Transcript: Dunbar’s Number and social bonding

Below is a transcript of a conversation with Robin Dunbar, Emeritus Professor of Evolutionary Psychology, about the research he’s led on social group size and social bonding and the limit to the number of meaningful relationships humans can maintain (often referred to as “Dunbar’s Number”).

This is part of a call out to read Robin Dunbar’s recent books ahead of a future seminar podcast discussion about his latest research, building on the foundational ideas explained in the transcript.

Share your question by voice memo in the new Discord group linked here or email by Monday September 12, 2022.

Professor Dunbar will be our guest, answering your questions.

Reading list

Start with the transcript on this page linked here (approx. 10,000 words / 30 mins):

Transcript of Robin Dunbar on the Wide Open Air Exchange

Then this short article (approx. 1,000 words / 3 mins):

Robin Dunbar Explains Why His ‘Number’ Still Counts

Then if you’re interested to learn more source these recent books:

Friends: Understanding the Power of our Most Important Relationships

How Religion Evolved: And Why It Endures

The following podcast transcript has been lightly edited for readability. Care has been taken to be as accurate as possible in this presentation, however people tend to speak differently to how we write and there will be a degree of translation in any transcript through choices about punctuation and paragraphs.

NB. This transcript is intended as an efficient way of getting across the foundational ideas as of 2017, as an entry point to further reading and research about developments over the past five years.


Wide Open Air Exchange podcast

Guest: Professor Robin Dunbar

Host: Christine Gallagher

Recording location: Professor Dunbar’s college office in Oxford

Original publication date: April 1st 2017

These are hyperlinks to sections of the transcript if you want to skip to a topic of most interest when viewing this from the webpage:


Explaining “Dunbar’s Number” and layer sizes

Friendship bonds

Reproduction and brain size

Endorphins and social bonding

Romantic relationships and more on reproduction

Concluding remarks


Hello, I’m Christine Gallagher and this is the Wide Open Air Exchange.

My guest today is a world renowned anthropologist and professor of evolutionary psychology, Professor Robin Dunbar. He’s a research professor in the Department of Experimental Psychology here in Oxford.

Professor Dunbar has written or edited dozens of books, published hundreds of journal articles, and has a bunch of honours and fellowships and awards and associations of which there are far too many to mention.

“Expert” is a word which I rarely use. I think it’s thrown around far too much. It’s used to describe people who I think should be called “specialists”, at most. But it would be pretty safe to say that Professor Dunbar is an expert in his field, or fields really.

And there’s a very good chance that you’re already familiar with some of Professor Dunbar’s work, particularly the idea that there’s a limit to the number of meaningful relationships humans can maintain: around 150. It’s a theory which has been dubbed, “Dunbar’s Number”.

You might be thinking, “well, hang on, I have 500 Facebook friends”.

But ask yourself, with how many of them are you in meaningful, regular contact and how many of those would you turn to in a moment of crisis.

Professor Dunbar’s theory—and the empirical work that he’s led—shows that we can actually predict that number pretty precisely, which is quite incredible.

I’m not going to say anymore right now because you’re about to hear Professor Dunbar explain this in his own words, except to say that it’s something Professor Dunbar predicted based on the relationship between brain size and group size in other primates. 

Some of what Professor Dunbar does is comparative studies of primates, and he’s made other observations about social bonding to do with grooming, which you’ll hear about.  If you’ve ever felt pleasure from having someone stroke your hair, or lightly touch your skin, you’ll be able to relate to this. 

As I said, Professor Dunbar is in the Department of Experimental Psychology here in Oxford and he explains some of his findings about the role of endorphins in group bonding through things like singing and dancing and laughing together. 

You’re going to hear about friendships, family relationships, and a bit about romantic bonds as well. It’s fascinating stuff. Some of it I find quite challenging in terms of what it means for my existing views on love and relationships and I’ll reflect more on that at the end.

For now, without further ado, I’m going to bring you into my conversation with Professor Robin Dunbar from the University of Oxford.

Explaining “Dunbar’s Number” and layer sizes

Christine Gallagher: Dunbar’s Number and the 150—you’re so famous for it now. Could we maybe start there with, in a nutshell, what that is.

Robin Dunbar: Dunbar’s Number, in a nutshell, is the number of meaningful relationships you can have.

Those are relationships with history, that are reciprocal relationships, so you know how they fit in your social world and they know how you fit in their social world.

And they’re relationships based on trust and obligation, so you feel you could ask a favour of them.

CG: Somewhere to stay, or whatever it might be.

RD: Exactly, and they’re very unlikely to say “no”. They might grumble, but they’ll say “sure”. 

That number turns out to be about 150, give or take. For some people it’s a bit smaller, for some people it’s a bit bigger. The average seems to be about 150. 

It turns out that for monkeys and apes and humans—the grouping size that’s become identified as “Dunbar’s Number”—or “Dunbar’s Numbers” across monkeys and apes—is just one of a series of layers. 

If you think of it in terms of your social world, you sit within the centre of a series of circles that expand outwards from you and which have very, very specific sizes.

And it turns out to be the same sizes in monkey groups, so what we’re doing in the course of evolution is adding layers on the outside.

