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

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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