Have you heard of Dunbar’s number? My sister certainly has. Years ago, when her new neighbour asked her if she wanted to pop round for a coffee, she said: “No thanks, I’ve got enough friends.”

Now, before you start judging my sis for being unnecessarily blunt, she does have a point. And that’s where Dunbar’s number comes in. You see, it suggests that a person can only maintain a maximum of 150 social connections at once. So why is this and where does the idea come from?

The Origin of Dunbar’s Number

Robin Dunbar is a British anthropologist and an evolutionary psychologist. Back in the 1990s, he was interested in the number of social connections a person could have.

For instance, how many people do we know; how many do we actually care about and is there a common link to this number? Do some people have more connections, others less? Now, remember, this was the 90s so well before social media and ‘likes’ and ‘friends’ and all of that.

Dunbar began his research by examining the patterns of sending Christmas cards.

Dunbar and the Christmas Card List

Sending cards at Christmas seems fairly innocuous, but there is a certain amount of investment involved. You make the list of people, you choose and buy the cards and the stamps, and you look up the addresses. Then you write them all out and post them. It all takes time and effort. Dunbar reckoned that most of us would not go to all this trouble and effort for just anyone.

After collecting data from thousands of households, Dunbar found a remarkable coincidence. Of every household he collected data from, the average number of cards sent was always around 150. There was also a fairly unanimous split in who the cards went to. For example, around a quarter were sent to close relatives, two-thirds to friends, and the remaining small percentage to colleagues.

But why did the number 150 keep cropping up? It was a mystery. Dunbar carried on researching. But this time he turned his attention to primates and social groups.

Why Is Dunbar’s Number 150?

Dunbar discovered a link between a primate’s brain and the size of their social groups. Specifically, their brain mass and the primate’s preferred group sizes. He looked at different primate species and catalogued social activity.

In particular, time spent grooming (the equivalent of socialising for humans), the size of the neocortex (the area of the brain related to language and cognition), and group size. He found that in primates, the smaller the size of the brain, the smaller the size of the groups were formed. As brain mass increased, so did group size.

Dunbar proposed that brain size was the overriding factor in deciding the number of social connections a primate could successfully manage. Dunbar then collected data across all primate species, including humans.

He proposed that humans can only comfortably maintain 150 social connections. Larger numbers require stricter social rules and a larger neocortical processing capacity.

So what exactly does Dunbar mean by 150 and social connections?

Dunbar characterises the number 150 as:

“..the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar.”

There is strong evidence, throughout history, that shows 150 is an average size for social groups. Indeed, it is the optimum number for a group. For when numbers start to exceed this size things tend to collapse or fail to function effectively.

Dunbar’s Number applies to many social groups

Even our earliest ancestors, the cave-dwellers, the hunter-gatherers, lived in groups consisting, on average, of 150 people. The earliest villages consisted of around 150 people. From African tribes to Roman legions, we are always drawn back to this magic number of 150.

Perhaps stranger still, Dunbar and his magic number of 150 can be seen in many other aspects of human social groups, not just our personal lives. For example, offices, campsites, hotels, military organisations, and even book clubs. Indeed, research proves time and time again that if numbers exceed 150 the group fails.

150 only applies to primates and humans

So why 150? It appears that 150 is the prime number for evolutionary survival. Primates, in particular, live in social groups, and this helps them to survive. In our ancestor’s time, humans were prey, not predators. We didn’t have sharp teeth, razor-like claws, or strong muscles.

Whereas it suited other predators to hunt alone, for humans to stay alive, we needed to form groups. We used our shared knowledge and cunning. We planned and formulated ways of attack. For us, staying in strong, social groups was a matter, literally, of survival.

Now, look at other animals. For instance, the tiger, a predator at the top of the food chain, or a penguin, prey near the bottom. Tigers are solitary animals. They survive without the need of a group and therefore hunt alone.

On the other hand, penguins are at risk from many predators, including extreme weather conditions. As a result, it is in their best interest to form huge groups. In fact, some of the largest penguin colonies have consisted of up to 180,000 to 200,000 birds.

Of course, tigers and penguins are very different from primates and humans. Penguins may form groups but they are not social in the way that human groups are. For the penguins, it is all about staying alive. For humans, it is more about emotional, psychological, and spiritual connections.

And this is where it gets interesting. Because it takes a lot of effort to maintain all this emotion, and our brain can only manage so much. However, have we changed in the 21st century?

Has Social Media Changed Dunbar’s Number?

Now, in today’s society, there is nothing unusual for a person to have hundreds, if not thousands of friends on Facebook. So is it possible that Dunbar’s number no longer applies in our modern world?

Dunbar first proposed the number 150 in the 1990s. The 2020s is a very different place. We communicate online. We meet for the first time online. We date online. Surely, Dunbar’s number must have increased a little to keep up with our modern society?

I mean, this doesn’t make sense for a modern age. People communicate in seconds across the globe. Our social reach has expanded as our grasp of technology has stretched our imaginations. Also, I would have thought that our brain capacity would have increased substantially since our ancestors first set up villages over 250,000 years ago.

Well, not really. And that’s because it is all to do with our emotional capacity.

“It is as though we each have a limited amount of social capital and we can choose to invest it thinly in more people, or thickly in fewer people. But you can’t exceed these limits.” Dunbar

So what do these social connections look like? Dunbar arranges them in ever-decreasing circles. Our closest friends are in our inner circle and our acquaintances are in the furthest circle.

Most people, on average, have:

  • 5 loved ones
  • 15 best friends
  • 50 good friends
  • 150 meaningful contacts
  • 500 acquaintances
  • 1500 people you recognise

So we may know thousands of people, but Dunbar states that the 150 number is the important cut-off.

“The 150 layer is the important one: this defines the people you have real reciprocated relationships with, those where you feel obligations and would willingly do favours.” Dunbar

Because humans are complex creatures, maintaining these relationships take effort and time. And that’s why we only have the capacity for 150 social connections.

Of course, people move in and out of our lives at any given point. There are also huge differences between the social connections of an extrovert and an introvert. An extrovert may have a larger social network. However, they tend to spread themselves out thinly across a wide network of people. Introverts have a smaller social pool of contacts. But they like to spend more quality time with a few special friends.

There are also interesting differences between the genders. For example, men have a wider spread of contacts throughout their social circles. Whereas women have more contacts within their inner circles.

Final Thoughts

So is there really any advantage to knowing that humans have a limited capacity for maintaining social connections? Well, I think so yes. I realise it is all about time and effort. If we only have space for 150 connections then we should make sure those connected to us are worth the effort, and that we make the effort to preserve them.

Oh, and my sister? She did pop round for that coffee after all. They’ve been good friends ever since.


  1. www.ncbi.nlm.nih.gov
  2. https://royalsocietypublishing.org

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This Post Has One Comment

  1. Christopher D Hill

    If there was a way to upvote this article I would! Space is limited so you can’t go on-and-on, but if I could, I’d add lots of benefits from the truths you shared, specifically quality over quantity relationships, honesty about the depth/degree of relationship and the difference between kindness/respect and deeper relationships–there is no limit to the number of people you can be kind/respectful toward!

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