We’ve all heard of Dunbar’s number; this essay (part of a series on the topic) doesn’t rehash the basics.
Instead, the author revisits Dunbar’s original research, and reaches the interesting conclusion that
… 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.
He uses examples from online games, developer forums, and Wikipedia to support his thesis that…
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.
He then raises the interesting idea that there is not one but two peaks of ideal group size—one at ~45-50, one at ~5-8; and that the chasm between those peaks is a dangerous point for a growing organization (for reasons he goes into).
Linked at the bottom of the essay are other pieces on the same topic, which I encourage everyone to read (and may comment on specifically later on).
 Archived version of the first missing graph: Primate Neocortex Size vs. Social Group Size