Dunbar's Number

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. ...

This number of 150 has become "Dunbar's Number" and has been popularized by various very popular business books such as Malcolm Gladwell's The Tipping Point: How Little Things Can Make a Big Difference (summary), Duncan J. Watts' Six Degrees: The Science of a Connected Age (review) and Small Worlds: The Dynamics of Networks between Order and Randomness (review), and Mark Buchanan's Nexus: Small Worlds and the Groundbreaking Science of Networks (review), the ideas from which are the foundation of the various Social Network Services that I've discussed elsewhere in this blog. ...

Lately, Dunbar's number has been taken as a mean size for online networks and groups ... However, Dunbar's work itself suggests that a community size of 150 will not be a mean for a community unless it is highly incentivized to remain together. We can see hints of this in Dunbar's description of the number and what it means:

The group size predicted for modern humans by equation (1) would require as much as 42% of the total time budget to be devoted to social grooming. ...

My suggestion, then, is that language evolved as a "cheap" form of social grooming, so enabling the ancestral humans to maintain the cohesion of the unusually large groups demanded by the particular conditions they faced at the time.

Dunbar's theory is that this 42% number would be true for humans if humans had not invented language, a "cheap" form of social grooming. However, it does show that for a group to sustain itself at the size of 150, significantly more effort must be spent on the core socialization which is necessary to keep the group functioning. Some organizations will have sufficient incentive to maintain this high level of required socialization. In fact the traditional villages and historical military troop sizes that Dunbar analyzed are probably the best examples of such an incentive, since they were built upon the raw need for survival. However, this is a tremendous amount of effort for a group if it's trying not just to maintain cohesion, but also to get something done. ...

This all leads me to hypothesize that the optimal size for active group members for creative and technical groups -- as opposed to exclusively survival-oriented groups, such as villages -- hovers somewhere between 25-80, but is best around 45-50. Anything more than this and the group has to spend too much time "grooming" to keep group cohesion, rather then focusing on why the people want to spend the effort on that group in the first place -- say to deliver a software product, learn a technology, promote a meme, or have fun playing a game. Anything less than this and you risk losing critical mass because you don't have requisite variety.

Thus Dana Boyd gets it right when she uses the word MAXIMUM in her eTech Talk, not average or mean:

(Dunbar) found that the MAXIMUM number of people that a person could keep up with socially at any given time, gossip maintenance, was 150. This doesn't mean that people don't have 150 people in their social network, but that they only keep tabs on 150 people max at any given point.

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