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The Dunbar Number

  • Written by Professor
  • March 11th, 2013
  • 4 min read



Bear with me; this may take a while to get to the relevant point.

Robin Dunbar is Professor of evolutionary anthropology (the study of how humans came to be modern) at Oxford. Growing up in Tanzania his focus in graduate school was the social life of the gelada, a baboon like monkey found in the highlands of Ethiopia. These monkeys create friendships and bond with each other by grooming—looking for ticks on each other. But, Dunbar found, the amount of time one monkey spent grooming another was not based on physical size, but on the group size. The larger the group, the more time one monkey spent grooming another. In other words, as the group expanded, the monkeys lost touch with their closest ones, and resorted to grooming in order to rekindle friendships.

Dunbar began to wonder what other characteristics related to group size, and found that brain size was crucial. His argument was that big brains evolved to solve the problem of social life. I’ll spare you the science but the bottom line is that the larger the brain the larger a group the species can manage. However, even the largest brains are not large enough to handle vast sizes of groups.

So Dunbar then asked himself the next question: given the size of human’s brains, what is the size of group we are able to handle? Now this was in the days before Facebook and e-cards, so Dunbar took a very simple measurement. He asked, in England, how many Christmas cards the average person sent. He reasoned that sending a card took an effort, and one would only send a card to people that were in a relatively inner circle.

The answer was 153. This confirmed studies Dunbar and other anthropologists had already concluded. In most societies around the world, going back to ancient times, the optimal size for any community was 150; more than that the group tended to split up into multiples. Many military units are built on this number. Many companies will not allow managers to manage more than this number. In studying multiple primates, and correlating brain size with group size, Dunbar has found a clear direct relationship between the two, and has calculated that the optimum number of relationships for a human is 148.

This is, of course, an average, with the range for most people lying between 100 and 230.

Several years ago Dunbar worked with one of the original founders and technocrats of Facebook, who was leaving to start his own company. That company, called Path, enables people to connect with each other, like on Facebook, but with a group size limited to 150. This founder felt that as one’s group got beyond 150 people, it started to include too many strangers, and people would begin to self censor the type of material they would post. By limiting the group to 150 (assumed close friends) one’s posts could retain their intimacy.

A Twitter user regularly interacts with between 100-200 people, while the average user on Facebook has 190 “friends” (larger than the Dunbar number of 150, but let’s admit we all have about 30% or so friends on Facebook who we really don’t know that well.)

Dunbar’s research is not only connected to this number. He also has researched the amount of time an average friendship can last without face to face contact. The answer is between 6-12 months. More specifically, close friendship declines on average by 10-15% a month without regular contact. (Obviously a critical issue is how close the friendship was before non-communication began. If it was, for example, your twin brother who you hadn’t seen in ages you might be able to start right off immediately upon seeing him again).

This kind of gets me to what I hope is the relevant point. I have long believed that long distance relationships do not work, and especially those between a foreigner who has met, let’s say, a Thai lady during holidays. One reads enough subs about someone who comes to Thailand, meets the love of his life, spends two glorious weeks in her company, returns to his home land, and wonders why, despite his best intentions and the regular sending of money, the relationships falters.

Now Professor Dunbar from Oxford has supplied us with the answer. It’s in our primal neo-cortexes! Somewhere back in pre-historic evolutionary times, when we were descending from a common apelike ancestor and trying to avoid mating with Neanderthals while at the same time inventing agriculture so as to get away from hunting and gathering, something developed in our brains so that thousands of years later we are unable as a species to sustain a long distance relationship with a Thai lady.

And no, skype doesn’t count.

The plain fact of the matter is, and we all knew this even without hearing from Oxford, that only regular contact can sustain a relationship. If we are not ready and able to see our love at least once a month, things will deteriorate quickly (10-15% month?).

Anyway, I thought you might like to know.

Take care

Professor