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Understanding Dunbar's Number (The 150-Person Theory)

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  1. Definition

    A theory that there is a cognitive limit to the number of social relationships a person can maintain with stable trust, and that this upper limit is roughly 150 people.

  2. Origin: Primate Brain Size Research

    British anthropologist Robin Dunbar derived this number in the 1990s by analyzing the correlation between neocortex (the brain's outer layer) size across various primate species and the size of the social groups those species form. He then applied this correlation to humans, using the size of the human neocortex to estimate a figure of roughly 150.

  3. Why It's Back in the Spotlight in the Social Media Era

    With social media now allowing people to be connected to hundreds or even thousands of "friends" or "followers," Dunbar's Number β€” the idea that there is a real limit to how many relationships can be meaningfully maintained β€” has drawn fresh attention as a framework for explaining the limits of online relationships.

  4. Academic Pushback and Criticism

    In 2021, researchers at Stockholm University in Sweden re-examined Dunbar's original analysis using a larger dataset and more advanced statistical methods, concluding that the confidence interval around the estimate was so wide that there wasn't enough basis to pin the cognitive limit at exactly 150 β€” statistically, the true number could be anywhere from 100 to 250.

  5. Dunbar's Own Rebuttal

    In response to this criticism, Robin Dunbar has argued that the re-examination study had flaws in its statistical methodology, and that three decades of observational data across many cultures have consistently supported a figure around 150. This means Dunbar's Number remains an actively debated hypothesis in academia today.

  6. Practical Use and Cautions

    Some companies and organizations reference this figure when designing department sizes, but it is best understood not as a precisely validated fixed value, but as a rough indicator showing that there is a realistic limit to how many relationships a person can maintain.

What Is Dunbar's Number?

Dunbar's Number is a theory holding that a person can only maintain around 150 stable, trust-based social relationships, based on cognitive limits. It is frequently cited today as a concept for reflecting on what "genuinely meaningful relationships" actually mean, in an age where social media lets people connect with hundreds or thousands of others.

Another Effect of Group Size on the Individual

While Dunbar's Number addresses the limit on how many relationships a person can maintain, there's also a phenomenon where an individual's effort actually decreases as group size grows. See our guide to the Ringelmann effect (social loafing) to learn more.

Frequently Asked Questions

Is 150 an exact, precise figure?

No. When researchers at Stockholm University in Sweden re-examined the underlying analysis in 2021, they found the confidence interval was very wide, concluding there wasn't enough basis to treat 150 as a precise cognitive limit. However, Dunbar himself disputes the statistical methods used in that re-examination, so the exact number remains debated.

If I have a lot of social media followers, does that disprove the theory?

Not necessarily. Dunbar's Number isn't simply about how many people you know β€” it refers to the number of deep, trust-based relationships you actively and continuously communicate with. Even someone with many followers often has a far smaller number of people they actually interact with closely.