What’s your Dunbar number?

Over 30 decades of research supports Dunbar’s initial theory. But how does this theory fit with online networks?

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How many friends can one person have?

According to evolutionary psychologist Robin Dunbar, the answer is 150.

Dunbar developed his theory while studying non-human primates. He argues that the number of relationships an individual can maintain is limited by the size of their brain, specifically the neocortex, the part of the brain responsible for language and abstract thought. Because humans live in socially complex societies, we evolved relatively large brains to allow us to handle the processing demands of keeping up with these complex and ever-changing networks.

It turns out that our brains can effectively process and maintain a network of 150 meaningful connections at any given time. This is known as Dunbar’s number. Anything beyond this isn’t possible because of our cognitive processing capacity. There’s only so much information you can keep in your head and juggle at the same time.

Dunbar’s number is remarkably consistent over the course of history and across groups.

Modern hunter-gatherer societies had about 150 people. It’s also the size of military fighting units, Christmas card lists, telephone calling networks, and the ideal number for church congregations.

But social networks are more complicated than this 150 number. They’re actually made up of a series of layers.

The inner most layer has just five people. These are your loved ones—romantic partner, best friend, and close family members.

The next layer up represents your good friends. It’s the 15 people that you can turn to in times of need, the ones you can talk to about most things.

Next up are the 50 people that you call your friends. They’re the people you’d invite to a group dinner (not all at the same time) and can rely on for more casual favours, like recommending you for a job.

The layer after is Dunbar’s number of 150 people that you consider meaningful connections. They’re the people you’d invite to a large party, like a wedding. Dunbar himself defines meaningful connections as those people you know well enough to greet without feeling awkward if you ran into them in an airport lounge. 

There are also layers beyond 150.
Dunbar's Number

The layer above 150 is your 500 acquaintances. When you run into them, you have a little chit chat but that’s about it. Finally, there’s the layer of 1500 people that you recognize—the people that you can put a name to a face.

Essentially, these layers reflect our frequency of contact with people and how emotionally close we feel to them. We spend more than 60% of our social time with our closest friends, the layer of five, and we give decreasing amount of our time to the people in the layers beyond. These layers tend to be pretty stable, but people can shift between them as you get closer or more distant.

Of course, all these numbers are ranges. For example, Dunbar’s number is actually a range of 100-250 people. Where someone falls within a range depends on who they are as a person.

Extroverts tend to have a larger network while introverts tend to have a smaller, but denser network.

Women also typically have more people in their inner most layers. Several neuroimaging studies have also showed that the size of someone’s social network is correlated with the size of their brain.

So over 30 decades of research supports Dunbar’s initial theory. But how does this theory fit with online networks?

So far, research shows that online relationships are constrained by the same numbers as offline relationships. For example, online gaming communities have the same layers as real-life groups. Research into Facebook also shows that although many people have more than 150 Facebook friends, these represent the outer most layers of connection: the 500 or 1500. In fact, on average, people consider only about 75 of their Facebook friends as real friends.

What this means is that online and offline networks work largely the same.

The online world simply represents another means of communication with people rather than representing a fundamentally different type of relationship.

Intimacy just doesn’t seem to be possible beyond 150 relationships. Real friendships require cognitive resources and time. And these can’t be stretched too thin without our connections suffering.

That being said, there may be generational differences.

People aged 18-24 have much larger online social networks than people aged 55 and older. It’s possible that real-world connections may be less important for younger people, who have never known the world without the Internet.

It makes sense that we have an upper limit on how many people we can be friends with. What remains to be seen is whether this limit will change as more of our lives happen online.

Sonia Krol, PhD - Social Psychologist and Resident Friendship Guru @ Pally

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