These start at the very, very inner core with about 1.5 people. Some of us have two, some of us have one, kind of intimate-intimate.

CG: I see, just one other—your partner or best friend—most of us have one person.

RD: That’s right, your most important person, a best friend. Or you might have a partner and a best friend, which is how you get two and then end up with an average of 1.5.

And then, they’re embedded within a circle of 5 intimate friends and they’re the shoulders to cry on.

CG: That’s the inner core? 

RD: That’s the inner core.

And then they’re embedded in turn within a layer of about 15 which we sometimes call the sympathy group, because all the people if they died tomorrow you’d be very upset about.

CG: (laughs) sorry to laugh, but yes that’s a good way of putting it.

RD: That is also a very standard size for lots of social activities. So most team sports are in that sort of region of 10, 11, 12-ish in size. 

And if you think about the ones that are pushing that limit of about 15, as rugby does, it’s clearly getting a bit tricky to keep everybody together so you divide it into two sub-games of the front/forwards and the backs.

CG: Wow!

RD: Isn’t that extraordinary.

CG: It is. And when you talk about the size of the brain, is that the capacity for bonding?

RD: Yes

And just to finish off that series, we know that the circles then run out to 50, and to 150 — officially “Dunbar’s Number”. 

Beyond that for at least two more layers, to 500 which we think of as acquaintances, and then 1,500 which we think of as – well actually it’s very specific, it’s the number of faces we can put names to.

And that turns out to be the average size of tribes in small scale societies, and that’s your language unit. Tribe is defined as all the people who speak the same language.

CG: Then because the world that we live in, especially now through social media we have followers and friends on Facebook and all of that, those numbers can be larger technically, the people who are in our networks, or who we’re in contact with.

But are you saying that you can’t actually meaningfully socialise with as many people as you think that you’re in touch with?

RD: Yes, these numbers seem to be incredibly fixed and we pick them up on Facebook, we pick them up on telephone databases (these are big national telephone databases), and Facebook, and we pick it up on Twitter. 

If you look at the kind of postings that go on among the Twitterati, let’s say the followers of a Twitter account when they’re talking to each other, you pick these layers up and it’s simply a consequence of how much time you devote to different individuals. 

So each layer has a very specific frequency with which you have to contact that person, or a person, for them to be in that layer. If you contact them less then they will slip down into the layer below. Family are a bit more robust to this but friendships you really seem to have to work at. 

Eventually if you stop contacting them, especially friends, and if you stop seeing somebody or stop contacting them regularly after they’ve gone away, they eventually drop down through the layers and drop over the 150 and become an acquaintance — somebody I once knew but I can’t remember very much about them.

CG: Yes. We lost touch. I see.

From my own experience, I’ve had that happen where there are people who were very intensely in my life in particular circumstances or moments, or places that I lived, but then I move on. And people who I maybe would have turned to for a place to sleep, or whatever, are no longer that person anymore.

RD: And they wouldn’t feel that obligation now.

If they’d been very close, if they’d been in your inner five, let’s say, 15 years ago or something or 20 years ago, then they probably would say “Oh, okay yeah lovely to see you, let’s start again”, as it were.

If they’d been in the outer edges of the 15 layer, where it’s not quite such an intimate relationship, they would probably go “well, I don’t really know who you are anymore, and I never really did”.

CG: So, history is important.

RD: Yes.

CG: But you have to have been in the inner core to be able to move back?

RD: No there’s a lot of movement up and down, both among your friends and your family. Remember this network, at any given layer is a mixture of friends and family and they turn out to be about 50/50 with our current rates of reproduction. 

In traditional small scale societies, your community size—let’s say your 150ish—would all be family, you’d all be related directly or indirectly to each other. And of course then you’re having about five surviving children per parent, as it were, and that will actually produce this number very nicely. We’ve reduced that to a shade either size of two, and that leaves this big hole and we’ve filled that hole with friends.

But family still have priority, so much so that we can show in our data that people who come from big extended families have fewer friends, because they get priority.

CG: Wow, because that number is so consistent? Isn’t that incredible.

RD: Yes. I mean we’re not sure, still, exactly how all this works or which layer is the kind of crucial hinge for the whole system. The rest seem to fall out as a consequence of the way networks are structured and that one of them is the kind of crucial network that everything is pitched around.

But it seems that we have a real limited capacity for the number of people we can keep in a given circle which is a constraint in our mind — a brain constraint as it were.

And secondarily then you have this time constraint wrapped on top of that because the strength of a relationship is directly related to how often you see the person. 

You devote something like 40 percent of your total social capital, or social effort, or social time (it kind of doesn’t matter how you measure it) to the five people in the inner core of your network and then 20 percent to the ten people in the next layer.

So those first two or three circles—out to fifteen—account for 60 percent of your total social effort.

CG: Incredible. So for someone who is living away, like I am, (sorry can’t help but think of my own personal circumstances, I’m from Australia and living in England), will those circles of mine not change that much because I’m not forming new ones in the same way. How does that work?

RD: They are very, very fluid in the sense that, in one study we did, over eighteen months, there was a 40 percent turnover in who was in each circle. This might not be dropping out completely but 40 percent of people changed circle. Now if you were in the inner 5 to start with, that might just mean you move down to the 15. If you were in the outer 150, you might well drop out all together. 

