Ever heard of Robin Dunbar? He’s the guy who famously discovered Dunbar’s Number: 150.

Wait, let me clarify that. Robin Dunbar did not discover the number 150. What he did discover is that 150 is roughly the largest number of friendships most of us can maintain at one time.

Robin Dunbar is a British anthropologist and evolutionary psychologist. He’s currently a professor at the University of Oxford.  His latest book, Friends: Understanding the Power of our Most Important Relationships, explores how we make, keep and lose our friends.

Friends: Understanding the Power
of our Most Important Relationships
By Robin Dunbar
Little, Brown, London, 2021

I was already familiar with Dunbar’s Number from my work at various technology companies. So I was eager to read this book.

Friends looks at friendship from every conceivable angle. Why do we need friends? Who is most likely to become our friends? How do friendships change as we age? How do friendships differ between men and women? How does friendship work psychologically in the mind, and physiologically in the brain? Do online friendships really work at all? And of course, how many friends can we have?

The book is based on decades of research by Dunbar, his students, and collaborators around the world. In fact, there’s so much research cited in the book I found it overwhelming.  So in this review I’ll just highlight a few themes that I thought were the most interesting or important.

Who’s a Friend?

First off, how do we define friendship? And how do we tell the difference between a friend and a mere acquaintance?  Dunbar doesn’t give a concise definition, but he does list some key characteristics. Friends are:

• Much like family, except that you get to choose your friends.  Of course, for many people, family members are also among our closest friends.
• People with whom we exchange obligations and favors. They’re people we call upon when we need support, and people we help without hesitation when they need us.
• People we want to spend time with and are willing to make the effort to do so.
• People we’ve come to know a lot about.

Casual acquaintances are different, Dunbar says, because you wouldn’t go out of your way to be with them or to help them.

Friendships come in various strengths, and the strength of a friendship can certainly change over time. Dunbar outlines this nicely in the circles of friendship diagram below.

People often move from the outer to the inner circles of our friendships, and back again over time. And there are some friends who become “sticky,” remaining in one of our inner circles even when we don’t see them as frequently as we used to.

The 7 Pillars of Friendship

Dunbar cites research indicating there are seven factors that make it likely two people will become friends. He calls these the 7 Pillars of Friendship. They are:

• Speaking the same language or dialect
• Growing up in the same location
• Having the same educational or career experiences
• Having the same hobbies and interests
• Having the same moral, religious and/or political views
• Having the same sense of humor
• Having the same musical tastes

You don’t need to tick all these boxes with another person in order to become friends but, Dunbar says, the more of them you do share, the more likely it is that you will want to spend time with them, help them and become emotionally close to them. In other words, the more likely they’ll fit into one of your inner friendship circles.

Birds of a feather do flock together. In fact, there’s a name for this: homophily.

I think it’s interesting that humor and music ore on Dunbar’s list. More about them in a moment.

The Neurochemistry of Friendship

It turns out that a lot of the rewards of friendship, and many of the well-known physical and psychological health benefits of friendship come down to endorphins. Endorphins are hormones involved in pain and stress reduction. Dunbar details how being with friends and doing things with them releases endorphins leading to reduced stress and sometimes even feelings of euphoria.

This is particularly true when we touch each other, Dunbar notes. In most cultures there are complex rules about how and where we can be touched and by whom. Yet hugging, a pat on the back, stroking of the arm, or just a handshake are all forms of touch that stimulate our endorphins and serve to reinforce the bonds of friendship.

Other activities, like laughter, singing and dancing, storytelling, and religious rituals can have similar effects. You’ve probably experienced that wonderful feeling of unity that sometimes comes from them. They also let us scale up our friendship activities since they can bring several people, sometimes many people, closer together at the same time.

By the way, Dunbar doesn’t have much time for the idea that gender differences in the way we handle our friendships – for example friendships between females tend to be more intense but also more fragile than male friendships – are the result of socialization.  He’d probably side with the idea that gender differences, at least in this area, are more innate. He cites research that shows consistent gender differences across cultures and even among other primate species.

A Theory of Mind

One of the key things that makes friendship possible, Dunbar argues, is that humans can develop a theory of mind. This is our ability to form ideas about the mental states of other people. It’s an ability we develop around the age of five. I found this to be one of the most fascinating parts of the book.

A trivial example: if I tell you a joke and you laugh, I understand that your mental state is happy, at least in that moment. (And we both probably get a little endorphin hit too.) With increasing levels of sophistication, we can understand or keep track of people’s mental states at several levels of indirection. When you tell me that you had fight with your friend. I know that you’re upset, but I can infer that your friend might be upset too. If that friend tells their sister about the fight, I can imagine that the sister would feel sympathetic even if I’ve never met your friend or their sister. 

This ability to form theories of mind, Dunbar writes, is foundational to language and to building social relationships. It’s an ability that no other species has.

“There is one very big difference between animals’ exchanges and human conversation, however. From the honeybee’s ‘There’s nectar in the third patch of flowers to the right’ to the monkey’s ‘Watch out – there’s an eagle about to launch itself at us’, animals are limited to comments on factual matters. Our conversations are mostly about the social world we live in, a mental world that doesn’t exist in material form because it only exists in people’s minds.” [p. 175]

Empathy would be impossible without a theory of mind.

Online Friends

Dunbar is a little skeptical about whether online communication, particularly social media, can truly lead to deep inner-circle friendships.  He cites research showing that most of the people we connect with online are people we already know in the real world.

He seems to agree that online connections can help us maintain friendships when they might otherwise fade away if we can no longer meet face-to-face. I wrote about this topic myself a few years ago in a post called We Don’t Say Goodbye Anymore.   

But Dunbar thinks that new friendships formed online will be very fragile unless they are reinforced by spending time together in real life.

Friends was published in early 2021 and must have been written in 2020. So it doesn’t contain any research about how the COVID-19 pandemic and our increased use of online video have affected friendship. That’s something to look for in the coming years.

Unsolicited Feedback

Friends is jam-packed with research findings, much of it from Dunbar’s own work. In fact, the book is really a literature review – a comprehensive survey of the academic literature about friendship.

Unfortunately, the writing is monotonous and dull. There are probably a couple of hundred studies cited in the book. Dunbar describes all of them using the same basic template: “In collaboration with Professor A of the University of B, I and my clever postdoc C – who later went on to become a professor at D University – designed an experiment E to see if hypothesis F was true. Thanks to the savvy computing skills of another student G, we were able to analyze our data and arrive at conclusion H.” This pattern repeats with only minor variations on almost every page of the book.

Friendships are so important in our lives, and for our well-being. Friends shows this clearly, based in large part on Dunbar’s own profound body of work. It’s a shame he couldn’t enliven the text with human stories to illustrate the research. US Surgeon General Dr. Vivek Murthy does a much better job of this in his book Together, which I reviewed here.

Thanks for reading.