What is Dunbar's number?

It’s the theory, from anthropologist Robin Dunbar, that humans can sustain only about 150 stable, meaningful relationships — a cognitive limit on the size of our genuine social network.

Is Dunbar's number exactly 150?

No — it’s an approximate, contested figure that varies by person and context, with tighter inner layers (around 15 close friends, 5 intimates). Treat it as an order-of-magnitude guide, not a precise cap.

Why does Dunbar's number matter for organisations?

Because once a group grows past roughly 150, informal trust and coordination break down — people no longer all know each other — so the group needs formal structure, rules, and hierarchy to keep functioning.

Dunbar's number

Anthropology & social systems

Dunbar's number is the theory that humans can maintain only about 150 stable social relationships — the cognitive limit on the number of people with whom you can sustain meaningful, reciprocal connections. Beyond it, groups need rules and hierarchy to hold together.

How it works

Treat ~150 as a rough ceiling on genuine relationships, with tighter inner layers (close friends ~15, intimate ~5). When a community, team, or company grows past it, expect informal trust to break down and formal structure to become necessary.

How to use it

Understanding why companies and communities need formal process once they pass ~150 people.
Recognising the limits of your own real social network versus your follower count.
Designing groups and teams around the natural layers of human connection.

Worked example

A startup runs smoothly on trust and informal coordination at 50 people. Past ~150 it descends into confusion — people no longer know each other — and must add management layers, process, and explicit roles to function. The number marks the phase change.

Where it fails

The figure is approximate and contested — it varies by person and context, and the precise "150" shouldn’t be taken literally. Social media also complicates "relationship." Use it as an order-of-magnitude guide, not a hard cap.

The deeper point

It reframes scale as a cognitive limit, not just a logistical one: an organisation past ~150 isn’t merely bigger, it’s a different kind of thing, where trust must be replaced by structure. The number marks where "we all know each other" quietly stops being true.

Frequently asked

What is Dunbar's number?: It’s the theory, from anthropologist Robin Dunbar, that humans can sustain only about 150 stable, meaningful relationships — a cognitive limit on the size of our genuine social network.
Is Dunbar's number exactly 150?: No — it’s an approximate, contested figure that varies by person and context, with tighter inner layers (around 15 close friends, 5 intimates). Treat it as an order-of-magnitude guide, not a precise cap.
Why does Dunbar's number matter for organisations?: Because once a group grows past roughly 150, informal trust and coordination break down — people no longer all know each other — so the group needs formal structure, rules, and hierarchy to keep functioning.

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Editorial synthesis © ReadGlobe 2026, drawing on the mental-models tradition (Charlie Munger, Farnam Street) and the primary sources for each model. · Last reviewed 2026-05-29.