The Oxford evolutionary psychologist Robin Dunbar is best known for his namesake “Dunbar’s number,” which he defines as the number of stable relationships people are cognitively able to maintain at once. (The proposed number is 150.) But after spending his decades-long career studying the complexities of friendship, he’s discovered many more numbers that shape our close relationships. For instance, Dunbar’s number turns out to be less like an absolute numerical threshold than a series of concentric circles, each standing for qualitatively different kinds of relationships. He’s also studied the seven factors people use to evaluate whether someone has the potential to become a friend, and the average number of hours it takes for an acquaintance to become a close friend. All of these numbers (and many non-numeric insights about friendship) appear in his new book, Friends: Understanding the Power of Our Most Important Relationships. (The book is out in the United Kingdom, and will be released in the United States in January.)
The book is a timely arrival, as vaccination rollouts and eased social-distancing restrictions spur people to rekindle—or reevaluate—their friendships. I recently spoke with Dunbar about what we can learn about our own friendships from all these numbers, how friendships evolve over the years, and his predictions for post-pandemic social life. Our conversation has been edited and condensed for length and clarity.
Sheon Han: Could you explain what Dunbar’s number is?
Robin Dunbar: Dunbar’s number is the number of meaningful and stable relationships you can have at any one time. That includes extended family as well as friends. In fact, people who come from large extended families have fewer friends because they give priority to family members. The number 150 is an average, but there’s a lot of variation. The range of variation is somewhere between 100 and 250.
Han: An intriguing concept from your book is the “circles of friendship,” which I see as a more granular way of sorting friendships than Dunbar’s number. Could you describe what those circles are?
Dunbar: Dunbar’s number really isn’t a single number. It should be a series of numbers. When collecting data on personal friendships, we asked everybody to list out everybody in their friendship circles, when they last saw them, and how emotionally close they felt to them on a simple numerical scale. Relationships turned out to be highly structured in the sense that people didn’t see or contact everybody in their social network equally. The network was very clumpy.
The distribution of the data formed a series of layers, with each outer layer including everybody in the inner layer. Each layer is three times the size of the layer directly preceding it: 5; 15; 50; 150; 500; 1,500; 5,000.
The innermost layer of 1.5 is [the most intimate]; clearly that has to do with your romantic relationships. The next layer of five is your shoulders-to-cry-on friendships. They are the ones who will drop everything to support us when our world falls apart. The 15 layer includes the previous five, and your core social partners. They are our main social companions, so they provide the context for having fun times. They also provide the main circle for exchange of child care. We trust them enough to leave our children with them. The next layer up, at 50, is your big-weekend-barbecue people. And the 150 layer is your weddings and funerals group who would come to your once-in-a-lifetime event.
The layers come about primarily because the time we have for social interaction is not infinite. You have to decide how to invest that time, bearing in mind that the strength of relationships is directly correlated with how much time and effort we give them.
Han: Although the average size of each layer is more or less fixed, some factors cause slight variations. For example, when researchers looked at a sample of Dutch students, they found that extroverts have more friends in every layer than introverts do. In your book, you write that there’s a trade-off between time and intimacy that plays into that.
Dunbar: Introverts seem to be risk averse. They prefer to have fewer friends so they can invest more time in each. Extroverts are more socially confident, so they prefer to have more friends at the expense of investing less time in each. They probably feel they can wing it with someone else if one friend says no [to something]. These are just two equally good ways of solving the same problem.
Han: How do these numbers fluctuate as you get older and circumstances change?
Dunbar: Who your friends are changes constantly. You don’t throw away all your friends and start again, but you have this kind of churn going on. When you’re younger, in your late teens and early 20s, the churn rate can be very high indeed. Losing and gaining is largely a consequence of who you’re exposed to. Have you moved away to a new place for school or for a job? Have you just been exposed to a new group of people? That stabilizes by [about] the 30s, in most cases, [when people start having children], because babies are the killer for any kind of social life for everyone.
But the number starts to decline into old age—mainly by virtue of progressively losing the outermost layers. It ends up, if you live long enough, with just the innermost layer of 1.5.
Han: Here is a notable quote from the book: “Falling in love will cost you two friendships.”
Dunbar: If you meet a new person, fall in love, and get married, then you’re investing a lot of time and mental energy in that relationship. And from our data, it seems that you essentially sacrifice two people. Think about it in these terms: You meet this new person, so you now have six in your inner circle, so somebody has to go. But the new person is taking up to two rations. So you end up losing two people, who drop into the next circle, who push two people from that circle out into the third circle. It’s a domino effect.
Han: You identified “seven pillars of friendship” that people use to evaluate how likely they are to become friends with someone.
Dunbar: The seven pillars are seven dimensions of who you are that form the basis of friendship through homophily, which is the tendency for like to associate with like. “Birds of a feather flock together.” Our friends are very similar to us.
Han: Among the more predictable pillars, such as sharing the same language, growing up in the same location, and having similar worldviews (moral, religious, and political), I was surprised to find that having the same sense of humor was included. Even more counterintuitively, so was having the same musical taste. Do weightier factors, such as your moral view, matter more than seemingly less important ones?
Dunbar: No. The seven pillars are what economists call substitutable—each is as good as any other. A three-pillar friendship can form with any combination of three pillars. However, liking the same music seems to be especially good for relationships with strangers.
Han: Unsurprisingly, the amount of time spent together is a crucial factor for forming and maintaining friendships. A study by Jeffrey Hall, which you mention in your book, outlined how many hours it takes for someone to go from an acquaintance to a casual friend, then to a meaningful friend, and finally to a best friend.
Dunbar: It takes about 200 hours of investment in the space of a few months to move a stranger into being a good friend. This fits with our data, which suggests that close friends are very expensive in terms of time investment to maintain. I think the figures are a guideline rather than precise. It just means friendships require work.
Han: As I was reading the book, I thought, It’s one thing to be aware of findings from friendship research, and another to use those findings to actually improve our lives.
Dunbar: I think the important lesson here is: You should not try and over-rationalize what you do in the light of this. If you do that, you will get it wrong, for sure. That’s because these processes are unconscious, maybe because we learned them, or maybe they’re psychologically inbuilt like instinct. But either way, they are very difficult to unlearn. That’s important because they’re shortcuts to allow us to operate fast in a very complicated, dynamic social world. We have to be very, very flexible.
If you try and apply rules consciously, everything in all these natural sequences just falls apart. So it’s best not to overthink. The science side is illuminating and interesting in explaining why we do the things we do. But it’s not always helpful to use it as a guidebook.
Han: It should be a heuristic, not like computer instructions.
Dunbar: Yes, exactly.
Han: Do you think the pandemic will have any long-term negative impact on how we form friendships?
Dunbar: Not really. Maybe for some subgroups who are very timid in their interactions with other people. They may become more timid because they have not had enough practice. And for old people who don’t have the energy or the motivation anymore to replace friendships. Lots of data show that social-network size shrinks [from] about age 65 onward.
But I think anybody else will look back on it as, Well, that was a really frustrating time. But we’re back to normal. If you’re forced to interact with people, then you become better again.
As we come out of lockdown, you’ll have made some assessments: Well, it’s convenient that I haven’t seen so-and-so much. Because actually, we probably weren’t terribly good friends.
Han: It’s a healthy pruning.
Dunbar: Yeah, it’s a healthy pruning. Don’t be fazed by loss of friends, because it’s an opportunity to go off and make new friends, which may turn out to be even better.