“SOCIAL GROUP SIZE FOR HUMANS
“While the observed social group size of chimpanzees is 55, the social group size that Dunbar calculated from the neocortex size of humans is 150. How can that be, when we now live in huge cities, often with millions of people? However, think about it. Most of those people you never even have cause to interact with. Remember: Our ancestors were hunter-gatherers, and people didn’t start to settle in one place until agriculture was developed about ten thousand years ago. Today the typical size of hunter-gatherer clans, related groups that gather together once a year for traditional ceremonies, is 150. This is also the size of traditional horticultural societies and modern-day Christmas card lists in personal address books.
“It turns out that 150 to 200 is the number of people who can be controlled without an organizational hierarchy. It is the basic number used in military units where personal loyalties and man-to-man contact keep order. Dunbar states that it is the upper limit of the size of modern business organizations that can be run informally. It is the maximum number of people an individual can keep track of, whom he can have a social relationship with and would be willing to help with a favor.”
p. 94, emphasis added