Dunbar's Number

By Sean Carroll | February 4, 2008 6:53 pm

I never knew this. (Via xkcd.) Wikipedia defines Dunbar’s number:

Dunbar’s number, which is very approximately 150, represents a theorized cognitive limit to the number of individuals with whom any one person can maintain stable social relationships, the kind of relationships that go with knowing who each person is and how each person relates socially to every other person. Group sizes larger than this generally require more restricted rules, laws, and enforced policies and regulations to maintain a stable cohesion. Dunbar’s number is a significant value in sociology and anthropology. It was proposed by British anthropologist Robin Dunbar, who theorized that “this limit is a direct function of relative neocortex size, and that this in turn limits group size … the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained.”

In the context of the impending Super Duper Tuesday showdown, I can’t help but thinking of this in terms of politicians. Various famous political figures are occasionally described as having uncanny abilities to connect quickly with a wide variety of people, remember faces, and convince casual acquaintances that they are your best friend. Perhaps their neocortices have the unusual ability to maintain relationships (or at least appear to) with far more than the conventional 150?

  • http://www.allenvarney.com Allen Varney
  • http://www.allenvarney.com Allen Varney

    Sorry, screwed up the link to my “Dunbar’s Number” Escapist article: http://www.escapistmagazine.com/articles/view/issues/issue_84/478-Dunbars-Number

    Wish there was a comment preview on this blog…

  • http://officeofredundancyoffice.blogspot.com Ken Clark

    Oooooo, the Moneky Sphere, which I think, is a better term for it in this case.

  • http://anand.ws/ Anand

    I have read about this limit in In Search of Excellence but I couldn’t recall if they called it Dubnar’s Number.

  • http://blog.domenicdenicola.com/ Domenic

    Yeah, I’d heard of it as the Monkey Sphere.

  • http://www.jollybloger.blogspot.com Jolly Bloger

    Ken beat me to it, but I was going to point out that this is another name for the too.

  • http://www.spayced.com spayced

    Interesting, this is the same number given for every other social animal: dogs, hyenas, etc..

  • MedallionOfFerret

    As in all things, science fiction may have the answer. In the novel “Geodesica: Ascent” by Sean Williams & Shane Dix, the future politician Melilah Awad’s Farleyfile system is explained this way:

    “Two things she had learned from her political career were that society depended on interpersonal relationship networks, and that everything could turn on a simple misidentification. In her fifties she had set about ensuring that her memory remained in good working order in order to avoid the latter. Anthropologists had known since the twentieth century that people were born with a ceiling on their social groups. Once Melilah exceeded the Dunbar and Hill number of 150, and rapidly acquiring still more associates, it became progressively harder to keep track of everyone she had to know. As she passed her hundredth birthday, it became only worse. Some of her contemporaries option for cortical grafts and other memory prosthetics. She, motivated by a gut-level — some would say irrational — desire to keep her body as free of technology as possible, opted for other means.

    “She had found that using a name once every two months was enough to keep her memory fresh. Any longer than that between reminders, and the associations faded, rendering recall unreliable. So she programmed a complete list of her occasional acquaintances and enemies into a database and programmed it to cycle through them ever sixty days, shuffling the order each time.

    “A hundred years on, she was still in the habit of skimming fifty or so names a day, just to keep on the ball. By flicking through the images, voice files, place names, and more obtuse clues, she felt confident of recognizing any of the several thousand people more or less likely to interact with her at any point in the near future. The list had grown and shrunk down over the years, and many people had dropped off it entirely because of death or distance. Sometimes she added names to the list only to be surprised that they had been on it once before, decades ago, and she had forgotten them completely. That only reinforced the need for the list and the effort to keep it fresh in her mind.”

  • zeferino

    Some people are really really good at pretending they know you but really have no clue. I worked as a Legal Assistant for years and learned this pretty quickly. Once you pretend to recognize someone, they volunteer information about your last interaction and all you have to do is realistically pretend to remember while gaining valuable information that may or may not jog your memory or at least provide relavent conversation topic(s). I’m sure politicians (often attorneys) are the best pretenders yet.

  • Snoop

    I remember a fun talk given by a theoretical chemist back when I was in grad school. He had spent a lot of time studying aggregation processes, clustering etc. and found that this Dunbar’s number (or thereabouts) was also the maximum size of a typical aggregate (any bigger and it splits), suggesting a more fundamental basis for this number than just the number of social interactions a human can maintain.

