n doing that for a long time in marketing. And it's been popularized in recent years by people like Malcolm Gladwell, author of "The Tipping Point," who very explicitly draws the analogy between the spread of behaviors or beliefs and diseases.
That sounds reasonable.
At a certain level, it is. But it's tempting to go a step further and apply the same mathematical models that have been developed to understand the spread of a disease to the spread of ideas or products.
What's wrong with that?
There are all sorts of models of how things spread, and they're often incompatible with each other. Moreover, we have very little data to test any of these models. For example, if you want to trace the spread of an idea, you have to be able to observe that person A has that idea, and then that person tells B, and then person B has the idea, and now person B tells person C. Mapping this out in a population of millions of people with hundreds of thousands and millions of things to observe is a tremendously difficult process.
So how do you approach it?
We've done a lot of work using Twitter data – news, media, images - where we can, to some approximation, observe everything. And we can also track exactly where everything is at a given point. People who study diffusion are generally looking for a critical threshold where ideas go from not spreading to spreading like wildfire.
And what have you found?
Initially, we found that nothing really spreads like that. In a study of millions of observations, 99 percent of everything that happened, every "adoption" of an idea, was within one degree of the source. Which is almost the opposite of spreading. In other words, I tweet something and you re-tweet it. Your followers see your re-tweet, but they don't do anything. It's the re-tweet that we pay attention to. When we described this result to our colleagues, however, they often didn't like it.
People are convinced that certain things "go viral," and that's what interests them. So they would say, "We know some things spread. Look at ‘Gangnam Style' or the use of Hotmail or Facebook. Maybe you didn't see anything spread, but you must only have looked at things that weren't any good." So then we did a second study where we really looked at everything on Twitter – a billion observations – for an entire year. And sure enough, we found that some things do spread quite a lot. But they are very rare, one in a million events. And even they don't look like "social epidemics."
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Why not? Surely "Gangnam Style" is a social epidemic?
Epidemic models assume that ideas become popular by viral word of mouth. But in the media world, information can also spread because some major channel or site picks it up – it gets on the front page of Yahoo! or the "New York Times" or whatever – and a million people or a hundred million people see it. And a bunch of them re-tweet it. That would still be popular, but we wouldn't say it had spread virally. So when something becomes popular, is it a "broadcast" or is it "viral"? Intuitively, you might guess one or the other. But when we looked, we found tremendous diversity: some popular things are pure broadcasts, and some display pure viral spreading. We also found about every conceivable mixture of the two. There's no typical way in which things become popular.
What do your findings tell you?
Unlike diseases, where in general you only have one thing spreading at a time, in ideas, it's always a contest. Everything that's spreading on Twitter is fighting for oxygen with everything else that's spreading on Twitter. It's hard to be exposed to one particular idea because there's just so much other stuff to pay attention to. Occasionally something is able to rise above the noise, and everybody hears about it and pays attention to it. But that is extraordi