The Chemistry of Social Networks

Question: How have human social networks changed over the \r\nmillennia?

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

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

One of the key ideas about human social \r\nnetworks is that in the addition of ties between people and specific \r\npatterns of ties that obey particular mathematical rules the whole \r\nbecomes greater than the sum of its parts. The collection of human \r\nbeings have properties that do not reside within the individuals, and \r\nthis collection of human beings is now able to do things that they \r\npreviously were not able to do. And one of the illustrations or examples\r\n that I most like to give about this is something that most people are \r\nfamiliar with from high school or college chemistry and that is the \r\nexample of carbon. So you can take carbon atoms and you can assemble the\r\n carbon atoms into graphite and here we put particular hexagonal pattern\r\n of ties and you get sheets of graphite and this graphite is soft and \r\ndark. Or we can take the same carbon atoms and assemble them differently\r\n into a kind of a perimetral structure with the ties between them, the \r\nbonds between the carbon atoms and we get diamond, which is hard and \r\nclear and these properties of softness and darkness or hardness and \r\nclearness first of all differ dramatically, not because the carbon is \r\ndifferent. The carbon is the same in both, but rather because of the \r\nties between the carbon atoms. And second these properties are not \r\nproperties of the carbon atoms. They’re properties of the group, \r\nproperties of the collection of carbon atoms. Therefore, when we take \r\nconstituent elements and assemble them to a larger whole, this larger \r\nwhole can have properties that we could not have foreseen merely by \r\nstudying the individual elements and properties which do not reside \r\nwithin the individual elements.

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

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

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

Recorded March 31, 2010
\r\nInterviewed by Austin Allen