[both laugh]

RD: But the issue seems to simply be, there’s a limit to the number of people you can have in that inner core in the centre.

If somebody new turns up, and that might be because you move away, or it might be because somebody turns up in your social world and you think: “Oh they might make a good friend” for some reason, you bring them right into the core. Now that means the inner core of 5 is now 6, it’s overloaded, and so somebody drops out because you don’t actually have the time to do 6 people.

CG: It blows my mind that you can measure this. Wow.

RD: And be so precise.  And of course the knock on consequences then, is if somebody gets knocked down…

CG: Oh then somebody from that layer gets knocked down…

RD: And somebody eventually ends up getting pushed out all together.

CG: Amazing. 

Friendship bonds

Christine Gallagher: Something else I’ve heard you speak about is the different ways that the sexes, what’s the right phrase…

Robin Dunbar: Bond their relationships.

CG: Yes, you were saying women in conversations and men in activity.

RD: Yes it’s very clear that’s the core to building relationships, aside from the family ones which are slightly odd in that somehow kinship kind of makes those relationships special so you don’t have to work quite so hard at them and they’re a bit more tolerant of your bad behaviour.

CG: The whole, “you can’t choose your family”.

RD: You can’t choose your family but on the other hand they will tolerate many more social indiscretions on your behalf, like forgetting birthdays, not calling them up when you’re supposed to call them, all these things, whereas friends won’t. Really you have to work hard at friendships. 

But overall this is going to be true of family relationships, too. Where a family member sits in the circle, or which circle they sit in, is in the end going to be a consequence of how much time you invest and how much effort. 

Well you kind of assume this is all to do with chatting and stuff because language is so important to people, to humans, and this is how we organise our social world. By chance this longitudinal eighteen month study we did, we were interested in what happens when people move away from home and therefore their home networks are put under pressure because they’re not able to see them so often, how do they keep them going. 

To cut a long story short here, it turned out that what keeps girls’ friendships going (and this is specific to friendships, in this case), despite this spatial, geographical separation is talking together in some way. It could be on the telephone, I guess it could be on Facebook but basically it’s talking together and obviously ultimately the best form of that is face to face.

CG: So we have Skype and all that kind of thing.

RD: All those things, yes, they’re kind of well designed to fill that need.

But it turned out when we looked at the guys’ friendships, talking together had absolutely no effect at all, at all, on whether their friendship survived or didn’t survive, over this eighteenth month period.  

What did affect their friendships was doing stuff together. Now of course that was also true of the girls, they go shopping or they go to a party together or whatever, or out for a coffee. But it was much, much more important for the guys, and the guys had no back-up whatsoever from conversation. 

So the sort of Mickey Mouse version of this is, the way girls service their relationships and keep them going, and keep them well oiled and working, is talking together, and the way blokes do it is to bang their heads together. 

CG: [laughing]

RD: And it might be the guys they play five a side with football, they meet Friday night, or go mountain climbing.

CG: So does that mean if a man wants to bond with a woman he should make an effort to speak with her and vice versa? If a woman is trying to bond with a man, whether it’s friendship or romantic or what not, she should say, “hey let’s go and do something together” rather than talk?

RD: That’s a very interesting question, actually, to which we actually don’t know the answer, and that’s because we’ve kind of never really looked at that. But in some sense the answer is “yes“. 

Obviously it depends what the nature of the relationship is.

If it’s a romantic relationship, there’s other stuff that usually happens in a romantic relationship—namely sex—that is a very, very good bonding agent. 

If it’s friendships, then it’s going to be kind of more difficult, I think, to match up the dynamics of the way the two sexes kind of build their friendships…

CG: Really?

RD: …which may be one reason why a woman’s best non-romantic friend tends to be another woman, not a bloke. Sometimes it is another bloke. There are technical issues with having another bloke as a best friend because then if she has a romantic partner he’s possibly not always so keen on this and so you can see a natural push, perhaps, towards women.

But I suspect it comes back to the fact that, because the whole basis of our friendships is this concept of homophily—being very similar in lots of different ways, sharing the same interests, same sense of humour, same kind of political views or moral views or whatever—that’s so important in creating the strength of a friendship, even in a family relationship.

Family relationships that you feel emotionally closest to, tend to be the ones that you share or have the most similar views and attitudes and likes and dislikes.

CG: That’s intuitive, I suppose.

RD: Yes, yes.

CG: Speaking of shared traits, you have so many papers but I think there was one, was it: “Age, Music and Ethics”?

RD: Yes, so we’ve done a number of studies on this, and it turns out that there are six major pillars of friendship as far as we can understand, that the more of these you share in common, the stronger the relationship will be. One of them isn’t personality, as far as we know.

CG: Really?

RD: Yes, it’s much more to do with cultural things and we think the reason for this is that what they actually identify is a very small community. In a kind of small scale society it would be your local community, your village.

Why this is important, is that, for two reasons really.

One is you know how people think, so you can make all sorts of assumptions in the process of interacting with people and doing stuff, in general, which you know they will be happy with. They’re not going to suddenly say, “hang on a minute, we don’t do that kind of thing here”.

But also at the same time, because of that, and because it is in origin identifying your birth community essentially, or the community you grew up in, what that means is you know you can trust these people because you know how they think.

