Definition
Dunbar's number is the proposed upper limit on how many people one person can keep stable, lasting social relationships with. British anthropologist Robin Dunbar arrived at it by studying primate brain size and group size, and he argued that the number humans can handle is about 150. Not everyone agrees with that figure, though.
Why 150, of all numbers?
Monkeys and chimpanzees build friendships by grooming each other's fur. Grooming takes a lot of time, so when a troop gets too big, an animal can't be close with everyone. On top of that, each animal has to keep a complicated map in its head: who is friends with whom, and who has the most power.
British evolutionary anthropologist Robin Dunbar noticed a pattern. The larger the share of a primate's brain that is neocortex, the larger its group tends to be. The neocortex is the part of the brain that handles thinking, memory, and complex social communication. The idea is that a bigger neocortex lets an animal keep track of more friends.
When he plugged the human neocortex ratio into that formula, the answer came out to about 150. In other words, the claim is that the brain's natural capacity for truly close relationships is around 150 people. But in 2021, Swedish researchers ran the numbers again using the same approach. Their estimate ranged from just a few people to several hundred, which is too wide to pin down any single number. So there is pushback.
Even 1,000 Social Media Friends Won't Change It
These days, a few taps on your phone can connect you with hundreds or thousands of online friends. Plenty of people have thousands of followers. But no matter how long your friend list looks, Dunbar's number doesn't grow much.
In fact, a 2011 study of Twitter conversations found that no matter how many followers someone had, the stable relationships they kept talking with topped out at around 100 to 200 people. Most of the names on the list were basically strangers.
Deep relationships take steady time and emotional energy. Technology keeps advancing, but the brainpower and hours in a day we have to spend are the same as our ancient ancestors had.
To Be More Precise: The 5-15-50-150 Secret
The number 150 doesn't mean you're equally close to all 150 people. Robin Dunbar described relationships as a set of concentric circles, with each circle about three times bigger than the one inside it.
At the very center are your 5 closest people, the ones who would help you without hesitation in a crisis. Beyond them are 15 best friends you see regularly and confide in, and then 50 good friends you hang out with often.
Then comes 150: people you might invite to a wedding, and people you could run into on the street and greet warmly without any awkwardness. Outside the 150 is a layer of about 500 acquaintances, and the number of people whose names and faces you can match up is thought to be about 1,500.
Common misconceptions
Dunbar's number of 150 is the total number of people you can remember in your lifetime.
The number of people whose faces and names you can match up is thought to be about 1,500. The 150 is the maximum number of people whose lives you know about and with whom you can keep up emotionally meaningful contact.
Where you meet it
Dunbar analyzed hunter-gatherer societies and found that the groups living together were about 30 to 50 people, while the larger clan or community units averaged around 150. Some researchers dispute this interpretation, though.
It's often compared to a military company, the unit where one commander can personally know and lead the soldiers. A company is roughly 100 to 200 people.
In one sentence
Because of the limits of the human brain (the neocortex) and of time, the claim is that we can keep real bonds with only about 150 people.