Dunbar Himself on Magic Numbers + Gladwell on Dunbar

Wednesday, January 01, 2003

Robin Dunbar and the Magic Number of 150

From http://www.commonsenseadvice.com/index.html

The Magic of 150

Your brain is hard wired to pay attention to about 150 people. Try to have a relationship with any more than that, and your life will turn to pure crap. Just ask the Military, Gore-Tex, or Krippendorf’s tribe. They’ll all tell you the same thing. One fifty is the way to go. They’ve known for hundreds of years that people work best in groups of 150 or less. Now it’s your turn.

The human cortex, responsible for complex thought and reasoning, is overgrown in humans when compared to other mammals. Scientists have argued for years about why this is the case.

One theory holds that our brains evolved because our primate ancestors began to gather food in more complex ways. They began eating fruit instead of grasses and leaves. This involved traveling long distances to find food, and required each species to maintain a complex mental map in order to keep track of fruit trees. More brainpower might have been needed to determine if a fruit was ripe, or to discern proper methods for peeling fruit or cracking nuts.

The problem with this theory is that if one tries to match brain size with the eating habits of primates, it doesn’t work. Some small-brained monkeys are eating fruit and maintaining complex maps and some larger brained primates are eating leaves.

What does work, apparently, is group size. If one examines any species of primate, the larger their neocortex, the larger the average size of the group they live with.

Anthropologist Robin Dunbar has done some of the most interesting research in this area. Dunbar’s argument is that as brains evolve, they become larger in order to handle the unique complexities of larger social groups. Humans socialize the largest social groups because we have the largest cortex. Dunbar has developed an equation, which works for most primates, in which he plugs in what he calls the neocortex ratio of a particular species – the size of the neocortex relative to the size of the brain – and the equation gives us the maximum expected group size for each species. For humans, the max group size is 147.8, or about 150. This figure seems to represent the maximum amount of people that we can have a real social relationship with – knowing who another human is and how they relate to us.

Dunbar has gone through anthropological literature and found that the number 150 pops up over and over again. For example, he looked at 21 different hunger-gatherer societies around the world and found that the average number of people in each village was 148.4.

The same pattern holds true for military organization. Over the years, through trial and error, military planners have arrived at a rule of thumb for the size of a functional fighting unit – 200 men. They have realized that it is quite difficult to make any larger a group than this to function as a unit without complicated hierarchies and rules and regulations and formal measures to insure loyalty and unity within the group. With a group of 150 or so, formalities are not necessary. Behavior can be controlled on the basis of personal loyalties and direct man-to-man contacts. With larger groups, this seems impossible.

Further is the religious group known as the Hutterites, who for hundreds of years, through trial and error, have realized that the maximum size for a colony should be, low and behold, 150 people. They’ve been following this rule for centuries. Every time a colony approaches this number, the colony is divided into two separate colonies. They have found that once a group becomes larger than that, “people become strangers to one another.” At 150, the Hutterites believe, something happens that somehow changes the community seemingly overnight. At 150 the colony with spontaneously begin dividing into smaller “clans.” When this happens a new colony is formed.

Another good example of our hard wired social limits is Gore Associates, a privately held multimillion-dollar company responsible for creating Gore-Tex fabric and all sorts of other high tech computer cables, filter bags, semiconductors, pharmaceutical, and medical products. What is most unique about this company is that each company plant is no larger than 150. When constructing a plant, they put 150 spaces in the parking lot, and when people start parking on the grass, they know it’s time for another plant. Each plant works as a group. There are no bosses. No titles. Salaries are determined collectively. No organization charts, no budgets, no elaborate strategic plans. Wilbert Gore – the late founder of the company, found through trial and error that 150 employees per plant was most ideal. “We found again and again that things get clumsy at a hundred and fifty,” he told an interviewer some years ago.

