What's Your Christmas Card List Got to Do With the Development of the Human Brain?


It is safe to say that our primate ancestors and the early humans they begot never picked out sparkly snowflake paper, wrote up a missive about Og and Jane's many achievements in the last cycle and handed out copies to all their friends, relations and hunt/gather coworkers.

But, according to anthropologist Robin Dunbar, Ph.D., the social relationships that were forged during the dawn of humanity still influence everything from Christmas card lists to Facebook networks. I saw Dunbar lecture at the 2008 Nobel Conference in Minnesota, and called him recently to find out more. Dunbar, head of the Institute of Social and Cultural Anthropology at the University of Oxford, says the size of the human neocortex puts a limit on the size of our social networks--a limit that can be seen in examples throughout history.

The discovery has its origin in studies that compared the size of non-human primate social groups with the animals' brain size. The idea is that larger social networks are good things: Offering physical protection against enemies, shared strength and ingenuity to accomplish difficult tasks and a safety net in case you, personally, don't hunt or gather up enough food. But managing those networks takes brain power. If you don't have enough, your clique can't ever get very big.

But say you're the one guy primate with a slightly larger brain and, thus, slightly bigger social network. You'd have a better chance of surviving adverse conditions. And, you'd have a better chance of meeting women who'd be interested in your monkey butt. The fact that a larger brain means a larger social network was probably one of the evolutionary pressures that turned humans into the big-brained species we are today, Dunbar said.

In the early 1990s, Dunbar applied the ratio between primate brain size and social network size to modern humans. By his calculations, 150 people is about the largest social network each human can maintain. You might know more folks than that, but the 150 will be the ones you really have an important relationship with--the ones you really care about.

And this is where things get kind of freaky. To verify his idea, Dunbar started looking at the size of documented social networks throughout human history. He found 150-person groups all over the place: It's the size of traditional villages in England prior to the Industrial Revolution; the size of religious communities; and the size of basic military units. And then there's the Christmas card lists.

Christmas cards are a big deal here in the UK, more than in America," he says. "It's expensive and so you think carefully about who you want to send your cards to. We found that the average list is typically about 150 people. There might be fewer households than that, but if you add up the people each household represents, you get 150."

In fact, the 150 limit is so pervasive that if you have a large, close-knit, extended family, you'll likely have fewer non-relative friends, Dunbar said. One way or another, he found that the size of a person's network balances out to be, roughly, Dunbar's Number.

The other really interesting thing: Dunbar's Number seems to also represent the invention of the "friend". Recently, Dunbar and his colleagues started looking at the brain size and social networks outside of the primate realm. They found that, in non-primates, large brain size is correlated with species that form groups of just two--monogamous pair-bonds that mate for life. Anyone who's married can tell you that maintaining a relationship takes a lot of brain power. But why, then, do we see a different pattern in how primates use that power compared to these other animals?

What we think primates have done is that, early on in evolutionary history, they've taken the same machinery that builds pair-bond relationships and used it to create friendships," Dunbar said. "The machinery is there to allow you to build these deep relationships and it's simply a question of who and how many you apply that to.

Watch Robin Dunbar's presentation from the 2008 Nobel Conference

Image courtesy Flickr user tiswango, via CC


  1. I read about someone who studied how people act towards each other vs community size. He studied towns in africa and found that people regularly greeted each other up to a certain population (don’t recall that value) and when it got bigger they stopped.

    I’ve noticed myself doing the same thing on trails when hiking. I’ll greet folks regularly (and so do they) until the number of people on the trail gets too dense then I stop. Seems like there is a burn-out factor.

    1. This is analogous to the “Texas 2 finger Salute” – when approached by an oncoming vehicle (typically pickup) on a lonely 2-lane highway, it is customary to raise the two fingers you have draped over the steering wheel in salutation to the other driver. If there are too many cars on the road, one will skip this courtesy.

  2. “a safety net in case you, personally, don’t hunt or gather up enough food”

    Neanderthal socialism. Of course libertarians are gonna claim we evolved past that stage. I just find it comforting that taking care of one in another, in a community, is very human and has been with us from the beginning.

    1. Oddly enough, I think the Libertarians are probably doing the exact same thing– that 150-member “tribe” of family and close social contacts still gets the benefit of the doubt, shared resources, etc.
      Even an ardent Libertarian probably wouldn’t charge his own kid for lunch, or turn on a taxi meter when giving his best friend a ride.

