It's called Dunbar's number: an influential and oft-repeated theory suggesting 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 into primate brain sizes and their social groups, has since become a ubiquitous part of the discourse on human social networks.
But just how legitimate is the science behind Dunbar's number anyway? According to a new analysis by researchers from Stockholm University in Sweden, Dunbar's famous figure doesn't add up.
"The theoretical foundation of Dunbar's number is shaky," says zoologist and cultural evolution researcher Patrik Lindenfors.
"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."
Dunbar's number was originally predicated on the idea that the volume of the neocortex in primate brains functions as a constraint on the size of the social groups they circulate amongst.
"It is suggested that the number of neocortical neurons limits the organism's information-processing capacity and that this then limits the number of relationships that an individual can monitor simultaneously," Dunbar explained in his foundational 1992 study.
"When a group's size exceeds this limit, it becomes unstable and begins to fragment. This then places an upper limit on the size of groups which any given species can maintain as cohesive social units through time."
Dunbar began extrapolating the theory to human networks in 1993, and in the decades since has authored and co-authored copious related research output examining the behavioral and cognitive mechanisms underpinning sociality in both humans and other primates.
But as to the original question of whether neocortex size serves as a valid constraint on group size beyond non-human primates, Lindenfors and his team aren't so sure.
While a number of studies have offered support for Dunbar's ideas, the new study debunks the claim that neocortex size in primates is equally pertinent to human socialization parameters.
"It is not possible to make an estimate for humans with any precision using available methods and data," says evolutionary biologist Andreas Wartel.
In their study, the researchers used modern statistical methods including Bayesian and generalized least-squares (GLS) analyses to take another look at the relationship between group size and brain/neocortex sizes in primate brains, with the advantage of updated datasets on primate brains.
The results suggested that stable human group sizes might ultimately be much smaller than 150 individuals – with one analysis suggesting up to 42 individuals could be the average limit, with another estimate ranging between a group of 70 to 107.
Ultimately, however, enormous amounts of imprecision in the statistics suggest that any method like this – trying to compute an average number of stable relationships for any human individual based off brain volume considerations – is unreliable at best.
"Specifying any one number is futile," the researchers write in their study. "A cognitive limit on human group size cannot be derived in this manner."
Despite the mainstream attention Dunbar's number enjoys, the researchers say the majority of primate social evolution research focuses on socio-ecological factors, including foraging and predation, infanticide, and sexual selection – not so much calculations dependent on brain or neocortex volume.
Further, the researchers argue that Dunbar's number ignores other significant differences in brain physiology between human and non-human primate brains – including that humans develop cultural mechanisms and social structures that can counter socially limiting cognitive factors that might otherwise apply to non-human primates.
"Ecological research on primate sociality, the uniqueness of human thinking, and empirical observations all indicate that there is no hard cognitive limit on human sociality," the team explains.
"It is our hope, though perhaps futile, that this study will put an end to the use of 'Dunbar's number' within science and in popular media."
The findings are reported in Biology Letters.