Dunbar's Data Suggests Our Big Brains Grew to Handle Social Life
Extending the primate pattern to people points to about 150 meaningful relationships.
Half as Interesting · Why You Can Only Have 150 Friends (According to Science)Primates, the animal family that includes monkeys, apes, and people, have unusually large brains for their body size. Researchers have proposed that diet and digestion helped drive that growth, since finding and processing food takes energy and planning. A second proposal puts the pressure elsewhere.
It is called the Machiavellian intelligence hypothesis, Machiavellian here meaning concerned with scheming and social strategy. The suggestion is that individuals that were better at navigating group life left more descendants, so social skill favored larger brains over time.
Robin Dunbar, an evolutionary psychologist, looked for support in anatomy in 1992. Across different primate species, he compared the size of the neocortex, the outer region tied to complex thought, with total brain size, and set that ratio beside the typical size of each species' social group.
He found a correlation: species with a larger neocortex ratio tended to belong to larger typical groups. Dunbar read that pattern as support for the social idea, with more neural capacity going hand in hand with larger groups.
A correlation of that kind does not prove that social demands caused brains to grow. His comparison measured brain ratios against group size, and it did not test the food explanations, so it leaves those ideas open rather than ranking one demand above another.
Extending the primate pattern to the human brain gave an expected average group of 148, which Dunbar rounded to 150. He described that as the circle of people a person could greet without embarrassment during a chance meeting, such as a late night airport stopover.
For Dunbar, the number gave the social idea something concrete to check against human life. It remains a hypothesis, but one tied to measured brains and counted groups, that our large brains grew in part to handle life with one another.
Questions people ask
What does the neocortex do?
The neocortex is the folded outer layer of the mammalian brain. It supports perception, memory, planning, and, in humans, language and reasoning. Dunbar used its size relative to the whole brain as a rough index of capacity for complex information, including social information.
What counts as one of the 150 people?
He described it as the people you would feel comfortable approaching and greeting if you ran into them unexpectedly while traveling. The video gives the example of a 3 a.m. airport stopover. It is meant to capture meaningful contacts, not every face you recognize.
Do scientists still take the 150 figure seriously?
Later researchers have debated the exact number and whether online friendship, migration, and cultural differences stretch or shrink the circle. Some studies find smaller active networks and larger acquaintance layers. The broader claim that primate brain structure tracks social complexity has received continued attention even as the round number is questioned.