So you know that when they say, “I’ll do something for you”—they really will.

Or, “if you do me a favour, I’ll pay you back next year”—they actually really will rather than doing a runner.

Reproduction and brain size

Christine Gallagher: So when we talk about evolutionary psychology, it would make sense then in terms of the future of your own family line. 

Robin Dunbar: Yes this goes back to the fact that, like all monkeys and apes, our major adaptation is this capacity to live in social groups and to effectively share the risks of the worst the world can throw at you in order to be able to reproduce more effectively.

So all monkey and ape societies are basically implicit social contracts. They’re kind of informal agreements between individuals to live together and share, principally for primates, the costs of predation; that is, protection against predators.

By sort of ganging together they’re able to reduce those risks. But in order to gang together, you have to really be prepared to give up some of your short term interests, in order to gain this bigger, long term benefit which is protection from predators. 

CG: So does that mean that things like empathy are only evolutionary out of self interest? I guess we’re getting into very philosophical territory, when we talk about love and empathy. 

RD: Everything in evolution is out of self interest.

But what you have to bear in mind, and this is simply because evolution is driven by the underpinning genetic processes, so it is in a sense—particularly for humans and probably primates, because they’re so social—it’s about lineage survival rather than individual survival and reproduction, per se.

Because what evolution is about is leaving great, great grandchildren. That’s the driver for it. That’s the selection net that filters all these things that go on in the daily lives of an organism. 

Now the problem for evolution is that evolution is a kind of Heath Robinson affair which sort of makes do most of the time.

Partly this is because we’re constantly torn, any organism is torn between many different requirements. You’ve got to eat but at the same time you’ve got to avoid being eaten by someone else and often the two are incompatible. 

I suppose the classic one is the difference between mass reproducers and folks like us and the upper primates that only produce one baby at a time normally.

So you can maximise your genetic fitness, as biologists would say, following the salmon strategy where you just pump out a million eggs. You know that most of them are going to get eaten, but as long as a few survive that’s all you need.

CG: It’s a numbers game.

RD: It’s a numbers game, totally, and at that point you don’t have to invest anything in looking after the babies, you just spawn away.

CG: It’s literally survival of the fittest.

RD: That’s right, you go off and swim down on the beach and nature takes its course. But because you kind of overwhelm nature and the predators that normally go round eating these eggs of yours, some get through. 

The alternative choice, and this is a classic sort of set of alternatives in evolutionary biology, is you invest very heavily in your offspring. But if you’re going to do that you can only have a small number of offspring. And if you limit to having one at a time, you can be massive in your investment. 

Now if you want a big brain you have to have massive parental investment, in our case through a very long period of gestation, followed by lactation, or indeed—especially so for primates—through this long period of socialisation of childhood where you are learning, and perhaps being taught, the social skills you need.

Because it’s not good enough just to have a big computer in the brain, somehow you’ve got to put the software in there—and the software comes from learning through experience. 

CG: But we haven’t always had the concept of a teenager, when did that develop?

RD: No that’s probably quite recently. It’s kind of arguable as to whether there are teenagers in especially species like the apes, I would say. Most primates undergo puberty which is usually taken to be sort of the defining point for adulthood because that’s when females can start to reproduce. But they will still carry on growing after that. 

So you have this sort of seminal period, as it were, between the end of the juvenile period of puberty and full adulthood. Very often males don’t stop to reproduce until the end of that period, in fact even longer. Females get going pretty much straight away, so they’ll have their first babies, in monkeys and apes, while they’re effectively what we would think of as teenagers.

CG: And did that used to be the case for humans as well, were girls having babies at 12 and 13?

RD: Remember 12 and 13 for puberty now is a consequence mainly of our relatively good quality food, right. 

CG: So it’s earlier than it used to be, aha.

RD: Yes much earlier than in traditional societies where diet is a bit unpredictable and a bit rough and probably not so accessible. Now we process a lot of our food and make it easier to extract nutrients and it’s why we all end up getting overweight, basically. 

CG: It’s too easy to eat. 

RD: It’s too easy. Whereas if you’re sort of chewing on a bit of old bark, you can get nutrients out of it but you’ve got to eat an awful lot of bark to get the same amount of nutrients as you’d get out of a Mars Bar. So in small scale societies puberty tends to be much later, 15, 16, quite often.

CG: On lineage and the idea of the evolutionary or biological urge to think a few generations ahead to have great, great grandchildren — I’m in my late 30s and I don’t have that urge to have children that most women my age do, but I love being an aunty.

And I know this might have just been a single paper that you did, the one about how childless people tend to be more inclined to look after their nieces and nephews… 

RD: Gosh you’ve done your research

CG: Is there something in evolution where you need some people to not have children, within the 150 social circle?

RD: Not necessarily. This all goes back to what would be called “inclusive fitness” — Hamilton’s theory of inclusive fitness. 

Traditional views of the way the genetics of evolution works is that you pump babies out and—whether you’re a tree or a human it makes no odds—you try and produce offspring and try and do your best and ensure they grow up and can produce offspring in turn. Because if they’re not producing grandchildren for you it’s a waste.

CG: So it’s as important to make sure that the child you have is going to have a child, as it is to actually have the child in the first place?

