Can you catch an idea like you catch a cold? The language of business often borrows terms from epidemiology like viral marketing. But does information actually move through social networks like a contagious disease?

In their article, “Tracing information flow on a global scale using Internet chain-letter data,” appearing in the March 25 issue of Proceedings of the National Academy of Sciences, computer scientists David Liben-Nowell and Jon Kleinberg investigate the path of political chain letters. One petition attempted to organize opposition to the impending war in Iraq; the other, was a petition to continue public support for National Public Radio (NPR) and the Public Broadcasting System (PBS). Perhaps you received one of these? If so, you were asked to append your name to the bottom of the list and forward to as many friends as possible. Even if most of the email messages went into the trash, a few would get through and continue their travels.

In computer science parlance, the first person to send the email petition is the root, and an individual who signs the bottom is a child. If this person then forwards the email, she becomes a node. By analyzing multiple copies of these chain-letters, one can understand the paths they take.
For example:

A simple binary tree of size 9 and height 3, w...Image via Wikipedia

If chain-letters functioned like epidemics, we would see short paths and many nodes. But this is not what the researchers found. The researchers report that the median path length was nearly 300 long, and over 90% of the nodes have only one child — more like a weeping willow tree than a dandelion gone to seed. You can see a diagram of their tree structure. The authors report:

the structure of a small world, in which most people are connected by short paths, need not be at odds with a world in which an antiwar appeal, embedded in an e-mail chain letter, can pass through several hundred intermediaries before arriving in one’s inbox.

In other words, the spread of information is much more complex than the spread of a virus, and the concepts of epidemiology don’t always match well to the flow of information in social networks. Now if I could only get rid of this summer cold … (ah choo!)

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Phil Davis

Phil Davis

Phil Davis is a publishing consultant specializing in the statistical analysis of citation, readership, publication and survey data. He has a Ph.D. in science communication from Cornell University (2010), extensive experience as a science librarian (1995-2006) and was trained as a life scientist.