Dunbar’s Number: Why We Can’t Have More Than 150 Friends
“Dunbar’s number is suggested to be 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. Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group.
No precise value has been proposed for Dunbar’s number. It has been proposed to lie between 100 and 230, 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 lack a persistent social relationship, a number which might be much higher and likely depends on long-term memory size.
Dunbar’s number was first 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.” On the periphery, the number also includes past colleagues such as high school friends with whom a person would want to reacquaint oneself if they met again.” (Wiki)
“Robin Dunbar used the volume of the neocortex – the ‘thinking’ part of the brain – as his measure of brain size, because this accounts for most of the brain’s expansion within primates. He found that both measures of social complexity correlated with relative neocortex volume in primate species. Subsequently, he predicted the social group size of some other monkey and ape species from their neocortex volumes – with impressive accuracy.
Thanks to his ground-breaking work, and the follow-on studies which it stimulated, numerous features of primate social behaviour can now be predicted from neocortex volume – from the time devoted to social interaction, the level of social skills and the degree of tactical deception practiced to community and coalition size. We can also predict when social groups will split up because their size is unsustainable; Robin Dunbar’s research shows that the volume of the neocortex imposes a limit on the number of relationships that individual primates can sustain in their mental model of their social world.
The human dimension
Humans are primates, too – so do they fit into the pattern established for monkeys and apes? This is the key question which Robin Dunbar sought to answer by using the same equations to predict human social group and clique size from neocortex volume. The results were… ~150 for social group size, and ~12 for the more intimate clique size. He subsequently discovered that modern humans operate on a hierarchy of group sizes. “Interestingly”, he says, “the literature suggests that 150 is roughly to the number of people you could ask for a favour and expect to have it granted. Functionally, that’s quite similar to apes’ core social groups.” “
— The ultimate brain teaser, University of Liverpool - Research Intelligence, 17 Aug 2003
Q: How did you come up with this concept?
I was working on the arcane question of why primates spend so much time grooming one another, and I tested another hypothesis – which says the reason why primates have big brains is because they live in complex social worlds. Because grooming is social, all these things ought to map together, so I started plotting brain size and group size and grooming time against one another. You get a nice set of relationships.
It was about 3am, and I thought, hmm, what happens if you plug humans into this? And you get this number of 150. This looked implausibly small, given that we all live in cities now, but it turned out that this was the size of a typical community in hunter-gatherer societies. And the average village size in the Domesday Book is 150 [people].
It’s the same when we have much better data – in the 18th century, for example, thanks to parish registers. County by county, the average size of a village is again 150. Except in Kent, where it was 100. I’ve no idea why.
Q: Has this number evolved at all?
The Dunbar number probably dates back to the appearance of anatomically modern humans 250,000 years ago. If you go back in time, by estimating brain size, you can see community size declining steadily.
Q: Why did we evolve as a social species?
Simply, it’s the key evolutionary strategy of primates. Group living and explicitly communal solutions to the problem of survival out there on the plains or in the forests… that’s a primate adaptation, and they evolved that very early on.
Most species of birds and animals aren’t as intensely social. Sociality for most species hovers around pair-bonds, that’s as complicated as it gets. The species with big brains are the ones who mate monogamously… The lesson is that there is something computationally very demanding about maintaining close relationships over a very long period of time – as we all know!
Q: How can we grow the Dunbar number?
We’re caught in a bind: community sizes were designed for hunter-gatherer- type societies where people weren’t living on top of one another. Your 150 were scattered over a wide are, but everybody shared the same 150. This made for a very densely interconnected community, and this means the community polices itself. You don’t need lawyers and policemen. If you step out of line, granny will wag her finger at you.
Our problem now is the sheer density of folk – our networks aren’t compact. You have clumps of friends scattered around the world who don’t know one another: now you don’t have an interwoven network. It leads to a less well integrated society. How to re-create that old sense of community in these new circumstances? That’s an engineering problem. How do we work around it?
The alternative solution, of course, is that we could evolve bigger brains. But they’d have to be much bigger, and it takes a long time.
Q: What about the role of the web in this?
(…) Words are slippery, a touch is worth a 1,000 words any day.”
— Aleks Krotoski, Robin Dunbar: we can only ever have 150 friends at most…, Guardian, The Observer, 14 March 2010
How Many Friends Does One Person Need?
We are the product of our evolutionary history, and that history colors our experience of everyday life — from the number of friends we have to how religious we are. Renowned evolutionary anthropologist Professor Robin Dunbar visits the RSA to explain how the very distant past underpins all of our current behaviors, and how we can best utilize that knowledge.
— Robin Dunbar, British anthropologist and evolutionary psychologist and a specialist in primate behaviour. He is currently Professor of Evolutionary Anthropology and the Director of the Institute of Cognitive and Evolutionary Anthropology of the University of Oxford and the Co-director of the British Academy Centenary Research Project, Robin Dunbar: How Many Friends Does One Person Need?, FORA.tv, RSA, London Feb 2, 2010. (Illustration source: The magic number, RSA Journal)
☞ Robin I. M. Dunbar, Neocortex size as a constraint on group size in primates, Department of Anthropology, University College London
☞ B. Goncalves, N. Perra, A. Vespignani, Validation of Dunbar’s number in Twitter conversations, Cornell University, May 2011
☞ Is the Social Brain Theory Applicable to Human Individual Differences? Relationship between Sociability Personality Dimension and Brain Size (pdf)
“Our study intends to examine whether the social brain theory is applicable to human individual differences. According to the social brain theory primates have larger brains as it could be expected from their body sizes due to the adaptation to a more complex social life. Regarding humans there were few studies about the relationship between theory of mind and frontal and temporal brain lobes. We hypothesized that these brain lobes, as well as the whole cerebrum and neocortex are in connection with the Sociability personality dimension that is associated with individuals’ social lives. Our findings support this hypothesis as Sociability correlated positively with the examined brain structures if we control the effects of body size differences and age. These results suggest that the social brain theory can be extended to human interindividual differences and they have some implications to personality psychology too.”
☞ Internet users now have more and closer friends than those offline, Ars Technica, June 16, 2011
☞ Douglas Fox, The Physics of Intelligence, Scientific American, July 2011 (Note at Lapidarium)
☞ William Deresiewicz on the meaning of friendship in our time