The Science of Dunbar's Number

This is from a blog on social software, collaboration, trust, security, privacy, and internet tools, by Christopher Allen The Dunbar Number as a Limit to Group Sizes. Chris has taken a very clear look at network ecosystems and assessed effective group participation. Definitely worth a deeper read and think!


"Lately I've been noticing the spread of a meme regarding "Dunbar's Number" of 150 that I believe is misunderstanding of his ideas.

The Science of 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.

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."


It is worth your while to read the rest of his blog because he addresses group satisfaction and the best group numbers for successful teams. You can also post your comments on his blog.