RD: That’s right, so this is a general principle of evolution. 

In the 60s Hamilton came along and said, well, actually, when you think about it, what this process is about is not actually producing individuals, let’s say children or grandchildren, it’s about promoting the replication of your genes into future generations.

And where that’s the case because you share genes with your relatives, you can do that just as efficiently—in other words you can contribute to the future gene pool just as easily in principle—by promoting the successful reproduction of your close relatives according to how close they are.

CG: That’s an exciting idea to me [laughing]. So I could still play a role in the great, great grandchildren and the lineage by taking an interest in my nieces and nephews. Okay.

RD: Absolutely. So that was his explanation to explain why you get worker bees and you only get one queen because the workers who are all females, genetically, sacrifice reproduction. They become infertile but of course they work very busily to produce the sister’s, the Queen’s, offspring for her. 

Now the complication that comes in with humans is that we are capable of managing multi-generation time processes: we can see and understand what our grandchildren are, and we can try and invest in them. And particularly once you have heritable resources, like land that you can pass on, this then becomes a big issue in managing scarce resources. 

The classic case would be once you have agriculture, and you’ve got literally parcels of land that you can quote unquote “own” that you can pass on to your descendants, in due course, the problem you have to worry about is that if you keep splitting that land up into ever smaller parcels over each generation, eventually the parcels will get so small that they’re no longer capable of supporting a family.

CG: Wow, which we see happening in cities all over the world.

RD: It happens all the time. Throughout history, you can see this happening. 

The Tibetans provide an example of this because they’re living on relatively poor quality land high up in the Himalayas, and they really cannot afford to partition their farms into smaller bits.

What this causes is the parental generation then to manipulate their children in order to ensure that the family farm doesn’t get overloaded. And you see them doing this in all sorts of ways, which involve taking some children out of reproduction.

The way the Tibetans do this, for example, is to marry a girl to all their sons simultaneously. So it’s polyandry or in a few cases polyamory. The farm retains its shape and size because all the brothers farm it together.

CG: And there are only so many children she can have; she can only be pregnant for a certain amount of time.

RD: That’s right. Interestingly, for her, she gets serial husbands because obviously the youngest is like eight very often when he’s married and has limited interest. Whereas the oldest son will usually be the one that’s at reproductive age, as it were, maybe early 20s or something. So as he ages and gets less and less competent in this department his younger brothers come up.

CG: Amazing. So she’s always got someone in the prime of their sexuality. Wow.

RD: On the other hand, as is often the case, and if you talk to people like the polygamous Mormons, the sects of the Mormons that have retained polygamy, these polygamous or polyamorous marriages are very, very hard work, and they cause the women a lot of problems.

The downside of this system, of course, is that all of her sisters end up not being able to marry, and they become drudges in the family household. 

CG: What do you mean by drudges, sorry – they’re working?

RD: Kitchen maids, basically. And very often have a miserable life. So that’s the downside.

CG: So it serves an evolutionary purpose but it doesn’t mean that the life of those people is necessarily very good.

RD: Yes. That’s the parents trying to manage the system. And you see this all over the place.

We did an analysis of 19th century Irish farming families from one of the Irish counties and what they did there was ship their spare sons off into seminaries to get them out of the system because now the church is looking after them.

CG: Wow, and they’re celibate once they go in.

RD: Yes. They’ll usually typically keep a son to inherit the farm and a second son to help him out on the farm but any others it’s “into the seminary with you, boy”.

CG: We hear so much about nature/nurture and they’re really so connected aren’t they? All of those nurturing things, those shaping things and family dynamics, and all of those societal things, they come from what you have identified as being natural group sizes. Right.

RD: Yes. The analogy that works quite well, I think, is to think of it as a game of football or game of rugby. Somebody has drawn out the lines on the pitch, and the pitch has to be a certain size, and you have to have lines here, and some goals there, and there are some rules about what you can do and what you can’t do. 

And that’s, if you like, the genetic platform that we all have.

But then how you actually play the game on the day depends on who the opposition is, and the kind of moves you actually make and all that is environment and learned.  

Now you can have a major shift in a genetic platform, if you like. The obvious example of that, here, would be let’s have a round pitch and do Aussie Rules. 

CG: [laughing] That’s great.

RD: You can change the shape of the pitch and that, of course, changes the game and allows you to do something different. That might allow you to invade a new kind of habitat or something than if you stuck with the old pitch. That at least allows you to have it on Melbourne Cricket Ground. 

CG: [laughing]

RD: You know, I think that analogy kind of works well. It just kind of reminds us that there are some things that are givens, that come with us and we can’t do much about them, but a lot of the rest of the stuff is highly flexible.

That’s the whole point of having a big brain and the whole point of our parents investing this huge amount of time—first with gestation, and then with lactation, and then with childhood—in producing this offspring with this massive brain that’s capable of making these moment by moment tweaks of how to get the best out of situations. 

CG: So there’s some agency. It’s not all pre-determined for us. 

RD: Absolutely not. 

Endorphins and social bonding

Christine Gallagher: We are running out of time.  It would be good to hear about endorphins and social bonding, if you don’t mind, and how that connects. We have touch, dance, singing, laughter, all of those sorts of things. 