      It’s how you interact with the folks _outside_ that tribe that makes the difference between the socialist (hey, let’s treat them like they were part of our tribe) and the libertarian (Outside of the tribe? Screw ’em).

  3. Libertarians have no problem with communities and social sharing. It’s the means by which individuals are compelled to be a part of a particular community, and the means by which those communities are empowered to enforce their will on the individual, that can define tyranny.

  4. But ants have social networks with millions of members! This superior intelligence is why I, for one, welcome our new insect overlords.

  5. I have to admit the concept of the fact I had a very large family growing up might have contributed to me having less adult friends fascinating. Both my sister and I have 4-5 really close friends, but other people we know seem to have a lot more. I’ve always wondered about that.
    My mother on the other hand was an only child, and when ALL of her friends come over, they have to do serious planning. She regularly speaks to at least three times the number we do.

  6. Interesting. ^^
    But 150???
    Come one … if I add up all the people, I have ever known in my entire life, I won’t get a total count of 150! :D

  7. I am sorry for Anonymous #15, I have over 700 friends on Facebook. I have a handful I have never met, but am connected to through my college.

    On the other hand, I have very few close friends. I know lots of people, but I prefer to keep company with only a few.

    I remember reading somewhere about a person telling a friend, “I currently have the 15 friends I need, but when I have an opening I will let you know.”

  8. i for one, have exactly 100 facebook contacts. but that’s a pretty arbitrary number. i cut out everyone who i accepted, but i don’t really know them, or i didn’t really like them the short length we spent in school/work. and then about 95/100 of those *friends* are acquaintances, or people i might say hi to but not much else.

    i really only have a handful of close friends and that’s it. small family, too. maybe i’m just an antisocial socialist.

  9. Fiddle faddle. I clearly have processing power above and beyond that of an average chimpanzee as evidenced by my ability to type. However, I form absolutely no relationships with other entities (sapient or otherwise). Explain that, Mr. Scientist.

  10. This concept struck me as either dead-on, or a weird coincidence. For the first years after we got married, my wife and I limited our Christmas card list to a maximum of 120 addresses, which was four pages of 30-count Avery address labels. Just seemed like a good number, and it included all the “important” people in our lives. Each year would always bring in a small number of new addresses, but we could almost always manage to edit away more or less the same number, to keep it at 120. (Seemed sort of like a waste to “spill over” into only two or three labels of a new Avery sheet–but I admit it was a totally artificial limit.) But when we became parents, somehow the list swelled a bit, and edits became much more difficult to make (unless someone actually died). So we added a fifth 30-count Avery sheet to our psychological boundary, meaning that for the past few years our Christmas card list has been settled comfortably at exactly 150.

  11. I wouldn;t say ‘crap’ as one other poster here did becuase in the interests of making 10 friends I have tried to curtail my swearing. I’m not good at reading how to put stuff together but visually .. whizz band(That was apropos of nothing.. but maybe there out there there are a coupe of people who might make up my aim of 10 friends?)I wonder did the primates think of singing and music as they saw it as a way of making more friends. And do Rock Concerts make us friends?

  12. I wouldn;t say ‘crap’ as one other poster here did becuase in the interests of making 10 friends I have tried to curtail my swearing. I’m not good at reading how to put stuff together but visually .. whizz band(That was apropos of nothing.. but maybe there out there there are a coupe of people who might make up my aim of 10 friends?)I wonder did the primates think of singing and music as they saw it as a way of making more friends. And do Rock Concerts make us friends?

  13. and hey, no one mentioned the size of an average wedding… anywhere between 150 and 300… which is coincidentally 150 times two.

  14. Improvements in communication technology have made it easier to maintain large social networks, e.g. the telegraph, telephone, email and facebook.

    I have more than 150 friends, and had more than that two years ago when I was at Unviersity. But there’s no way I could talk to all of them without Facebook, skype and email. (this is proper friends, not facebook friends, which is around 350)

    So, I think this theory might be fine, but does not account for the generation that has grown up with facebook. Imagine in 30 years when people who were 15 when Facebook launched will be 45 and could have gone through school, university, a career, hobbies, armed forces service, parenting and more. They could easily maintain a over 150 friendships.

  15. So, I generally do not like people (most are idiots) and limit my “inner circle” to… well… me — does that mean that I have a small brain, and in fact, an idiot myself?

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