The Discovery
In the 1990s, British anthropologist Robin Dunbar noticed something strange. Across every primate species, there was a tight correlation between the size of the neocortex (the "thinking" part of the brain) and the size of the social group the species maintained.
Chimpanzees, with their large neocortices, live in groups of about 50. Gorillas cluster around 30. Baboons, about 40. The relationship was almost perfectly linear.
Dunbar plotted the human neocortex on the same graph. The prediction: 148. Rounded up, 150.
The maximum number of stable social relationships a human brain can maintain. Not approximately. Not culturally. Biologically.
The Evidence Is Everywhere
W.L. Gore & Associates — the company that makes Gore-Tex — discovered this independently. When any factory exceeded 150 employees, productivity dropped. Communication broke down. People stopped knowing each other. Their solution: never let a factory grow past 150. When it does, split it. They've done this for decades.
The Layers
Dunbar didn't just find a single number. He found a series of nested layers, each scaling by roughly a factor of three:
🧠 Your Social Brain — Map Your Layers
Use the sliders to estimate how many people are in each of your layers. Watch how they compare to Dunbar's predictions.
Your Social Brain vs Dunbar's Layers
Why Social Media Doesn't Count
The average Facebook user has 338 "friends." The average Twitter user follows hundreds. LinkedIn connections can number in the thousands. Does this mean we've broken Dunbar's number?
No. A 2016 study by Dunbar himself found that of those 338 Facebook friends, users could only identify about 150 by name and backstory — and only about 4 as people they'd turn to in a crisis. Social media doesn't expand your social brain. It creates an illusion of expanded social capacity while the actual cognitive limit stays fixed.
"The interesting thing about social media is that it allows you to keep in touch with people you would otherwise lose touch with. But it doesn't allow you to increase the number of people you can maintain a meaningful relationship with."— Robin Dunbar, 2016
Each relationship you maintain requires tracking: their name, face, personality, history with you, current status, mutual friends. Your neocortex has a fixed budget for this. When you add someone, you're implicitly dropping someone else.
The Organizational Implication
When a group exceeds 150, something breaks. People start forming cliques. Communication becomes formal. Trust decreases. Bureaucracy appears — not because someone chose it, but because the human brain literally cannot track that many relationships informally.
This is why every scaling startup hits a wall around 150 employees. It's why military units don't exceed company size for combat operations. It's why churches split. It's not a management failure — it's a neurological limit.
What This Means for You
You have roughly 150 relationship "slots." They are currently occupied. Every new meaningful connection displaces an old one — gradually, imperceptibly, but inevitably.
The question isn't whether Dunbar's number is real. The evidence is overwhelming. The question is: are the right people in your 150?