Robin Dunbar: So the big issue, in the end, is how we and the monkeys and apes live in these bonded social systems. And that’s how we create these implicit social contracts as we have these deep bonded relationships with each other that prevail on us to do these favours for each other. 

And the question is, how do they create that. 

Primates use social grooming, and what social grooming does is trigger the release of endorphins in the brain.

Turns out we have a special set of neurons, all mammals do, not just primates, which respond only to light slow stroking on their hairy skin. You don’t have them on the bare hands and soles of the feet, it’s only on the hairy skin.

They’re very different neurons to all the other neurons that go from your skin to the brain, the ones that tell you about pain and sensations and all of that kind of stuff. They’re very, very slow and they’re one way only but they kick the endorphins system in.

And the endorphins are an opiate so they make you feel kind of woozy and happy and contented with whoever you’re doing it with, and this creates an intense bond when you do enough of it. 

CG: And is that why you would feel more attached to either your mother or a parent who has done that while you were young, or a lover because of the soft touching, slow stroking, and all of that kind of thing?

RD: Yes, absolutely so. And in fact, we have shown for humans that this light stroking actually does kick the endorphin system and floods the brain with endorphins.  

Now the problem with grooming is it’s very time intensive, because back to this idea that the strength of your relationship depends on how much time you invest in it.

And the key to that time investment is actually how much time, for monkeys, they spend grooming each other, stroking each other, and building up this endorphin flood. 

The problem with that is because time is limited in real life there are only so many individuals you can groom at that level of intensity and that limits group size for them. And it limits group size to about 50 in monkeys and apes.

So when we evolved into much bigger groupings—up to 150 during the course of human evolution—we needed to break through that glass ceiling and find other ways of triggering the endorphin system. 

CG: Because we can’t go around touching each other all day long. 

RD: Well no, exactly.

CG: We don’t have time. It’s not practical.

RD: You’ve got to spend time finding food and resting, and all this kind of stuff, during which time you can’t be grooming or cuddling. 

But it reminds us that this process is still important for us. And it’s still a very intimate one-on-one ratio. That’s the problem with grooming, it’s a one-on-one activity, and it still is with us.

Physical touch is so intimate. And indeed, how much and where we allow people to touch us is directly related to the emotional intimacy of our relationship with them.

So the problem then really is, well, the only way you can break through that glass ceiling is to be able to do it with more people simultaneously. Because you can’t invest more time.

CG: As in, find ways to release endorphins as a group rather than between two people. 

RD: That’s right, yeah. 

CG: Which is where the singing and dancing and laughter comes in?

RD: And the sequence seems to have been that laughter kicked in first.

But laughter turns out—despite what we think when we go to stand up comedy shows—laughter turns out to be really very constrained in the size of the group in which you get laughter: it’s actually only about three people.

When people laugh only about three people do it together. No matter how many people are around the table you will see only about three people are ever laughing.

CG: Another interesting number.

RD: Yes, because you laugh as well.

So what happens with the grooming is it’s only the receiver of grooming that gets the hit and you as the giver don’t get it.

But when you laugh, because of the way we’ve kind of changed how we laugh and the way it’s done in monkeys and apes generally, especially apes, it makes it much more stressful on the body and kicks the endorphins out.

But because you laugh, as well as the people you’re talking to, it increases the efficiency.

All three of you in the laughter group conversation get the hit. So now it’s three times more efficient than grooming is, so that allows you to increase group size.

Singing and dancing then come in later. Singing without words, chorus singing if you like, and I suppose if you’re doing line dancing you can have an infinite number of people all doing the same step.  

CG: [laughing] and is that important, that you’re moving in the same way. 

RD: Yes, behavioural synchrony turns out to ramp up the endorphin hit for reasons which we don’t actually understand. But it doubles it, for no extra effort. 

For both singing and dancing there probably is ultimately a limit somewhere on the number of people you can get perfectly synchronised. And I think if you had a dance line of 100 people you’d probably find the ends were not entirely synchronised.

CG: But at a dance party you can have lots of people, not in a line but all together, say jumping up and down to a band or something

RD: Yes. So we’ve actually got a study on that, which we did last year, in the pubs and dance clubs around Oxford. We compared conversation groups in pubs and dance groups in clubs, so free dancing, as opposed to organised square dancing or line dancing.

And it turns out that the number of people you can dance with simultaneously is exactly the same as the number of people you can have a conversation with simultaneously. 

CG: That’s amazing !

RD: And that’s because the limit is set by the number of other minds you can manage simultaneously, and that’s about four other minds.

CG: So if I put myself in a crowd of 1000 people all dancing, I’m only going to actually feel like I’m dancing with four or five people around me. 

RD: Well three or four. 

CG: Three of four, sorry, because I’m one of them.

RD: Yes. But the advantage that dancing has over conversation is that the problem with a conversation is it depends on the stream of thought, and you can’t dip in and dip out of conversation as once you’re there you’ve got to keep going with it to see where the conversation is going. 

CG: You’ve got to invest in that. 

RD: You’ve got to invest. And you never know when the laugh is going to come. And that’s why we use laughter so much, or jokes so much in conversations.

With dancing you’re getting the hit from the dancing and the synchrony of behaviour. That’s why you kind of synchronise with somebody when you turn to them. 

