It’s called the Dunbar number: an influential and often repeated theory that suggests that the average person can only maintain about 150 stable social relationships with other people.
Proposed by British anthropologist and evolutionary psychologist Robin Dunbar in the early 1990s, Dunbar’s number, extrapolated from research on the brain size of primates and their social groups, has since become a ubiquitous part of human social media discourse.
But how legitimate is the science behind Dunbar’s number? According to a new analysis by researchers from Stockholm University in Sweden, the famous figure of Dunbar is not added.
“The theoretical basis of Dunbar̵
“The brains of other primates do not handle information exactly like the human brain, and the sociality of primates is mainly explained by factors other than the brain, such as what they eat and who their predators are.”
Dunbar’s number was originally based on the idea that the volume of the neocortex in primate brains acted as a constraint on the size of the social groups in which they circulated.
“The number of neocortical neurons is thought to limit the body’s ability to process information, and this then limits the number of connections a person can observe at a time,” Dunbar explained in his 1992 main study.
“When the size of the group exceeds this limit, it becomes unstable and begins to fragment. This then sets an upper limit on the size of the groups that each species can maintain as cohesive social units over time.”
Dunbar began extrapolating theory to human networks in 1993, and in the decades since has authored and co-authored numerous related studies examining the behavioral and cognitive mechanisms that underlie sociality in both humans and other primates.
But as to the initial question of whether the size of the neocortex serves as a valid constraint on the size of the group outside non-human primates, Lindenfors and his team are less sure.
While a number of studies offer support for Dunbar’s ideas, the new study reveals that the size of the neocortex in primates is equally important for the parameters of human socialization.
“It is not possible to estimate humans using available methods and data,” said evolutionary biologist Andreas Vartel.
In their study, the researchers used modern statistical methods, including Bayesian and generalized least squares analyzes (GLS), to take a new look at the relationship between group size and brain / neocortex size in primate brains, with the advantage of updated primate brain datasets.
The results suggest that the stable size of the human group may eventually be much smaller than 150 individuals – with one analysis suggesting that up to 42 individuals may be the average limit, and another estimate ranging between a group of 70 to 107 .
Ultimately, however, the vast amounts of inaccuracy in statistics show that any method like this – trying to calculate an average number of stable relationships for each human individual based on brain volume considerations – is unreliable at best.
“Specifying any number is useless,” the researchers wrote in their study. “In this way, no cognitive limit can be drawn on the size of the human group.”
Despite the focus on Dunbar’s numbers, researchers say most research on the social evolution of primates focuses on socio-environmental factors, including nutrition and predation, infanticide and sexual selection – not so much calculations dependent of brain volume or neocortex.
Researchers also claim that Dunbar’s number ignores other significant differences in brain physiology between the brains of human and nonhuman primates – including that humans develop cultural mechanisms and social structures that can counteract socially limiting cognitive factors that might otherwise to apply to non-human primates.
“Environmental research on primate sociality, the uniqueness of human thinking, and empirical observations show that there is no hard cognitive limit to human sociality,” the team explains.
“We hope, though perhaps in vain, that this study will put an end to the use of the Dunbar number in science and the popular media.”
The findings were reported in Letters on biology.