Dunbar’s Number

This is a photo of The Cave of Hands in Argentina, which is about 10,000 years old. The crowding of the hands is reflective of the way that humans for a long time have formed large communities. What is fascinating is that many anthropologists think they know just how large these average societies were.

And the answer is 150 people. In the 1990s, British anthropologist Robin Dunbar came up with this number as the maximum amount we Homo sapiens could (on average) form intimate relationships with — 150 is actually known as “Dunbar’s Number”.

There is a lot of evidence to support this idea, but the most significant is the ratio of brain size to body size among primates that Dunbar studied — the bigger the brain, the larger the community. And we modern humans have (despite lots of evidence that we frequently do stupid things) very big brains.

150 allows us to mix into a lot of different social groups, from immediate biological families to groups of same-sex peers to multi-generational folks — with each distinct group having different goals, different ways of communicating, and different things they hold in esteem.

Dunbar’s Number has been replicated in many different venues of modern society (the key idea is that it refers to close relationships and not just people you slightly know). It also makes sense in light of the fact that for most of our history, we travelled in smallish bands. As a person who studies the distant past, I like to keep in mind the very low levels of humans that populated the planet for much of our history. One estimate has the total human population at 4 million in 10,000 BCE, and only 5 million in 5,000 BCE. Urbanization radically changed things in the next 5,000 years to 100 million people living in 1BCE.