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Question: How have human social networks changed over the millennia?

Nicholas Christakis: Well I mean for thousands of years people have been concerned with very basic questions about how and why humans form societies, how and why people form groups, but the social networks are different than groups in that in addition to a collection of individuals a network has a specific set of ties that you add to the individuals. And not just ties, but a specific pattern of ties. So, for example... the simplest example of this would be if you take a group of a hundred people that are waiting in line to get into a theater, for example. That’s a group of people, but it’s not a network. If you assemble those people into the simplest possible network, a linear network, like a bucket brigade to put out a fire for example now you have these hundred people and you’ve added 99 ties between these people, so and a specific pattern of ties. Each individual is connected to one individual on the left and one on the right. And now this network is capable of doing something which the group was not capable of doing, namely, putting out a fire rather more efficiently than a group of people. Or you could take the same group of people and take the same 99 ties, but organize them totally differently in where each individual now... in the form of a telephone tree for example, so each person calls two people, so you take the first person. They call two people. Each of those two people call two people and then you would get a completely different sort of branching pattern. Now instead of a linear network you have a more complicated network. 

In fact, the same kind of structure, archetypical structure was used by Bernie Madoff in a kind of a Ponzi scheme, but instead of distributing information outwards, money was sucked up and drawn inwards towards the center, so these would be artificial human networks. They have constituent individuals and they also have a specific pattern of ties. You add something more to the individuals, these ties. And in fact, as we argue in the book it’s the addition of these ties that makes the whole greater than the sum of its parts. It’s the addition of the ties that makes the population of people within it, the network, capable of doing things like putting out a fire or distributing information rapidly in the telephone tree example, that it wasn’t previously able to do. So a network of people is a collection of individuals and a collection of ties between them and a specific pattern of ties at that.

One of the key ideas about human social networks is that in the addition of ties between people and specific patterns of ties that obey particular mathematical rules the whole becomes greater than the sum of its parts. The collection of human beings have properties that do not reside within the individuals, and this collection of human beings is now able to do things that they previously were not able to do. And one of the illustrations or examples that I most like to give about this is something that most people are familiar with from high school or college chemistry and that is the example of carbon. So you can take carbon atoms and you can assemble the carbon atoms into graphite and here we put particular hexagonal pattern of ties and you get sheets of graphite and this graphite is soft and dark. Or we can take the same carbon atoms and assemble them differently into a kind of a perimetral structure with the ties between them, the bonds between the carbon atoms and we get diamond, which is hard and clear and these properties of softness and darkness or hardness and clearness first of all differ dramatically, not because the carbon is different. The carbon is the same in both, but rather because of the ties between the carbon atoms. And second these properties are not properties of the carbon atoms. They’re properties of the group, properties of the collection of carbon atoms. Therefore, when we take constituent elements and assemble them to a larger whole, this larger whole can have properties that we could not have foreseen merely by studying the individual elements and properties which do not reside within the individual elements.

And the same thing happens with human beings. We can take human beings and assemble them in different patterns and depending on the pattern in which we assemble human beings they have properties that we could not have understood just by studying humans. For instance, individual human psychology is not enough to understand some of these bigger properties and second these individuals depending on how they’re assembled can have different properties, so you take the same group of people and you assemble them one way and you get a bucket brigade, which has particular properties or you assemble them a different way and you get a telephone tree, which has yet again different properties. And so the pattern of ties between individual people is actually a kind of a resource that we all can use. It’s actually a reservoir of value. It’s a kind of social capital, actually.

And it’s not just the pattern of ties between people that matters.  It’s also what is flowing across those ties, so if you inhabit a network with a particular structure of ties, but it’s a trusting network versus a mistrustful network it has different implications for your life, or if you’re inhabiting a network where a pathogen is spreading versus where a pathogen is not spreading—different implications for your life. Something is spreading through the network. You’re connected to others and it affects you.

And we have been looking, James Fowler and I, at a variety of sort of counter intuitive examples of these kinds of phenomenon. For instance, we’ve looked at how things like obesity or your emotions, like happiness, spread through human networks. And we find that a lot of deeply personal things, things that people might not think of as being under the influence of others are affected, not just by their friends, but by their friends’ friends and even their friend's friend's friends. So people are used to think of things like fashions, for example. Like their taste in music or clothes might be affected by their friends or perhaps even they have this image that fashions can spread through the network or people might be used to thinking that germs, that right now they’re not sick, but their friend's friend's friend has a germ and that germ is going to spread to their friend’s friend and then to their friend and then eventually inexorably to them, but what they may not realize is that other sorts of phenomenon like who they vote for or how big their body is or even how happy they are also can behave in similar ways, and that is what James and I have been working on trying to understand over the last few years.

Recorded March 31, 2010
Interviewed by Austin Allen

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