You may have heard that the human brain can manage social relationships with about 150 people. Or perhaps you heard 200, or 50, or 104. Dunbar’s number, proposed by anthropologist and evolutionary psychologist Robin Dunbar in the 1990s, was derived by comparing the size of primate social groups to the size and structures of their brains. While the actual number has varied a bit, the concept has since become very influential.
A paper just published in the journal Biology Letters disputes this number, with updated analyses showing that human group sizes can’t be determined by the shape of our brains.
“The theoretical foundation of Dunbar’s number is shaky,” says Patrik Lindenfors, author on the study and associate professor of zoological ecology at Stockholm University, Sweden. “Other primates’ brains do not handle information exactly as human brains do, and primate sociality is primarily explained by other factors than the brain, such as what they eat and who their predators are.
“Furthermore, humans have a large variation in the size of their social networks.”
This study repeated Dunbar’s original analyses using newer data and statistical methods. The team found that while the average number of social interactions a human could manage was a little under 150, the error bars on that number were huge. They could only say that, with 95% confidence, the new Dunbar’s number falls somewhere between 2 and 520 – not a particularly useful estimate.
“It is not possible to make an estimate for humans with any precision using available methods and data,” says Andreas Wartel, another author on the study also from Stockholm University.
The number has informed policy and business around the world. In 2007, the Swedish Tax authority restructured its offices to remain within the 150-person limit. “This reorganisation would then be based on the implicit but hopefully unintended assumption that their employees have neither family nor friends outside of work,” says Lindenfors.
“I think Dunbar’s number is widely spread, also among researchers, since it’s so easy to understand. Our claim that it is not possible to calculate a number is not quite as entertaining.”
Ellen Phiddian is a science journalist at The Royal Institution of Australia.
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