The dunbar number
How biology is linked to organisation and collaboration
In my daily life among my friends as well as in my professional life working with leadership. I often wonder why we tend to group and cluster into stable or semi-stable groups of specific sizes.
We as humans are not born with teeth or claws, we simply lack the formidable armaments or defences of many of the other animals in the world. With one of major exceptions being that we are able to form long-lasting relationships with many others in very dynamic and flexible ways. I often think that this ability, combined with our intelligence is part of the explanation to why we are able to dominate the planet.
To me it indeed seems that we are a massively-cooperative-species in every way. From teaching our kids our own learnings, to the industrial division of labour that relies on larger and more diverse networks of relationships as our main strength.
It has long been a known fact among primatologists that non-human primates live in “grooming cliques”. These are closely-knit social groups of varying sizes where grooming is how the members socialise and stay close to each others. To the primatologists it has also been clear that the number of members in the grooming cliques are not randomised. And that primate species with bigger brains tend to have larger social groups.
The British professor in anthropology Robin Dunbar figured that the same principle should apply to all primates (including humans). And published in 1992 an article called “Neocortex size as a constraint on group size in primates” in the Journal of Human evolution. This article contained a model called “the social brain hypothesis” that predicted social factors about primates based on the size of their neocortex.
“The Tamarin monkey has a neocortex/brain size ratio of about 2.3 and an average social group of size of about 5 members. On the other hand, a Macaque monkey has a neocortex/brain size ratio of around 3.8 but a very large average group size of about 40 members.”
Dunbar then used data for 38 different primates to predict a human “mean group size” of 148 (casually rounded to 150) and an “intimate circle size” of 12-15.
He then famously said about the number 150 as the average social group size for humans:
“Putting it another way, it’s the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar.”
Dunbar also used this model to predict both how smaller and larger groups would be organised for humans, and the numbers he found were:
- The “Core group” = up to 5 people (family).
- The “Close Group” = 12-15 people (close kinship group).
- The “Acquaintance Group” = 50 people (band of related close groups).
- The “Personal Social Group” = 150 people (bands of common lineage — what Dunbar believed to be the biggest group of people one person can have close relationships with).
- The “Clan” = 450-500 people (cohesive sub tribal unit).
- The “Tribal Group” = 1500 — 2000 people (tribe).
These numbers have historically been used for many forms of organisation. With the number 150 being the typical size of a small human village through the ages, and with the Roman army arranging troops into 9 man “Contuberniums” as well as the Greek organising their army into 8-16 man “stichos”.
In modern times these numbers show up again, with 150 people being the number where many startups feel that the company becomes more rigid and loses the initial spirit. And with Percy Barnevik of ABB who restructured then then 200,000 person company into 5000 units of 40 people. Another example from today is Richard Branson of Virgin who thinks 60 people is the right size for a team to stay flexible, while still having a broad enough resource base to be able to work independently.
In my daily life I have for many years worked with musicians & bands in the recording studio as well as with designers, artists and programmers in software & video-game development.
And a common theme among the “high-performing” groups are always that they are small. The best bands I have worked with have consisted of as few as 5 people (the Dunbar “core” group) rather than working as individuals or being a larger group of people. And the most daring and efficient software & game-development teams I have worked with have been organised in 10-person cross-disciplinary “scrum-teams” (the Dunbar “close” group), rather than being organised in massive “departments”.
I would argue that any sort of organisation of human effort in groups should reflect the Dunbar numbers in some form, or at least consider them. They are still relevant today, even with modern technology enabling us to create organisations, groups and tribes over large distances.
So, if you work with leadership and organisation, look at the smallest groups of people in your organisation:
- Do they have common goals?
- Do they have a mandate to act?
- Do they have all the resources they need to achieve their goals?
If the smallest groups in your organisation have all these things, and they also are small enough to match the smaller Dunbar numbers (5 or less or 12 or less). My experience is that you will see that the groups your organisation are made of will be able to reach their goals fast and efficiently.
“Talent wins games, but teamwork and intelligence wins championships.” —Michael Jordan
