Here is a number you may have run into without knowing where it came from: 150. It gets quoted in management books, on team-size pages, in articles about why your group chat feels too big. It traces back to Robin Dunbar, a British anthropologist and evolutionary psychologist, who proposed in the 1990s that humans can hold roughly that many stable relationships at once. He got there by an odd route, extrapolating from the correlation between neocortex size and group size in non-human primates, then doing the sum for us.

The headline number is the least interesting part. What actually lands, for me, is what sits inside it.

Quick note before we go further: I’m not a psychologist or a neuroscientist, and this piece is reading and reflection, not advice. Dunbar’s number is a contested theory, not a settled law, and the wellbeing findings here come from observational research about groups of people, which is not the same as a rule about you. Treat the layers as a useful lens, not a diagnosis.

The number everyone quotes, and what it actually means

150 is not a roster of close friends. It’s the outer edge, the most people you can keep a genuine, two-way relationship with at all. Dunbar’s framing is that “the key to the 150 and the layers within is that they’re reciprocated.” Both people know each other. Both keep the thread alive. A follower count is not a friend, and 1,500 names you’d recognise is probably not 150 people who’d recognise you back.

He also stresses that 150 is an average with a lot of slack around it. As Dunbar himself puts it, the number is “always about 150, although there is some individual variation” — driven by personality, sex, and circumstance. It’s a soft ceiling, not a hard cap.

The layers inside 150 are where it gets personal

Inside that 150 sit nested rings, each roughly three times larger than the one inside it: about 5, then 15, then 50, then 150. The really good friends number about five, with wider rings of roughly 15, then 50, then 150 meaningful contacts. The five are the shoulders to cry on. The fifteen are the ones you’d confide in. The fifty are the good friends you’d happily have round.

The lopsided bit is how much of you goes to the centre. Dubar reportedly note that around 60% of your social attention, your time and your emotional capital, goes to the innermost fifteen. That inner core, Dunbar argues, makes “a big difference to your sense of wellbeing” and your engagement with life in general.

The rebuttal: is Dunbar’s number even real?

It would be tidy to stop there, but the brain part of the theory has taken real fire. In 2021, a team at Stockholm University re-ran the primate data using modern statistical methods and updated brain data, and concluded that a cognitive limit on human group sizes cannot be derived this way.  Co-author Andreas Wartel said that “it is not possible to make an estimate for humans with any precision using available methods and data.”

His colleague Patrik Lindenfors put it more directly: “the theoretical foundation of Dunbar’s number is shaky.” A third co-author, Johan Lind, suggested that “our brain can be trained in having more social contacts.” That last point is an interpretive analogy rather than a demonstrated result, so hold it loosely. The methodological objection stands, though: deriving a precise cap on friendship from neocortex size is harder to justify than the clean number 150 makes it sound.

What the framework is actually useful for

Here’s where I’ve landed. The exact figure could be 150 or far lower or a wide smear in between, and the brain-size derivation may not survive the statisticians. None of that touches the part that’s useful, which is the shape rather than the number. Concentric rings, with most of your warmth going to a small centre, and that centre mattering more for how you feel than anything in the outer layers.

You can’t see your fifty thinning to five in real time. It happens at the speed of people quietly drifting, missed coffees, the friend who moved and the call you kept meaning to make. The framework’s gift is that it names the inner ring as the thing worth protecting, the ring that does the emotional work. 

If any of this is sitting closer to home than it is interesting, talking to a qualified counsellor or therapist is worth far more than any number on a page.

The post In the 1990s, anthropologist Robin Dunbar argued there’s a natural ceiling on friendship set by the size of the human brain — around 150 people we can hold a real relationship with appeared first on Space Daily.