The Dunbar Number and Tribes, Big and Small

2010 November 9

Wikipedia defines Dunbar’s number (commonly cited as 150) as a “theoretical cognitive limit to the number of people with whom one can maintain stable social relationships. These are relationships in which an individual knows who each person is, and how each person relates to every other person.” Dunbar is director of the Institute of Cognitive and Evolutionary Anthropology at Oxford University and first wrote about this almost 20 years ago but the “tipping point” for disseminating his theory was Gladwell’s citation in his book, The Tipping Point in 2000. Wikipedia’s entry above is actually a quotation from The Tipping Point, which is an example of the downside of a crowdsourced encyclopedia. The ideal encyclopedia would have quoted Dunbar, the original source, not Gladwell.

I became interested in the Dunbar number when I was exploring creating a social network based on personal compatibility. It struck me that Facebook had two disadvantages: it recommended people primarily based on having friends (or acquaintances, or connections) in common and the number of “friends” easily grew to an impossibly large number.

So if Dunbar’s number suggests the ideal size of a community, what about smaller groups? During my research I came across the notion that in a hunter-gatherer society the ideal size for a hunting group was roughly a dozen. The blog, Signal vs. Noise, posted its thoughts on how small teams worked best, citing another British author, Antony Jay who wrote in his book, Corporation Man, that the ideal size for a team has been a “ten-group” for centuries. The basic unit for most armies is ten—and has been since Roman times. Most sports teams, juries … well, you get the idea.

It’s pretty obvious what the advantages of a team this size are: nimbleness, loose, flexible organization, quick decision-making, strong inter-dependence, well aligned interests, each individual plays a critical role. We’ve all experienced it. And when it works, it can be magic.

Dunbar’s number came from his research as an evolutionary anthropologist. Unlike other animals, primates bond by using their hands to groom each other and he found there seemed to be a natural size to a band of primates. Dunbar wondered whether the size of primate’s social networks was correlated with their brain size and found that “within the primates there is a general relationship between the size of the brain and the size of the social group.” He then tested his theory on human groups and found that, with remarkable consistency, for thousands of years early settlements and villages consisted of 150 people.

One feature of those societies was not just that you knew everyone in the group but that the group was also completely stable. No one left or entered.

This changed with the industrial revolution, which started the norm of social mobility we’re familiar with today. Once the village structure broke down, many of society’s self-regulating abilities died with it. Now we need policemen and courts of law. Obviously the Internet has given us the ability to connect with thousands of others, no longer limited by geography or social restrictions. But our brains are wired to live in these ancient societies. How do we adapt? Dunbar doesn’t think we can. If we accept his assumption that his number is related to brain size, in order to process significant relationships with 10 times as many people (1,500) our brains would have to be 10x larger. And we know that ain’t happening any time soon.

Obviously we’re living in a world in which we all have much larger networks than our parents did. But the point of the Dunbar number is that the number of meaningful relationships we can have is still limited. Dunbar defines meaningful as based on trust and obligation.

I’m interested in this because, as an entrepreneur, I’d love to find a way to connect people who have the potential to have a meaningful relationship. There’s no question that, in an era of 5,000 Facebook friends, people are craving tighter knit communities based on compatibility. But the question I’ve asked here before is, what is true compatibility? And how does it manifest itself online? It’s more than shared interests, it’s more than compatible personality types. Can you look at your close friends or your spouse and define what it is that connects you? It certainly isn’t picking lice out of each other’s hair.

A side note. When I lived in London in the early 80’s, I was a big fan of “Yes, Minister,” the popular BBC series. Turns out Antony Jay was a co-writer of that and the author of a book I enjoyed a few years later called Management and Machiavelli as well as Corporation Man. Interesting guy.

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