CG: Start to move in the same way, mirror each other. Yeah. 

RD: Now the problem is you can only handle so many people simultaneously, but in a freeform dancing environment you can move constantly and change your group so your total dance group is typically about eight. So you can double the number of people you have a little shot with. 

CG: Oh because you can turn. You can be dancing with someone over here and then turn away and dance with someone over there.  

RD: And if you actually count the number of people that during the course of a given dance, the length of a record realistically, typically it’s about seven or eight people that you actually have a shot with in the course of that particular dance. Whereas for a conversation, you’d be limited for that period of time to just four. 

CG: And does that apply to singing as well? Because you can sing in a group, does the group size make a difference when you’re having a sing along?

RD:  We’ve sort of tried to look at this. But I have to say we’ve done this with choral groups.

Now it seems that you can have very large numbers of people involved and get the same hit up to several hundred as you would get in say a group of ten.

But I suspect that’s because choral singing is highly structured and highly organised whereas if it was singing around the campfire probably the number would come down—a bit like dancing—and it’s who you can catch the eye of…

CG: To feel like you’re singing together 

RD: Yeah. And that probably is limited to the three or four or maybe five people immediately around you.

CG: This is fascinating. I know we are out of time. Thank you so much. It’s so fascinating.

Romantic relationships and more on reproduction

Robin Dunbar: Do you want to do romantic relationships?

Christine Gallagher: Yes, if we could. We did family, we did friendships didn’t we.

How does what we’ve spoken about apply to romantic relationships? Or is it something different when we talk about romantic relationships?

RD: I think lots of the same things apply but in a sense romantic relationships are slightly left field to the rest because they have another interest, ultimately.

Ultimately they were evolved to produce offspring—that was their purpose, if you like, from an evolutionary point of view.

Romantic relationships have that other interest and again the problem is evolution can’t work directly through the mind, so although it operates on the basis of the number of great, great grandchildren you produce even the human mind is not terribly good at thinking that far ahead. 

So evolution always operates through the emotional system and the motivational system and tunes that to get it right such that if you do that bit right it will produce these long term consequences of producing great, great grandchildren and so on. 

So that whole romantic machinery that we think of as falling in love is simply evolution’s way of getting the relationship functionally to the right end of producing offspring. Which of course since you are focused on the immediate interest of it—let’s say the sex—you can actually cut through the long term evolutionary processes and not produce offspring.

CG: Right, because a lot of people make efforts to not get pregnant because they like having sex but don’t want to have children yet.

RD: That’s right. Yeah. At the end of the day you have a huge amount of individual variation in the way people have tweaked on those things. So in one sense we’re driven by the sex component, but also at the other end of the stick there’s this kind of interest in and liking for offspring. 

So there’s a moment that comes very often, and not always but very often, where people go, “Yeah, well, maybe we should think about having an offspring as well”.

And then it is this sort of very complex trade off, as I said before, because you don’t have to produce offspring to contribute genes to the next generation but it’s an efficient way of doing it.

CG: So even if I’m not going to have children those drivers will still be there.

RD: Yes, they may or may not. The problem in the end—especially for women but also somewhat for men too, of course—is there’s a time line on this.

CG: Right, the biological clock.

RD: The biological clock.

Anyway in the end some people clearly push it too far and they go: “Oh, my God, I wish I hadn’t”. That’s not necessarily everybody, of course.

But all I’m trying to get around to saying is that the romantic relationships part of this system is just much more complex because partly it’s about friendship and companionship as well… 

CG: So do those shared traits and all of those kinds of things still apply?

RD: Yes, all of those kinds of things still apply. 

And probably relationships that last better are the ones where the couple have more in common or—because these are cultural—grow to have more in common.  

Now it may be that one sex gives away more than the other sex in the interest of making this work better, and there are all sorts of reasons for thinking that women will do that rather than men.

CG: As in, take on their partner’s interests and cultural values?

RD: Yep. They will adapt more to their partner’s kind of interests and so on. 

One is simply that girls are much more skilled socially than boys and that may be because they need to be, because actually they’re the ones that are driving this kind of intense relationship. And you can see some evidence of that, in humans in particular, that the whole romantic market is driven primarily by women’s interests. 

The problem is it’s a very complex choice they’re making, much more complex than the choice men make, because women are trying to trade off a number of different dimensions in which you can’t get a perfect solution because they’re incompatible.

At the very root of it in bare evolutionary terms, what they’re trying to find is somebody who’s got good genes mixed with their own.

Because the problem is that here you are with this perfect set of genes inherited from your parents, which you promptly split up with every kid and mix with somebody else’s, maybe less good genes. So what you’re trying to do is to find a partner whose share of genes are actually going to be…

CG: equally good if not better than yours.

RD: Yes but also because there’s a long period of parental investment, you need somebody who, kind of, doesn’t have to do the nappy changing but at least has to be interested, let’s say, and therefore also has to have some kind of commitment to the relationships in general.

CG: So you need for them to be invested to stay around for the child for at least the first 18 years or something like that.

