For most of human history, our forebears were content to live in compact, largely self-sufficient communities. Living and working beside strangers, as is now common in cities and towns across the planet, would have been unheard of; traditionally, everyone would have known their neighbors and the role they played in their tight-knit society.
And, according to a theory proposed by Robin Dunbar, an anthropologist and evolutionary psychologist at the University of Oxford, in 1993, there's a 150-person limit to the number of individuals with whom we can maintain meaningful social relationships, known as Dunbar's number.
But does Dunbar's theory hold up? Are humans limited to 150 friends? Decades since he first published his claim, Dunbar still sticks by his number, and other research has supported it. "There has been no change in the number of relationships," Dunbar told Live Science in an email.
However, some studies and experts aren't quite as confident.
"There is lots of variation in the size of people's social networks," Samuel Roberts, a professor of psychology at Liverpool John Moores University in the U.K., told Live Science in an email. "As with any theory, there is critique of Dunbar's number."
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Sarah Johns, a reader in evolutionary anthropology at the University of Kent in the U.K., believes that, while Dunbar's number is broadly accurate, it comes with several caveats.
"I wouldn't say it is absolutely 150 in every circumstance," Johns told Live Science in an email. "'Close and meaningful' is likely to result in a much smaller number, but this can also depend on individual factors, such as extraversion and introversion. However, Dunbar's number is supported quite broadly," Johns added.
Why is it 150?
"The constraint is largely cognitive," Dunbar said, adding that there's a link between social group size and the size of the neocortex — the part of the brain involved in high-level functions, such as sensory perception, emotion and language, which are associated with social behavior — in primates.
Johns agreed that the number of people we are able to form relationships with is largely a result of how our species was compelled to operate long ago. "It seems to be what humans can cognitively deal with," Johns said. "Beyond this number  you need more social rules and regulations to maintain relationships. Humans have to balance doing our own thing to survive and reproduce, but also to know what others are up to, who might help us out, and who might share food with us. 150 is the predicted number of people we can consistently track and have up-to-date information about."
According to Roberts, there is also something else to consider when it comes to our ability to maintain relationships beyond our cognitive limits: the time and effort required to do so. "If someone told you that they had 50 really close friends, you probably wouldn't believe them," Roberts said, "because there is some intuitive sense that maintaining these close friends requires a degree of effort in communicating and meeting up that is limited by time."
However, there are other theories on the "friend cap" that do not align with Dunbar's, with some suggesting that the number is far higher. A paper published in 1978 by anthropologists H. Russell Bernard and Peter Killworth concluded that the number was likely closer to 290, while a 2001 article comparing two different methods — known as the "scale-up method" and the "summation method" — settled on 291.
Additionally, a 2021 study published in the journal Biology Letters raised questions about the accuracy of Dunbar's number. According to the study, some empirical studies have found support for this number, while others have reported other group sizes. "It is not possible to make an estimate with any precision using available methods and data," Andreas Wartel, a researcher at the Centre for Cultural Evolution at Stockholm University and co-author of the study, said in a statement.
Dunbar is keen, however, to stress that 150 is the average number of — rather than a strict limit on — relationships an individual can maintain. Additionally, he suggests these 150 relationships are not equal in significance or degree of intimacy.
"It is important to appreciate that the 150 is simply one of a fractal series of layers in our friendship circles," he said.
According to Dunbar, these layers are defined by the "emotional intensity of the relationship" and also the time we tend to invest in each person. What's more, according to Johns, a "meaningful" relationship isn't even necessarily one that's built on love or even fondness.
"We might not even truly like all 150 individuals, but maintain the relationship as it helps us in some way, or because we need to continue to interact with them (they live next door or are our boss)," Johns noted. "It's not a limit that has a value judgment attached to it. It's just a feature of human evolution."
The population boom
Modern humans (Homo sapiens) have been around for at least 300,000 years, and for most of that time, there were no large towns and cities. Çatalhöyük, the ruins of which are in modern-day Turkey, is widely considered one of the first cities, with experts suggesting it was built around 9,000 years ago. According to historian Ian Morris, who wrote extensively about global population growth in his book "Why the West Rules — for Now" (Farrar, Straus and Giroux, 2010), Çatalhöyük was the largest settlement on the planet for almost 2,000 years, with a population that rarely exceeded 3,000.
Since then, the global population has expanded rapidly and, more recently, created huge, dense urban centers. In 1 B.C., there was only one city in the world with a population exceeding 1 million: Rome. In contrast, by 2030, our planet will have an estimated 662 cities with over 1 million inhabitants, according to the United Nations. And, on top of that, our modern ability to communicate with more people than ever regardless of their location means that, theoretically, almost everyone in the world has the potential to be a friend.
So, has modern society and technology changed the number of friendships we can maintain? Not according to Dunbar. "It hasn't changed anything," he said. "The limit is imposed not simply by one's ability to remember who is who, but the ability to understand the quality of the relationship, and work with that information when interacting with them."
However, Roberts is slightly less convinced regarding the internet's impact.
"One key unresolved theoretical issue is whether social media and messaging services fundamentally change these constraints by making it easier to stay in touch with people," he said.
Johns has a similar perspective. "Obviously, novel technology might open up the number of people we interact with daily," she said.
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Joe Phelan is a journalist based in London. His work has appeared in VICE, National Geographic, World Soccer and The Blizzard, and has been a guest on Times Radio. He is drawn to the weird, wonderful and under examined, as well as anything related to life in the Arctic Circle. He holds a bachelor's degree in journalism from the University of Chester.