cost to your non-existent reputation to say whatever you want - and it becomes easy to act like an ass, or flame people who are different, or anything else you want to do. When you start running into people who are from a different culture than you, and then arguments ensue leading to the website LearnToSpell.net getting posted.

So the key issue is that in large, public spaces, you end up with the lowest common denominator of communication. People then begin to drive other folks out, because the public space is a homogenizing force, rather than a diversifying one." (http://andrewchen.typepad.com/andrew_chens_blog/2007/12/public-and-priv.html)

Update on Research

Dennis R. Fox, "Psychology, Ideology, Utopia, and the Commons", writes:

"Edney (1980, 1981a) also argued that long-term solutions will require, among a number of other approaches, breaking down the commons into smaller segments. He reviewed experimental data showing that cooperative behavior is indeed more common in smaller groups. After estimating that "the upper limit for a simple, self-contained, sustaining, well-functioning commons may be as low as 150 people" (1981a, p. 27), he listed the following "functional benefits" of reducing group size:

  • Improved communication helps sustain necessary feedback;
  • greater visibility of member distress during scarcity enhances the probability of remedial action;
  • individual responsibilities are harder to avoid;
  • alienation is reduced;
  • and the role of money is reduced.

Also, with many small commons instead of one large one, shortages in one cannot endanger the whole, and free riders have limited impact. "The improved focus on the group itself, the greater ease of monitoring exploitative power, and the opportunities for trust to develop among individuals with face-to-face contact are also enhanced" (1981a, p. 28). " (http://www.dennisfox.net/papers/commons.html)

More Information

  1. Video: Robin Dunbar on his Number
  2. Dunbar's original essay is at http://www.bbsonline.org/documents/a/00/00/05/65/bbs00000565-00/bbs.dunbar.html ; also: The Social Brain Hypothesis
  3. See the related entry on the Power Law.
  4. Clay Shirky's commentary on Communities vs Audiences are very relevant.
  5. 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
  6. Also listen to a podcast by Christopher Allen on the Dunbar Number