Dunbar’s Theory Limiting Humans with 150 Friends, Stable Relationships Questioned

Scientists are seriously questioning a ubiquitous, influential theory suggesting that an average person could only keep 150 stable social relationships with other people.

Called the Dunbar's Number, proposed by British anthropologist and evolutionary psychologist Robin Dunbar in the early 1990s, the theory had been culled from studies in primate brain sizes and social groups. It has since been a popular reference for social discourse.

Dunbar's Number Doesn't Add Up, Say Researchers

In the study, "Dunbar's Number Deconstructed," researchers from Stockholm University are asserting that the theoretical foundation of the Dunbar's Number is "shaky."

First of all, they say primate brains should not be compared with human brains, since both handle information quite differently, as primate social activity is influenced by other factors than the brain, such as the food they eat or the predators they need deal with.

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Dunbar's number was originally conceived from the idea that the neocortex volume in primate brains restricts their social group size. The number of neocortical neurons, Dunbar thought in his 1992 study, would limit the individual's information processing capacity, thus limiting the relationships one can track simultaneously.

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When the size of the social group exceeds that number, it becomes unstable and starts to fragment, Dunbar, suggested. This, he said, would place an upper limit on the size of groups people can handle socially.

He then applied the theory to human networks in 1993 and would engage in several research studies on behavioral and cognitive mechanisms underscoring social activity in both humans and primates.

Tackling whether the neocortex size could limit social group size among humans, zoologist and cultural evolution researcher Patrik Lindenfors and his team at Stockholm University were skeptical.

Though many previous studies supported Dunbar, this new paper disputed the suggestion that the neocortex size in primates is actually linked to social parameters for humans, Phys.Org reported.

Utilizing Advanced Statistical Methods

In the new study, researchers utilized advanced statistical methods, such as Bayesian and generalized least-squares (GLS) approaches to study the connection between social group size and brain/neocortex sizes in primate brains using updated datasets.

Findings showed that stable group sizes might be much smaller than the original 150 individuals, with one result suggesting an average of only 42 individuals would make the ideal group size, and another suggesting 70 to 107.

Such varying numbers show that computing the average number of stable human relationships an individual can handle based on brain volume is unreliable, researchers said. As such, specifying one number, in Dunbar's case, 150, is "futile." They said the cognitive limit in social group sizes for humans could not be obtained in this way.

Researchers state that a large share of the research on primate social evolution is concentrated on socio-ecological factors, such as predation or foraging, sexual selection or infanticide, not on computations based on brain or neocortex size, SciTech Daily reported.

Brain Psychology in Humans, Primates are Different

In addition, Dunbar's number ignores other important distinctions in brain psychology in human and primate brains. This includes how humans nurture cultural mechanisms and social structures that can contradict socially restricting cognitive factors that would apply to primates.

They said ecological findings on primate social activity, the distinctiveness of human thinking, and empirical observations show no cognitive limit on human sociality, the researchers added.