A “Dunbar” refers to Dunbar’s number, the idea that humans can maintain only about 150 meaningful social relationships at any given time. The concept comes from British anthropologist Robin Dunbar, who in the 1990s noticed a pattern across primate species: the bigger the brain, the larger the social group. When he extrapolated that pattern to humans, the math landed at roughly 150.
Where the Number Comes From
Dunbar’s insight started with a simple observation: primates have unusually large brains relative to their body size compared to other animals. His explanation, known as the social brain hypothesis, is that primates evolved bigger brains specifically to handle the demands of complex social lives. Tracking who’s friends with whom, remembering past favors, detecting deception, navigating alliances: all of this takes serious cognitive horsepower.
The key structure is the neocortex, the outer layer of the brain responsible for higher-order thinking. Across primate species, the size of the neocortex relative to the rest of the brain predicts how large a group a species can hold together. Chimpanzees, with their relatively large neocortices, live in groups of around 50. Humans, with the largest neocortex ratio of any primate, land at about 150. That’s not a hard ceiling. It’s a statistical average, a rough zone where our brains can keep track of who everyone is, how they relate to each other, and what our own relationship is with each of them.
The Layers Inside the 150
Not all 150 relationships are equal. Dunbar found that people organize their social worlds into concentric circles, each one larger but emotionally thinner than the last.
- The inner 5: Your closest relationships. These are the people you talk to weekly and turn to in a crisis. Best friends, a partner, a parent, a sibling. This is your “support clique.”
- The next 10 (bringing the total to 15): Good friends you talk to roughly monthly and would genuinely miss if they disappeared from your life. Together with the inner 5, these 15 people form your core network.
- The outer 135: Acquaintances and weaker ties. You don’t talk to them regularly, but if you bumped into one at the grocery store, you’d stop and chat. You know their name, their face, something about their life.
Each layer demands a different level of time and emotional energy to maintain. Close friendships require frequent contact, sometimes daily phone calls or weekly meetups. Weak ties need far less investment; a monthly group gathering or occasional interaction keeps them alive. This is why the layers exist in the first place. You simply don’t have enough hours in the day to maintain 150 close friendships, so your brain triages, concentrating effort on the inner circles while letting the outer ones coast on lighter contact.
One interesting wrinkle: family relationships are cheaper to maintain than friendships. Research shows that kin ties stay at a given level of emotional closeness with less time investment than non-family bonds require. A cousin you haven’t called in six months still feels like family. A friend you’ve neglected for six months may not.
Where 150 Shows Up in the Real World
The number keeps surfacing in human organizations across history and cultures. The smallest military unit that can operate independently, the company, ranges from about 120 to 180 soldiers worldwide. That’s not a coincidence, according to Dunbar. It’s roughly the largest group where everyone can know everyone else well enough to coordinate without rigid bureaucracy.
Businesses have picked up on this too. In units smaller than 150 people, more friendships form, communication flows more naturally, and people share a stronger sense of common purpose. Above 150, that cohesion starts to weaken. Organizations need more rules, more hierarchy, more formal processes to compensate for the fact that people can no longer rely on personal relationships to get things done. The practical takeaway for companies is straightforward: semi-independent units of 150 or fewer tend to function more efficiently than sprawling departments of 300 or 500.
The Criticism: Is 150 Too Precise?
The number has become so well-known that it’s easy to treat it as settled science. It isn’t. A 2021 study from Stockholm University went back to the original method of predicting group size from brain anatomy and found that the approach produces wildly inconsistent results depending on which statistical method and variables you choose. Using Bayesian analysis on neocortex volume, the researchers estimated an average human group size of about 69, not 150, with a 95% confidence interval stretching from roughly 4 to 292. That range is so enormous that the researchers concluded pinpointing any single number is essentially meaningless.
This doesn’t mean the concept is wrong. Most researchers agree that humans have some cognitive limit on social relationships, and that it varies from person to person. The critique is more about precision: calling it “Dunbar’s number” implies a fixed threshold, when in reality it’s a fuzzy range influenced by individual differences in personality, social motivation, and how much time someone devotes to maintaining relationships. Some people naturally maintain larger networks. Others are perfectly content with 30 meaningful ties.
Why It Matters for Everyday Life
The practical value of Dunbar’s number isn’t in the exact figure. It’s in understanding that your social capacity is finite, and that maintaining relationships costs real time and energy. When you add someone new to your inner circle, someone else often drifts outward. When you move to a new city or start a new job, you’re not just building new relationships. You’re reallocating a limited budget of social attention.
Research on college freshmen shows this clearly: new friendships form through sheer frequency of contact. In the early stages, the single best predictor of how close a friendship becomes is simply how much time you spend together. That’s encouraging if you’re trying to build connections, but it also explains why old friendships fade when life pulls people in different directions. The bonds don’t break because of conflict. They decay because the time investment stops.
Thinking of your social world as a series of concentric circles, each with its own maintenance cost, can help you be more intentional about where you put your energy. The five people in your innermost circle absorb a disproportionate share of your social time. That’s by design. Those are the relationships that sustain you through difficult periods, and they only stay strong with consistent, frequent contact.