Dunbar's Number.
Why you can't have 100 real friends.

In the early 1990s, British anthropologist Robin Dunbar was studying primates when he noticed something striking: the size of a primate's neocortex — the part of the brain responsible for complex thought — correlated almost perfectly with the size of their social groups. Bigger brain, bigger group.

Out of curiosity, he plugged human brain size into the equation. The number that came out was 150. And then, somewhat to his surprise, that number started appearing everywhere.

Neolithic farming villages, hunter-gatherer tribes, Roman military units, modern army companies, Hutterite religious settlements — all tended to cap out around 150 before splitting. The number was eerily consistent across vastly different human contexts, spanning thousands of years.

What 150 actually means

Dunbar's number isn't really one number. It's a set of nested layers, each representing a different quality of relationship. Think of them as concentric circles, with the most intimate relationships at the center and looser connections on the outside.

Why the brain has this limit

Maintaining a real relationship is cognitively expensive. You need to track who someone is, what they care about, how they relate to others in your network, and where things stand between you. Dunbar calls this "social grooming" — the ongoing mental work of keeping relationships alive.

His research suggests this capacity is tied to the size of the neocortex. In social species, a larger neocortex allows larger, more complex social groups. Humans have unusually large neocortices — which is why we can maintain groups of 150 when chimpanzees max out at around 50.

What this means for social media

When researchers looked at actual social media usage, they found something consistent: despite having hundreds or thousands of "friends" or followers, the number of people any given user genuinely interacted with hovered around — 150. The platform expands your address book. It doesn't expand your brain.

Dunbar's research also found that friendships decay without active maintenance. If you stop contacting someone, they gradually drop down through the layers over months — from good friend to acquaintance, and eventually off the list entirely. Social media slows this decay somewhat, but doesn't stop it. At some point, an in-person connection is needed to truly sustain a relationship.

The real constraint is your innermost circle

The most striking finding isn't the 150 limit — it's the inner 5. Research shows that most people have only about 3 to 5 people in their genuine intimate circle at any given time. These are the relationships that require the most investment, provide the most support, and have the biggest effect on wellbeing.

This means that the question "how many friends do I have?" is probably the wrong one. The more important question is: who is in your 5? Are those slots being filled by people who actually show up? Are you investing enough in them to keep them there?

What to do with this information

Dunbar's number reframes the question of social life from quantity to allocation. You have a finite social budget — roughly 150 relationship "slots" with sharply diminishing returns as you move outward. The question is how you're spending that budget.

Most people's social energy leaks into the outer rings — acquaintances, social media connections, people from contexts that no longer exist — while the inner circles go underfed. The research suggests the reverse would serve wellbeing better: less breadth, more depth; fewer connections tended more carefully.