m 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 extraordinarily rare and somewhat arbitrary. If you're a social media strategist or a digital advertising agency, basing a marketing or other strategy on triggering a social epidemic is probably not the best use of your resources.
You are also investigating the nature of cooperation between people. How is that related to the spread of ideas?
The spread of information and the spread of cooperation are both examples of social influence, so they're related, at least in principle. But it's also important to understand that different types of influence are likely to spread in different ways. Me persuading you to change your political views is very different from me persuading you to click on a video.
People do not easily adopt ideas that involve changing their conception of themselves. Not accepting the idea of climate change might be tied up with your political ideology, your suspicion of government and your dislike of elite intellectual types. When is an idea something that you can adopt just because it's obviously right or obviously interesting? And when is it something that is going to be very difficult for you to adopt because it's tied up with all these other things?
What are the stakes?
This is not about cat videos going viral, even if sometimes that's what we study because it's what we have the most data about. Really, it's about changing people's minds, and their behavior; and one way or another, everyone – governments, corporations, marketers, policymakers – is in the business of trying to change people's minds. Understanding how that happens is one of the big questions of social science. It's frustrating that we've been thinking about this for so long and have very little in the way of concrete answers. But maybe this particular period of history is different. That's the hope.
Duncan Watts is a principal researcher at Microsoft Research NYC and an A. D. White Professor-at-Large at Cornell University. From 2000 to 2007, he was at Columbia University, then at Yahoo! Research. He is the author of "Everything is Obvious