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How 'Contagion' Became Contagious

Do ideas and emotions really spread like a virus?
(Photo: Marijus Auruskevicius/Shutterstock)

(Photo: Marijus Auruskevicius/Shutterstock)

Facebook wound up in hot water last month after revealing it had drawn on its own users as unwitting test subjects in an experiment on “emotional contagion.” Legal and ethical hand-wringing ensued, but it seems to have blown over as quickly as it blew up. Apart from a rare “expression of concern” from Proceedings of the National Academy of Science, the journal that published the research, it was just another day at the office for those who work at the intersection of science, technology, and the Internet.

Still, that word, ”contagion.” Reporters and researchers alike seem to have caught the contagion bug of late—whether it’s emotions, obesity, viral videos, or viral marketing, the epidemiological metaphors are rife. So where did that idea come from? Is contagion even the right metaphor—do emotions really spread like viruses? And why are we so keen on viruses, anyway?

WHILE THE PHRASE "EMOTIONAL contagion” originated in late 19th century, the idea dates back at least to the publication of Goethe's The Sorrows of Young Werther, which may or may not have set off a wave of suicides in 18th-century Europe. The concern surrounding the book reflected a contemporary fascination with germs and disease, and by the 1890s German philosopher Max Scheler had coined the term we still use today.

Social scientists didn’t pick up the idea until the 1950s, but it wasn’t long before they found evidence of contagion in everything from emotions to political preferences, effects that researchers often attribute to our unconscious tendency to imitate each other. But how far could imitation—or whatever it is that’s behind social and emotional contagion—really go?

In developing his ideas, Scheler was trying to capture a particular kind of emotional transfer. Imagine walking into a bar where everyone’s happily sipping martinis. The mood is contagious—it’ll put a smile on your face, even if you don’t know anyone there. What’s more, it’ll probably put a martini in your hand, an effect that Scheler’s French contemporary Gustave Le Bon and others called “social contagion.” Just like emotions, these philosophers argued, social and cultural traits could spread upon contact—no empathy or sympathy needed.

Social scientists didn’t pick up the idea until the 1950s, but it wasn’t long before they found evidence of contagion in everything from emotions to political preferences, effects that researchers often attribute to our unconscious tendency to imitate each other. But how far could imitation—or whatever it is that’s behind social and emotional contagion—really go?

SOCIAL AND EMOTIONAL CONTAGION remained largely academic subjects until the late 1990s, when email and the Internet made it possible to talk to a billion or so people at once. The New York Times first mentioned “viral marketing” in a 1998 story on Sony’s PlayStation, and the first viral videos—remember “Lazy Sunday” and the “numa numa” guy?—followed not long after.

But it may have been an offbeat 2007 study by physician-sociologist Nicholas Christakis and political scientist James Fowler in the New England Journal of Medicine that cemented contagion in the popular imagination—and at the same time launched a backlash among academics who wondered whether some were taking the virus metaphor too far.

According to that paper and many that followed, everything from obesity to sexual orientation could spread from person to person in much the same way as a virus. Not only that, but one person's influence typically extended out three degrees. In all manner of ways, Christakis and Fowler argued, you feel the effects of your friends' friends' friends.

The claims were immediately controversial. Quite a few viewed the idea as completely absurd, and perhaps it was: according to one purposely daft analysis, you could “catch” your height from friends and colleagues. And beyond reductio ad absurdum lay more serious challenges—and an unexpected way forward: quantum physics.

AT THE HEART OF Christakis and Fowler’s original claim was that an overweight person’s friends would themselves gain weight over time—on that point, everyone agrees. What researchers disagree on is the explanation. Perhaps it’s contagion; that is, one person’s obesity actually causes another’s. Or it could be that there's some other variable, something that explains why those most likely to gain weight are also most likely to become friends.

To better grasp the problem, suppose two friends, one skinny and one overweight, meet at a bar and decide to come back every week for steak and beer. Over time, the skinnier one gains weight. That might have happened because the skinny one copied the eating and drinking habits of the heavier-set friend. Or it might be that a shared desire to eat and drink brought the friends together in the first place, encouraging but not creating an already nascent behavior that led both to obesity.

So which is it? In 2010, Carnegie Mellon University statisticians Cosma Shalizi and Andrew Thomas proved that it's impossible to tell the difference between contagion and the second explanation, known as “homophily,” using conventional techniques. Without a way to either observe the two friends' taste for a weekly steak or control it experimentally, the two explanations will match the data equally well.

In a curious footnote to the obesity-as-virus controversy, University of Southern California computer scientists Greg Ver Steeg and Aram Galystan say there might be a way forward using an analogy with quantum theory. Using methods physicist John Bell developed in the 1960s to search for so-called hidden variables in quantum physics, Steeg and Galystan discovered that homophily alone can't explain the patterns of obesity researchers had seen. Still, that doesn't mean contagion is the right explanation, Ver Steeg says—the explanation could be that we’re all just getting fatter over time.

EVEN IF YOU'RE NOT dissuaded that things like obesity could spread through a social network, the epidemiological metaphor might not be quite right. A virus that manages to reach epidemic proportions usually spreads along a chain—patient zero infects the people he or she comes in contact with, each of them infects several more people, and so on.

Researchers often don't see that pattern with social and emotional contagion. In a microfinance study in rural India, MIT and Stanford economists at first thought that participation in a bank's microloan program spread like a virus. Once they controlled for who actually knew about the program, however, one person's participation had little to do with anyone else’s decision to take part.

In another just-completed study, Microsoft Research's Duncan Watts and colleagues looked at over a billion tweets spanning about a year in the Twitterverse. They found that while memes, videos, and products do sometimes hit the big time, it's not because they spread rapidly from person to person like an actual, biological virus would.

Instead, it's broadcasting.

Essentially all of the tweets Watts and team looked at failed to spread far enough to qualify as “viral”—“Everything is basically dying out," Watts says—but sometimes an idea will get picked up and retweeted by someone with tens of thousands of followers or more. Once in a while, broadcasters will even report on it, as was the case with Psy's K-pop satire "Gangnam Style." "It's unclear whether it was really viral," Watts says, or whether it took off galloping only after it appeared on NBC's Today Show.

WHY, THEN, DOES CONTAGION seem so contagious lately? Stanford University economist Matthew Jackson, a co-author on the microfinance study, thinks the rise of social media may have played a central role. "Social media has made it much more evident how connected [we] are to each other," he says. "That has changed people's perspective on why they act the way they do." And it's not just Facebook, Tumblr, or Twitter. In the business world and in central banks, contagion is "one of the top things they're thinking about," Jackson says. We’re connected, and as long as we don’t take the viral metaphor too literally, there’s a lot to be learned from that fact.

But as with viral videos, Watts suspects there's more—or perhaps less—to the story. The 2007 Christakis and Fowler paper concerned "something people really cared about," but maybe more importantly it was front-page news in the New York Times. At a time when everyone can hear everyone, he says, "we've forgotten the elephant in the room, the mass media. It's still the elephant."