Dunbar's number
150 relationships · social brain hypothesis
The rough limit on how many people someone can maintain stable relationships with, usually put near 150. Robin Dunbar proposed it from a relationship between primate group size and brain size, and the layers inside it matter more than the headline figure.
In practice
Past about 150 people a company stops running on who knows whom and starts needing written process, whether or not anyone decided to introduce any.
The common mistake
Treating 150 as precise. The number is an estimate with wide error bars, and the useful content is that the limit exists and has layers, not that it sits at any particular value.
Robin Dunbar proposed the figure in the 1990s from a relationship between the size of the neocortex and typical group size across primates. Extrapolated to humans it gives roughly 150 — the number of people with whom somebody can maintain a stable relationship, meaning they know who each person is and how each relates to the others.
The number is contested and the error bars are wide. What is more robust, and more useful, is the layered structure inside it.
The layers
Roughly 5 people in the innermost circle — intimate relationships requiring frequent contact. About 15 close friends. Around 50 in a wider group. Then about 150 at the outer boundary of stable relationships, beyond which are people you recognize but do not track.
Each layer is roughly three times the one inside it, and each requires less contact to maintain. The constraint is time rather than affection: relationships decay without contact and there are only so many hours.
Where the limit bites in an organization
Around 150 people, informal coordination stops working. Below it, a company runs on everybody roughly knowing everybody. Above it, people no longer know who does what, and process has to replace memory. Firms that grow through this point without introducing structure get the symptoms — duplicated work, decisions nobody can trace, new people who cannot find anything — and usually diagnose them as culture problems rather than arithmetic.
Your client list has the same ceiling. A genuine relationship with a client means knowing their business, which is a relationship in Dunbar's sense. Past some number they become accounts, handled by process, and that transition is a real change in what you are selling.
Teams have inner layers too. The 5 and 15 boundaries show up as the sizes at which a team stops needing meetings and starts needing them.
The use
It is a reason to expect a transition rather than a number to manage to. When coordination that used to be free starts failing, the population has usually crossed a layer, and the answer is to build the structure that replaces the informal knowledge rather than to ask people to communicate better.
Concept web
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What is Dunbar's number?
A rough limit, usually given as 150, on the number of people with whom someone can maintain stable relationships. Robin Dunbar derived it from a relationship between primate brain size and typical group size.
Is Dunbar's number accurate?
It is an estimate with wide error bars and is contested. The more robust part is the layered structure inside it — roughly 5, 15, 50 and 150 — and the fact that a limit exists at all.
What does Dunbar's number mean for a growing company?
That informal coordination stops working somewhere around 150 people, because nobody can track who does what any more. Written process has to replace memory, and firms that grow past the point without it get symptoms they usually misdiagnose as culture.