Take a lesson from this. If you are engaged in a large enterprise or are planning to work for one, realize that large groups rapidly reduce the efficiency of an operation. If each department is separated, especially if there are hundreds or thousands of people involved, complex systems of organizations will be required to keep everyone in check. Peer pressure is much more powerful than the somehow vague concept of a boss or punishment. People will work only hard enough not to get fired in a very large group, but will live up to the expectations of their peers in smaller groups where they have a personal relationship with each of their co-workers. Of course, a small group size is not by any means a guarantee of success. Small enterprises fail all the time. It’s just a concept — an idea to keep in the back of your mind as you vegetate in that basement cubicle.

For more information:
R.I.M Dunbar, “Neocortex size as a constraint on group size in primates,” Journal of Human Evolution (1992), vol. 20, pp. 469-493.

Dunbar has also written a good many spiffy books of which I would recommend:
Grooming, Gossip, and the Evolution of Language.

Malcolm Gladwell on Robin Dunbar’s work on Magic Numbers

“At a certain point, at somewhere between 10 and 15 people, we begin to overload, just as we begin to overload when we have to distinguish between too many tones.

It’s a function of the way humans are constructed. Man evolved to feel strongly about few people, short distances, and relatively brief intervals of time; and these are still the dimensions of life that are important to him.”

Perhaps the most interesting natural limit, however, is what might be called our social channel capacity. The case for a social capacity has been made, most persuasively, by the British anthropologist Robin Dunbar. Dunbar begins with a simple observation. Primates – monkeys, chimps, baboons, humans – have the biggest brains of all mammals. More important, a specific part of the brain of humans and other primates – the region known as the neocortex, which deals with complex thought and reasoning – is huge by mammal standards.

If you belong to a group of five people, Dunbar points out, you have to keep track of 10 separate relationships: your relationships with the four others in your circle and the six other two-way relationships between the others. That’s what it means to know everyone in the circle. You have to understand the personal dynamics of the group, juggle different personalities, keep people happy, manage the demands on your own time and attention, and so on.

If you belong to a group of 20 people, however, there are now 190 two-way relationships to keep track of. 19 involving yourself and 171 involving the rest of the group. That’s a fivefold increase in the size of the group, but a twenty fold increase in the amount of information processing needed to “know’ the other members of the group. Even a relatively small increase in the size of a group, in other words, creates additional significant social and intellectual burden.

Humans socialize in the largest groups of all primates because we are the only animals with brains large enough to handle the complexities of that social arrangement. Dunbar has actually developed an equation, which works for most primates, in which he plugs in what he calls the neocortex ratio of a particular species – the size of the neocortex relative to the size of the brain – and the equation spits out the expected maximum group size of the animal. If you plug in the neocortex ratio for Homo sapiens, you get a group estimate of 147.8 – or roughly 150.

“The figure of 150 seems to represent the maximum number of individuals with whom we can have a genuinely social relationship, the kind of relationship that goes with knowing who they are and how they relate to us. Putting it another way, it’s the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar.”

Dunbar has combed through the anthropological literature and found that the number 150 pops up again and again. For example, he looks at 21 different hunter-gatherer societies for which we have solid historical evidence, from the Walbiri of Australia to the Tauade of New Guinea to the Ammassalik of Greenland to the Ona of Tierra del Fuego and found that the average number of people in their villages was 148.4. The same pattern holds true for military organization. “Over the years military planners have arrived at a rule of thumb which dictates that functional] fighting units cannot be substantially larger than 200 men,” Dunbar writes. “This, I suspect, is not simply a matter of how the generals in the rear exercise control and coordination, because companies have remained obdurately stuck at this size despite all the advances in communications technology since the First World War. Rather, it is as though the planners have discovered, by trial and error over the centuries, that it is hard to get more than this number of men sufficiently familiar with each other so that they can work together as a functional unit.”

It is still possible, of course, to run an army with larger groups. But at a bigger size you have to impose complicated hierarchies and rules and regulations and formal measures to try to command loyalty and cohesion. But below 150, Dunbar argues, it is possible to achieve these same goals informally: “At this size, orders can be implemented and unruly behaviour controlled on the basis of personal loyalties and direct man-to-man contacts. With larger groups, this becomes impossible.”

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