After five years of teaching Earth science, I meet hundreds of students every year. Yet strangely, the ones I truly remember seem to cap out around 100 to 150. I used to think it was just my memory. It wasn’t.
What Is the Dunbar Number?
In a 1993 paper, British anthropologist Robin Dunbar made a striking claim: the upper limit of stable social relationships a human can maintain is approximately 150 people. This is known as “Dunbar’s Number.”[1]
Related: mental models guide
Dunbar analyzed the correlation between neocortex size and group size across primates. The larger the brain, the more complex a social group it can sustain. Plugging in the human neocortex size yields approximately 148 — rounded up to 150.[1]
The Social Brain Hypothesis
Dunbar’s research led to the “Social Brain Hypothesis.” The argument is that the human brain’s large size did not evolve for making complex tools or processing language, but rather for tracking complex social relationships.[1]
Tracking relationships isn’t simply memorizing names. It’s the ability to mentally simulate complex three-way dynamics — “A likes B, B is in conflict with C, and C is my relative.” This kind of social computation demands enormous cognitive resources.
Ever noticed this pattern in your own life?
I believe this deserves more attention than it gets.
The Structure of 150: Layers of Intimacy
Dunbar’s Number isn’t simply “you can have up to 150 friends.” There are distinct layers:
Last updated: 2026-04-08
Your Next Steps
- Today: Pick one idea from this article and try it before bed tonight.
- This week: Track your results for 5 days — even a simple notes app works.
- Next 30 days: Review what worked, drop what didn’t, and build your personal system.
About the Author
Written by the Rational Growth editorial team. Our health and psychology content is informed by peer-reviewed research, clinical guidelines, and real-world experience. We follow strict editorial standards and cite primary sources throughout.
References
Sources cited inline throughout this article.