RD: And on top of that because it’s so investment dependent having a partner who can put a lot of investment on the table functionally—in terms of modern societies or post agricultural societies—where you’ve actually got material such as farmland or for us obviously jobs, money in the bank, all these kind of things… 

CG: So the idea that someone is a good provider has an evolutionary purpose.

RD: Yes. Women are making this kind of very complex trade off between these various dimensions none of which can be maximised. The best you can do is optimise, or satisfy as the economist would call it, or as evolutionary biologists call it “Hobson’s choice strategy”. 

You cannot get the best of all possible worlds because these various dimensions pull against each other. If you’ve got lots of resources you’re probably not very reliable as a partner.

And it’s because it’s a market, in the end, that males know that if they’re very high status and have lots of money, which is an attractive component to women, they can find lots of women who will be interested in them so they’ll then tend to be less reliable as long term partners.

So you decide what’s in your best interest and make your choices accordingly.

And also because women, and this is true of all mammals, have much more at stake because they carry the burden of pregnancy, this is a long investment.

After fertilisation men can walk out but once you’ve fertilised an egg then female mammals in general have this long commitment ahead of them and if they pull out on that they’re wasting effort and resources. So, in the end it always comes back to the fact that they’ve got much more at stake.

CG: Well I know we’re out of time. Thank you so much. I will go away and think about who are my closest five, who’s my inner core, who may be slipping in and out of it, and how I can maybe trigger some endorphins for bonding with those people. And it’s absolutely fascinating. I’m sure we could go on and on. But I thank you so much for your time. It’s been great. Thank you. 

RD: It’s a pleasure.

Concluding remarks

Christine Gallagher: That was my conversation with Professor Robin Dunbar from Oxford University. Amazing work he does. Thank you very much Professor Dunbar for your time today. I know you had back to back meetings. I thought that was very generous of you. 

There’s so much more we could have discussed but I think this works as an introduction to Professor Dunbar’s work.  You can find him online. I think there are at least two TED talks and other clips and videos and interviews if you want to hear more. And as I said, he has a tonne of publications which are readable for non scientists, which is so refreshing.

I particularly like his book The Science of Love which I would have liked to discuss more but just didn’t quite have time to get to it. I’d recommend that to you if you’re interested in evolutionary explanations for attraction, attachment, falling in love, and the pain of separation and rejection. 

And speaking of pain, I think also some of Professor Dunbar’s work around endorphins is on the changing pain thresholds after endorphins are released which is another interesting area of his research. 

As I said at the beginning, I find evolutionary explanations for love challenging and I guess my previous views have been based not on scientific evidence and only on personal experiences. 

My idea of love has been something more like an energy which I’ve thought to be some kind of universal energy which connects people or through which people can share an experience together. 

And whilst I’m not religious, and I don’t believe in a god, I’ve thought maybe that’s what an understanding of God was before religious narratives were told and written down and used to establish churches and so on, and the equivalents for non western religions. I thought maybe this energy of love is what kind of powered empathy and understanding and human connections. 

But I’ve also been aware of scientific explanations about brain chemicals or hormones, which create a sense of love. So it is interesting to learn about Professor Dunbar’s findings about endorphins and bonding. Most of us have, I guess, some lived experiences which we can relate to those ideas. And that seems like fairly positive knowledge to have.

I wonder whether the findings with group bonding over singing and dancing and laughing, if that knowledge could be—or is being used—to treat depression. It seems kind of an obvious connection. Even if not formally, I guess, if you’re someone who’s quite isolated or lonely you could join a choir or line dancing group or something similar to get your endorphin hits and social bonding.

And I imagine the knowledge about group sizes informs all sorts of organisational management in businesses, sports clubs, the military even, maybe even the design of some of those online social network sites. 

Such incredible potential impact of Professor Dunbar’s work.

Another question I thought of on my walk home after meeting Professor Dunbar is about whether anyone has studied a correlational connection between his group size theory and the size of weddings. How many guests on average are invited to weddings?

And that got me thinking about how people’s social groups are affected by having a long term partner.

Would both people in a couple have 150 people each, or would they end up with pretty much the same 150 people with maybe just some differences in the makeup of their inner circles. Because, I guess, your in-laws and your spouse’s best friends end up in your 150 as you socialise with them fairly regularly and that would then maybe push out others who had been in your 150 because of your limited time for maintaining all of those relationships. 

Anyway, fascinating stuff, and food for thought. I enjoyed meeting Professor Dunbar and it’s always a good sign when you leave a conversation stimulated by ideas and thoughts. He’s a good communicator and a very nice person who made me feel very welcome. 

I know we could have had an interesting discussion about Professor Dunbar’s life and career. He said he spent his early years in Australia in Kalgoorlie which is famously a gold mining town and part of Australia’s Gold Rush history. I would love to hear what it was like growing up in what is pretty much the Australian Outback and about his pathway to becoming a world famous Oxford professor. So maybe I’ll have to see if Professor Dunbar would be up for part two sometime in the future. 

Okay, that’s about it for me. Do get in touch if you have thoughts, comments, criticisms. The email address is

You can subscribe to the podcast on iTunes, and now on Stitcher for Android users, just search for “wide open air exchange” on either of those, and I’ll see you next time.


2 thoughts on “Transcript: Dunbar’s Number and social bonding

Leave a Reply

Your email address will not be published. Required fields are marked *