Dunbar's Number: What It Actually Says, and What the Research Says Back
·6 min read
Dunbar's number is the proposition that a person can maintain roughly 150 stable social relationships — relationships where you know who each person is and how they relate to everyone else. It comes from the British anthropologist Robin Dunbar, who in the 1990s observed a correlation across primate species between the size of the neocortex and the typical size of the social group, and extrapolated the human figure from that relationship.
The number has since escaped into management books, army-unit trivia and social-media commentary, usually stripped of the two things that make it interesting: the layers underneath it, and a serious 2021 challenge to whether the number exists at all.
The layers are the useful part
The headline figure gets quoted; the structure underneath rarely does. Dunbar's model describes concentric circles, each roughly three times the size of the one inside it:
| Layer | Size | Who is in it | Typical contact | |---|---|---|---| | Support clique | ~5 | The people you turn to in a crisis | Weekly or more | | Sympathy group | ~15 | Close friends whose loss would genuinely distress you | Monthly | | Close network | ~50 | People you would invite to a large party | A few times a year | | Active network | ~150 | People you have a real relationship with | At least once a year | | Acquaintances | ~500 | People you know but do not track | Occasional | | Recognisable faces | ~1,500 | People you could name if you saw them | Rare |
Dunbar has described the 150 layer with a memorable test: the people you would not feel awkward joining for a drink if you bumped into them in a bar.
Two observations do more practical work than the number itself. First, the layers have costs: keeping someone in an inner layer requires contact at a rate the outer layers do not. Second, the layers are roughly zero-sum. Someone moving into your inner five generally means someone else moving out. That is not a moral failure, and treating it as one is the source of a lot of unnecessary guilt.
The 2021 challenge: is 150 a real number?
In May 2021, Patrik Lindenfors, Andreas Wartel and Johan Lind of Stockholm University published "Dunbar's number" deconstructed in the Royal Society journal Biology Letters. They re-ran the underlying analysis with modern statistical methods and updated primate datasets.
Their results are worth stating precisely, because they are more damaging than "the number is a bit off":
- Bayesian methods produced an estimate for average human group size of 69–109.
- Generalised least-squares phylogenetic methods produced 16–42.
- The 95% confidence intervals were 4–520 and 2–336 respectively.
When your confidence interval spans from four to five hundred, you have not measured a cognitive limit. As the authors put it, specifying any one number is futile. Lindenfors also argued the theoretical foundation is shaky: other primates' brains do not process social information the way human brains do, primate group size is driven substantially by diet and predation rather than brain size, and human social networks vary enormously between individuals.
Dunbar has defended the work in response, and the debate is not settled. But anyone citing 150 as an established scientific constant is overstating what the evidence supports.
So is Dunbar's number useless?
No — but it is useful as a shape, not a limit.
Whatever the exact figures, three things hold up independently of the primate-brain argument and are supported by simple observation of how people actually live:
- Attention is finite and unevenly distributed. You cannot be close to everyone, and you already are not.
- Relationships decay without contact. This is well documented and does not depend on Dunbar's model at all. Friendships do not usually end; they lapse.
- The inner circles are small. Whether the support clique is five people or eight, it is not fifty, and it is worth knowing who is in it.
That is the honest version: a rough map of how human attention distributes, not a law of nature with a fixed constant.
What to do with it
The practical exercise takes twenty minutes and is more revealing than the theory.
Write down the people who would be in your inner five and your fifteen — not who should be, who actually is, judged by who you spoke to in the last month. Then compare that list to the one you would have written five years ago.
Most people find two things. Someone has drifted out of the inner layers without any decision being made, usually because of a job change or a move rather than anything that happened between you. And someone is in an inner layer out of obligation and history rather than because the relationship is currently good.
Neither is a crisis. But both are the kind of thing that is invisible until you write the list, which is the entire point of writing it. (It is also, for what it is worth, the reason tools like ours exist — not to add people, but to make the drift visible while it is still easy to reverse.)
Frequently asked questions
What is Dunbar's number? The proposal that humans can maintain about 150 stable relationships, based on Robin Dunbar's 1990s work correlating neocortex size with social group size in primates.
Who is Robin Dunbar? A British anthropologist and evolutionary psychologist, emeritus professor at the University of Oxford, best known for the social brain hypothesis and this number.
Has Dunbar's number been debunked? "Debunked" is too strong; "seriously disputed" is accurate. The 2021 Stockholm study found that re-analysing the data with modern methods yields wildly different estimates and confidence intervals so wide that no single number is defensible. Dunbar disagrees with that critique.
What are the Dunbar layers? Approximately 5, 15, 50, 150, 500 and 1,500 — each circle roughly three times the size of the one within it, and each requiring less frequent contact.
Does social media increase your Dunbar number? The evidence suggests not meaningfully. Social platforms make it far easier to know of many people, but the inner layers — which require real time and attention — appear stubbornly similar in size. Having 900 connections and five close friends is the normal case, not a contradiction.
Is 150 the right number for team size? It gets cited constantly in management writing, and the citation is doing more rhetorical work than evidential. Organisations do struggle with informal coordination as they grow, but attributing a specific headcount threshold to primate neocortex ratios is not something the underlying research can carry.
Sources: Lindenfors, Wartel & Lind (2021), "Dunbar's number" deconstructed, Biology Letters · Stockholm University summary via ScienceDaily · Robin Dunbar's own writing on the social brain hypothesis.