Cobra Effect · People and organisations
Dunbar’s number
Why groups stop working the old way once they pass about 150.
6 cards, read aloud in 2:23, with a test and sources.
How many friends can one person actually have?
Not followers. Not names you would recognise. People you would not feel awkward joining uninvited at a bar. In the early 1990s an anthropologist went looking for the number, and he started with monkeys.
Robin Dunbar noticed that primates with bigger brains live in bigger groups.
More precisely, the bigger the outer layer of the brain relative to the rest, the bigger the troop. He drew the line through the monkeys and apes. Then he extended it to the human brain and read off the answer. About 150.
That is Dunbar’s number.
The rough size of group a human can hold together by knowing everyone in it. Dunbar found it in odd places. Villages in the Domesday Book. Hutterite farming communities that split when they pass 150. A famous manufacturing firm that is said to cap its factories at about that size.
It comes in layers.
Dunbar’s later work describes rings. About 5 people you would turn to in a crisis. About 15 close friends. About 50 good friends. Then 150, the people you know and who know you. Each ring is roughly three times the size of the last, and gets a fraction of the attention.
Hold it loosely. It is an estimate, and a wide one.
Dunbar’s own sums gave a range from about 100 to about 230. In 2021 a Swedish team reran the analysis with other methods and got anything from a handful to more than 500. They said the number cannot be pinned down at all. Dunbar disagrees. The argument is still going.
What to do with a number that might be wrong.
Use the shape, not the figure. Somewhere past a hundred people, a group stops running on who knows whom and starts needing rules, roles and a chart. Watch for that moment. When you stop knowing the name of the person you pass in the corridor, the tribe has become an institution.
Sources
- Dunbar’s number, Wikipedia. The 1992 paper, the layers, the villages and factories, and the 2021 challenge, with links to the original papers on both sides.
- How Many Friends Does One Person Need?, Robin Dunbar, 2010. Dunbar’s own popular account, in short essays. The number, the layers, gossip, grooming and why we laugh.
- Dunbar’s number deconstructed, Lindenfors, Wartel and Lind, 2021. The Swedish challenge, in Biology Letters. Same data, different statistics, and an estimate too wide to be useful. Short and open access.
Nearby ideas
- Brooks’s law. Adding people to a late project makes it later.
- Parkinson’s law. Work expands to fill the time you give it.
- The bystander effect. The more people who see a problem, the less each one acts.
- The fundamental attribution error. We blame people’s character and overlook their situation.
- The Abilene paradox. How a group can agree to something that nobody wants.
- The Pygmalion effect. What we expect of people quietly shapes what they become.
- Groupthink. How close, loyal teams talk themselves into bad decisions.
- Asch’s conformity experiments. Why people go along with a group they can see is wrong.