Is There an Ideal Size for Human Social Groups?

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Modern humans live in a bewildering array of population densities. Some of us live on farms in rural areas, while others live in small towns or suburbs. Many of us dwell in densely packed urban centers, and a few hardy souls live in isolation in the wilderness.

Clearly, we are quite flexible when it comes to the size of our communities, but is there an optimal group size that evolution has prepared us to be most comfortable with?

Let us look to the lives of our prehistoric ancestors for insight into this question.

The current consensus is that prehistoric humans lived intensely social lives in fairly stable groups. They had to cooperate with each other in these groups to succeed in competition with other groups, but they also had to recognize that in-group members were their main competitors for mates and for dividing up limited resources. Living under such conditions, our ancestors faced a number of consistent adaptive problems, such as remembering who was a reliable, trustworthy person and who was a cheater; knowing who would be a reproductively valuable mate; and figuring out how to successfully manage friendships, alliances, and family relationships.

And the people that they had to keep track of were by and large people whom they knew well; interactions with total strangers were probably an infrequent and temporary phenomenon back-in-the-day.

So, how large were these early human groups?

150 Is the Magic Number

As far as scientists can tell, our long-ago ancestors lived in relatively small groups of no more than 150 people. The case for this being the magic number for the size of human groups has been made so strongly by British anthropologist Robin Dunbar that it has come to be known as “Dunbar’s Number.”

Among our primate relatives, there is a consistent relationship between the size of a primate’s brain and the size of the social groups in which it lives. More specifically, it is the size of the neocortex that matters. To put it a bit more simply than it actually is, the neocortex is the largest, most wrinkly part of the brain. It is the part of the brain responsible for perception, language, and cognitive functioning, and it is also believed to be the place where consciousness resides. The ratio of the size of the neocortex to the rest of the brain nicely predicts the naturally occurring size of primate social groups, and for humans this prediction leads us to expect 150 to be the appropriate size for a stable human population.

Dunbar has mustered an impressive amount of evidence to bolster this conclusion: One-hundred-fifty turns out to be a fairly consistent size for a “clan” in a variety of different hunter-gatherer societies, and archaeologists have determined that this was also the approximate size of farming villages in the Middle East 7,000 years ago. It is still the standard size of small farming communities around the world today.

Historically, it has also been the ideal size for religious cults, church congregations, and military companies, and Dunbar maintains that it is also a good estimate for the maximum number of individuals one can realistically maintain any sort of meaningful social relationship with at any given time. Research even indicates that the ideal size for a high school (grades 9 through 12) is somewhere between 500 and 700 students. The exact middle of that range–600 students–would be the total if each grade in the high school had, you guessed it, 150 students.

Why Does 150 Work So Well?

Why, you may ask, does a group of this size work so well for humans? It appears that 150 is the largest that a group can get and still function in an egalitarian fashion without having a formal hierarchical organizational structure in place. In a social world of this size, you can know everyone else in a face-to-face, long-term kind of way.

If you have not already had personal dealings with an individual, one of your close friends or relatives almost certainly has. Peer pressure, gossip, and social ostracism would very quickly bring troublemakers back into line as everyone in the group was constantly monitoring everyone else’s reputations.

It would not have been necessary to have written rules, courts of law, police officers, or lawyers in these societies; group enforcement of informal social norms alone would have kept things humming along quite nicely.

People will continue to thrive in populations of all sizes, but it might contribute to human efficiency and well-being if we keep Dunbar’s Number in mind as we plan the size of schools, retirement communities, and departments nested within larger business and government organizations.