Dunbar’s number

Tonight (Thursday) on Catalyst Dr Jonica Newberry is going to show a segment entitled Falling in Friendship (cf falling in love), which will look at Dunbar’s number, the number of friends we can maintain in an enduring relationship. The Wikipedia spiel is as follows:

Dunbar’s number is a suggested 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.
This number was first proposed by British anthropologist Robin Dunbar, who found a correlation between primate brain size and average social group size. By using the average human brain size and extrapolating from the results of primates, he proposed that humans can only comfortably maintain 150 stable relationships. Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group. It has been proposed to lie between 100 and 250, with a commonly used value of 150. Dunbar’s number states the number of people one knows and keeps social contact with, and it does not include the number of people known personally with a ceased social relationship, nor people just generally known with a lack of persistent social relationship, a number which might be much higher and likely depends on long-term memory size.

Dunbar 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.” On the periphery, the number also includes past colleagues, such as high school friends, with whom a person would want to reacquaint themself if they met again.

Dunbar himself identifies trust and obligation as key elements in the relationships he is talking about. He cites the Domesday book as showing village sizes to cluster around 150 in 1086. He also sought evidence from parishes in the 18th century and from hunter-gatherer societies. On hunter-gatherers:

Dunbar noted that the groups fell into three categories — small, medium and large, equivalent to bands, cultural lineage groups and tribes — with respective size ranges of 30–50, 100–200 and 500–2500 members each.

Francis Fukuyama in his book The Origins of Political Order suggests that politics starts to become formal at the level of the tribe. Yuval Harari in Sapiens suggests that anthropologically groups tended to divide if they exceeded 150.

In military terms, 150 seems to be the ideal company size, historically and at present.

The first thing to note is that Dunbar himself considered the number exploratory due to the large error measure (a 95% confidence interval of 100 to 230).

People will ask, What about Facebook?

The obvious answer is that Facebook allows a large number of weak relationships to be maintained. It also maintains relationships that may otherwise lapse through distance. The suggestion is that it doesn’t radically alter Dunbar’s number.

Clearly, also, more relationships can be maintained if people spend more time on social grooming.

Alternative numbers have been suggested, but so far Dunbar’s number seems to hold sway.

7 thoughts on “Dunbar’s number”

  1. Brian: I think that modern society is different from more traditional societies.
    For example, a tribe of 150 people would have been made up of people who knew almost everyone in the tribe and very few, if any, people outside of the tribe apart from travelling traders and important people from nearby tribes. (OK, it is often more complex, particularly for groups that practice exogamous marriage or depend on other tribes for key commodities, shared religious events etc. But there is still a situation where there is a high level of overlap re who they know and have strong relationships with.) For the Aboriginal groups I am familiar with a great deal of effort is made to fit strangers into the group structures. The skill in this type of society is to remember the relationship, mutual obligation, avoidance rules etc. with every other
    On the other hand, in larger societies there may be much less overlap. For example, my wife belongs to a number of subcultures in Brisbane. These overlap very little with the subcultures that I belong to. Even within my wife’s group of subcultures there is not a great deal of overlap between members of her group of subcultures.
    All of which suggests that a number based on the size of villages in the middle ages may be ignoring the differences in the way individuals live in larger societies with better transport and communication.

  2. Granted our social worlds are differently configured, I think Dunbar is saying we still lack the brain size and time to meaningfully engage with more than 150 on an ongoing basis.

    As usual with Catalyst, the program wasted a lot of time being entertaining. It did resolve one thing – you count the kids, not just the adults. I recall Ronald Wright saying that we spent 98% of our time as a species in bands of 150, comprising 30 to 40 adults.

  3. Brian: Thinking about the mining towns we lived in I would have said the following;
    Alyangula – 500 when we started there (100 families) Place functioned as a single community a fair bit of the time. My comment was that “the village idiot had a place” and there was a sense of community at events. (Most people went to the events.)
    Hazel commented “If you collapsed in the street people would no who you were and be there helping you.” Lousy TV helped community cohesion.
    By the time we left the population (including families) had doubled. Still a community at times but the sub groups had become more important. (we even had a soccer competition but most people didn’t go.) Heavy networkers like Hazel would have known all the families + a lot of single men + a lot of people from the Aboriginal communities. Still hard for someone to fall between the cracks.
    Newman – 5000 people. At this size there there were a large number of people we had nothing to do with. People like Hazel related to most of her fellow movers and shakers plus a number of friends. Friends that had been there for years said that the advent of TV weakened the community substantially.
    Brisbane: People like Hazel are influential in their subgroups but most of the city are strangers. Hazel commented when we lived in Melbourne that she could collapse in the street and no-one would know who she was.
    It is interesting to compare the Groote Eylandt Aboriginal and European communities. In the case of the Europeans we have general rules for dealing with each other so we don’t really need to know who someone is before we can deal with someone. (Gender, age etc. may count but they are still general rules.
    By contrast many of the Aboriginal rules are relationship driven. (For example, the rules for dealing with my father’s sisters daughter are quite different to dealing with my father’s brother’s daughter.) A groote Eylandt Aborigine not only needs to know who someone is before knowing how to deal with them they also need to know how the person is related to a number of people. It is no wonder that the mental effort became more difficult as the Dunbar number is approached.

  4. There’s another odd statistic in that since we became ‘civilised’ after inventing agriculture apparently our brain size has diminished by about 10%. This is usually attributed to the fact that we are now more specialised and don’t need the same range of skills. It also may relate to social organisation being more structured and segmented.

  5. Brian: Most people don’t appreciate the amount of brain power and knowledge required to be a successful hunter gatherer living in a hunter gatherer society.
    I am a bit wary however, of linking human brain size to brain power. I know some very bright people with relatively small heads.

  6. Thanks for putting this up as a topic, Brian. Since that Catalyst program went to air, I have done a bit of reflecting on it. Every effective workplace group or military sub-unit or residential community in which I was a part did not exceed Dunbar’s Number in size or were up to a quarter or two-fifths smaller. The ones which were less effective or bothersome or given to factional disputes were invariably very much bigger than Dunbar’s Number.

    I can vouch for the effectiveness of the cohesive Company or Squadron sized group when it is a Unit and not a Sub-Unit on operational deployment. Take the number of personnel on the posted strength of a company or squadron – add to that the casualties and the reinforcements that keep the overall posted strength fairly stable from day to day – and add those with whom the company or squadron regularly interacts and the number does come quite close to Dunbar’s Number. Sorry, I can’t comment on the French demi-battalion which was at least twice the size of one of our companies or squadrons.

  7. Graham: I never been in the army but my impression is that armies can have effective platoons, companies, corps etc. A key point here is that armies have found effective (but often different) ways of operating at each of these levels even though a corp is well above the Dunbar number.
    Once you get above a very small group the whole thing needs some form of formal or informal structure to be effective.

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