How many friends can you really have?

You may have 11 million followers on social media and an extended network of colleagues and acquaintances. But a British evolutionary psychologist and expert on primate behavior became famous for deciding the upper limit of how many friends the human primate brain can handle.

It’s called the “Robin Dunbar Theory” or “Dunbar’s Number” or even the “social brain hypothesis,” first proposed decades ago, and it’s still being wildly debated.

Dunbar, now in his late 70s, told The Wall Street Journal this week that when you get beyond 150 friends, they become “one-way” relationships. “The key to the 150 and the layers within is that they’re reciprocated,” he said.

Others disagree, including a group of researchers from Stockholm University who said in a news release in 2021 that “a cognitive limit on human group sizes cannot be derived in this manner.” They didn’t find an upper limit based on brain size and said it couldn’t be precisely calculated.

Dunbar extrapolated the limit by considering the relative size of the neocortex and groups of nonhuman primates, specifically monkeys and apes. As BBC explained it a few years ago, “Dunbar concluded that the size, relative to the body of the neocortex — the part of the brain associated with cognition and language — is linked to the size of a cohesive social group. This ratio limits how much complexity a social system can handle."

But there are friends and then there are really good friends. And for that, you only have about five people. Expand the circle a bit and you might, if you’re blessed, also have 10 others you’re close to, giving you 15 good friends, 50 friends, and 150 meaningful contacts. OK, so that last group is friendly, but maybe not actually friends.

Beyond that, you can have 500 acquaintances and about 1,500 people you recognize, although they might not recognize you. That could include newsmakers like Robert F. Kennedy Jr. or Taylor Swift.

And folks shift where they fit into your mental social circle, as well. An acquaintance may become a real friend, while a different real friend could become someone you once knew.

So the inner circle includes the five friends or family members you confide in and talk to regularly. You see the good friends at least once a month and “about 60% of your social attention goes to these 15,” the Journal said Dunbar noted. The 50 include your 15 buddies and “you weekend backyard barbecue party group.”

He told the Journal that the 150 are those 50 and the 100 you’d invite to your biggest events, like a wedding, or who you’d feel fine about walking up to and slapping them on the back if you saw them at 3 a.m. in the Hong Kong airport departure lounge.

Dunbar told the BBC what determines where you fit is how often you see people. We tend to prioritize time with actual friends. His theory contains layers of sociability.

Testing the theory

Dunbar said that when a group gets too big, members start to peel off and form other, smaller communities, which was apparently a proxy for friendships. Or those bigger groups simply collapse. Per the BBC, using historical, anthropological and contemporary psychological data about group sizes, Dunbar and others “found remarkable consistency around the number 150.”

The theory has been tested many times, including around 2016, when MIT Technology Review reported on a study using Dunbar’s Theory based on cell phone usage. At that time, the theory was expanded a bit to suggest that introverts and extroverts might have on average different numbers of friends. But the researchers found the layer numbers were similar.

Per that article, “In total, the study shows good evidence for the existence of the innermost and outermost layers but with some variability for the size of the intermediate layers. ‘The clustering yields results that match well with previous studies for the innermost and outermost layers, but for layers in between we observe large variability,’” the researchers concluded.

Finding life’s meaning in relationships

So why does any of it matter?

Because as The Wall Street Journal reported, humans “are built to schmooze.” And relationships are wildly important.

They impact our mental and physical well-being.

“When people are more socially connected, they have increased survival rates,” Julianne Holt-Lunstad, a professor of psychology and neuroscience at Brigham Young University, told the Journal. “There’s reduced risk for cardiovascular disease, stroke, Type 2 diabetes, depression and dementia.”

In 2022, Deseret News reported on studies showing strong social connections are linked to better physical and mental health, coping skills, sleep quality and more. Relationships top genetics for helping people age well and live long.

Holt-Lunstad in that article said her own analysis of 148 studies — and research by others of a further 276 studies — all showed that.

It’s important to have all kinds of relationships, which provide different resources for different needs. Dunbar’s layers all matter to a good life.

“We have evidence that social connectedness is linked to immune functioning, to susceptibility to viruses and an ability to mount an effective immune response to vaccines, as well as health-related kinds of behaviors,” said Holt-Lunstad.

A famous, very long-term piece of research called the Harvard Study of Adult Development, which started following 268 Harvard sophomores in 1938 and continued to track them, doing the same with inner-city teens recruited from impoverished areas, found strong links between relationships and health.

“The surprising thing is that our relationships and how happy we are in our relationships has a powerful influence on our health,” Robert Waldinger, study director, a psychiatrist at Massachusetts General Hospital and a professor at Harvard’s medical school, told The Harvard Gazette in 2017. “Taking care of your body is important, but tending to your relationships is a form of self-care, too. That, I think, is the revelation.”

Having friends and spending time with them — in fact, relationships in general — is a real hedge against loneliness. And the World Health Organization, former U.S. Surgeon General Vivek Murthy and others have warned that there’s a global epidemic of loneliness.

Whether Dunbar’s number turns out to be right may not matter so much. Having enough friends and relationships for your own well-being does, the experts all seem to agree. So mind your circle.