    I also remember him connecting this to some French social theorist (Levi-Strauss?) who also found that the maximum size of a tribal society is about the same number. To maintain larger societies, we humans have to sort of subsume our individuality to some extent. It gets a bit blurry here, I remember something about “icons” being introduced to represent a new “unit” of social structure (maybe like a congressional district or something), and collections of these “icons” were best kept under Dunbar’s number too.

  • Anonymous

    Farley files go back to FDR, James Farley, Roosevelt’s campaign manager, kept files on everyone FDR has ever met, updating them as needed, and briefing Roosevelt whenever he met those people again. Politicians (who are not generally noted for their neocortical development) have been using Farley files ever since.

    As for Farley files and science fiction, see Robert Heinlein’s Double Star.

  • http://backreaction.blogspot.com/ B

    Hi Sean:

    Perhaps their neocortices have the unusual ability to maintain relationships (or at least appear to) with far more than the conventional 150?

    Without any irony, this might indeed be the case, and our present political
    system does just select the ‘fittest’ with regard to this ability. The sad thing is thought that this is not an adaption to external change, but it’s us ourselves who change the environment we live in faster than our brains can possibly adapt to it, so that most of us ‘fit’ only inappropriately. Evolution just takes time, and the human brain didn’t evolve to cope with 300 emails a day, 200 ‘friends’ on facebook, and all the information available on the www.

    Coincidentally, I came across Dunbar only yesterday in a book I am currently
    reading, “The Ingenuity
    by Homer-Dixon. I can really recommend it if you are interested in
    this kind of questions like, how well can we humans cope with the
    increasing complexity of our societies. Let me quote the paragraph
    following the explanation of Dunbar’s studies:

    “A capacity to monitor 150 other people was probably quite satisfactory for
    most human beings until the twentieth century. But today’s information
    technologies have extended our reach to a far vaster community – indeed, the
    community is, for all intents and purposes, infinitely large. And there’s an
    implicit expectation that we should be able to smoothly manage our complex
    interactions with all the members of this community. Given our evolutionary
    heritage, it’s not surprising that most of us fail to live up to this
    challenge. It’s not surprising, either, to hear that the rising incidence of
    depression and other anxiety-related mental illnesses in our societies might
    be partly caused by this failure.”

    (Comes with references that I’ve left out). For an review of his newer book see here. He does a great job bridging the gap between sociology, politics, and natural sciences (biology, physics, neuroscience etc).



  • Stephen Burns

    Hi Sean,

    Long time reader, first time writer…

    Despite living in LA, I have never met a successful actor (or unsuccessful one for that matter). But my girlfriend has met several, and she describes them as projecting such warmth and having such an ability to charm you to such an extraordinary degree that you would swear they were your best friend. I have heard Magic Johnson described in the same way. Perhaps the politicians you describe, who sound very charismatic indeed, are doing the same thing.


  • garyb50

    Just asked my wife (the intellectual in the family) if she’d ever heard of this and she said no. But when I started describing it she said, “you know, I saw something on TV just recently about Mennonites limiting the size of their communities to 150. When it grew bigger they split off to start another.”

    Don’t know if it’s true but it’s interesting.

  • http://www.allysonbeatrice.com/blog Allyson

    I’ve read something similiar (maybe from Clay Shirky?) regarding online communities, with the magic number being 300 as the maximium amount of active users a community can hold before breaking off into subcommunities (likely due to Godwin’s or Snacky’s laws).

    I think it was in a study of The Well. Must. Go. Google….

  • Damian

    The Tipping Point by Malocolm Gladwell devotes a chapter to Dunbar’s number.
    He says that a religious group called Hutterites live by it, splitting their communities in two once the population reaches 150.He also talks about a company (GoreTex from memory) that never has more than 150 employees in any plant.

  • http://www.savory.de/blog.htm Ole Phat Stu

    Dunbar’s number is 153 to be more precise.

    It says so in the Bible, already!

    “Peter cast out his nets and caught many fishes.
    And the number of fishes was 153” or words to that effect.

    Of course the Hebrews were into numerical symbology 2000 years ago, assigning different values to each letter. Interestingly, the sum of the digits of the Greek word PETRUS is 153. And the sum of the digits of the greek word for Nets is also 153; just them old scribes playing a numbers game.

    I wonder if the sum of the digits of Dunbar is 153 too?

  • dilbert dogbert

    A retired teacher friend commented on this in relationship to the optimum size of a high school. As and old country boy he said: In a small school everyone knows everyone and there usually only a couple of trouble makers who need to be kept track of. The trouble makers also, when running as a pack are not big enough to seriously disrupt the school. When school sizes go beyond about 300 the trouble maker population grows and the pack size gets big enough that now you need full time police. He worked in a high school in Stocton Ca that had a two cell lockup on campus.


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .


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