This is the first of two articles on the recent outbreak of social contagion studies.
If you've been following the news recently, you may be worried that a plague of social contagion has struck mankind. This week, a study found that drinking habits are socially transmissible. Last month, a paper said that both cooperation and selfishness can spread like a virus. In February, a study found that poor sleep and pot smoking are contagious among teens. All of these revelations come from the works of two scientists, Harvard's Nicholas Christakis and U.C. San Diego's James Fowler. They first brought fame to contagion in 2007 with a widely publicized paper suggesting that obesity is "socially contagious" and that it can spread like a pox from one friend to another, and then another, and then to one more. More contagions (depression and divorce) are in the works. In their 2009 book, Connected: The Surprising Power of Our Social Networks and How They Shape Our Lives, Christakis and Fowler write that connection and contagion are "the anatomy and physiology of the human superorganism," and that "everything we think, feel, do, or say can spread far beyond the people we know."
The studies have provoked excitement in the public health community but also some head-scratching. Many were surprised by the claim that obesity, for example, could be transmitted from one person to another. We thought we knew the major causes of fatness: genes, for one thing, along with eating too many calories and living a sedentary lifestyle. The finding that loneliness can be contagious also caught some readers off-guard—wouldn't lonely souls be hard up for people to infect? In fact, the "surprising power" of contagion should come as no surprise at all, as scientists have been diagnosing social illness—and using the contagion metaphor to describe it—since antiquity.
Long before the advent of germ theory, the word contagion—which means "to touch together"—was sometimes used to refer to the transmission of behaviors and ideas, especially dangerous ones. The Roman historian Livy told of how in 186 B.C. the debauched orgies known as the Bacchanalia had grown so wild that "the infection of this mischief, like that from the contagion of disease, spread from Etruria to Rome." The senate, concerned that the widening plague of contagious partying might swamp the commonwealth, sought to imprison and execute revelers.
The Middle Ages saw its share of weird, contagious episodes. On several occasions between 1017 and 1518, groups of people began dancing uncontrollably in the streets of Germany, Switzerland, and France—with the mania spreading Medusa-like "by the sight of the sufferers like a demoniacal epidemic," according to 18th-century physician Justus Hecker, who compiled accounts of the dancing contagions in his 1835 book The Epidemics of the Middle Ages. Convents were hot spots for nutty outbreaks, too: In 1491, nuns in Cambrai, France, started yelping like dogs, whereas the sisters in a Spanish nunnery took to bleating like sheep. (Naturally, there were also meowing nuns.) Today, these manias are seen as examples of hysterical contagion, which can spread from person to person like a panic virus. Hysterical contagion still occurs: See "the Bin Laden itch" and the dancing contagion that struck a Washington-state music festival last year.
The main ingredient for contagion, of course, is people. By the late 19th century, scientists could see from the madness of the French revolution, the peasant revolts of 1848, and the rise of chaotic mass politics that large agglomerations of humans were unusually susceptible to social whims. In 1895, the French psychologist Gustave Le Bon concluded that "[i]n a crowd every sentiment and act is contagious," with the gang mentality obliterating the personality to such a degree that the individual "is no longer himself, but has become an automaton who has ceased to be guided by his will."
This was a disturbing idea in an increasingly metropolitan France, where people were feeling over-stimulated by a flood of sensational newspapers and crime novels, carrying dangerous, contagious ideas. Coverage of London's 1888 Jack the Ripper murders had spawned a series of copycat killings, leading one sociologist to conclude that "epidemics of crime follow the line of the telegraph." At the time, scientists already believed murder, rioting, madness, suicide, yawning, facial tics, laughter, and crying were all communicable. Worse, the startling new science of hypnosis had revealed that the mind of man was fundamentally infirm, and susceptible to the power of suggestion. With bacteriology gaining ground, some scientists posited a germlike contagium psychicum— a microbe of the mind —as the vector behind so-called "mental contagion."
Still, it was believed that with a little sweat, these bugs could be resisted. In turn-of-the-century France, it became trendy for men to attempt to fortify their willpower against contagions via spa treatments and gymnastics. (Women didn't bother, since scientists said their minds were hopelessly mushy.) It was well known that "better" men caught contagions at lower rates than did the peasant classes. In the United States, the journalist Jacob Riis wrote of a "moral contagion" that infested urban tenements—"the nurseries of pauperism and crime"—and gave rise to "a scum of forty thousand human wrecks" who were blighted with corrupt habits. Early ads for hygienic products such as soap and toothpaste reinforced the association between contagion and the ethnic poor by depicting germs as dark-skinned, hook-nosed monsters.
Yet by the early 20th century, economists began to see that unique contagions could strike the elite, too; the rich man's disease known as conspicuous consumption was seen to spread via "pecuniary emulation." (Today this contagion is sometimes called Affluenza.) Not until midcentury did economists, sociologists, and psychologists begin to study contagion with rigor. One strand of research has examined the spread of relatively simple behaviors: things like coughing, applause, and face-rubbing. Another strand has looked at more complex contagions—speeding, baby-making, and suicide. A newer area of interest is emotional contagion, which has gotten a boost from the discovery of so-called "mirror neurons"—contagion receptors in the brain that supposedly facilitate the transmission of contagious anxiety, satisfaction and fear. Yet all the while, scientists have often struggled to agree on which processes make up contagion—imitation? learning? hysteria?—and which do not.
A little scientific fuzziness has not stopped the spread of contagion. Throughout recent history, the metaphor has proved irresistible to scientists, journalists, and public officials. In the 1930s, a Nazi disease infected Germany. Another malady threatened students in the wake of Brown v. Board, as scientists wondered about the viral effects of desegregation: Would black kids contaminate whites with antisocial contagion? In the 1970s, heroin users were called contagious. In the 1980s, the United States drug czar dubbed crack users contagious "agents of infection." Conspiracy theorists were said to be infectious, too; after the King croaked, "Elvis contagion" afflicted those who denied he was dead. A few years later the "Asian contagion" infected the marketplace. In New York City, first Amadou Diallo and later Sean Bell died in what police called outbreaks of "contagious shooting." After 9/11, scientists have said terrorist ideology (PDF) spreads like a virus. Today, the worry is Greek contagion.
In 2001, U.S. Surgeon General David Satcher issued a call to arms to confront America's growing "obesity epidemic." This image captured the attention of Christakis and Fowler, who began to wonder whether obesity might be literally contagious. Together the duo mined social network data from a long-running study of heart disease in a Massachusetts town that had tracked participants' physical and mental health for decades. In their data, they saw clusters of fat friends break out over time, and their statistical analyses suggested the cause was contagion. Evidence for other transmissible traits and behaviors emerge from the same dataset.
These new contagions appear sneakier and more potent than the mental microbes of old. Although Christakis and Fowler say they don't know for certain how contagions spread, they hypothesize that at least some of them propagate subconsciously. For example, when we see a fat person, obesity contagion can slip into our brains and reset our sense of what a normal person looks like: It gives us "permission," in effect, to grow fat. Loneliness contagion is even stealthier: When we see a frowning face, or observe a lonely person, the mirror neurons in our brains well up with misery, and we shrink into loneliness ourselves.
Perhaps the only thing more irresistible than these social germs is the contagion meme itself—in September, Christakis and Fowler's work was featured on the cover of the New York Times Magazine, and their book Connectedmade Oprah's fall reading list. The scientists seem to have hit on a simple recipe—socially contagious transmission, three degrees of separation—that has proved remarkably catching.
Contagionism is spreading more widely than ever before. But should we embrace the new socially infectious world that Christakis and Fowler are promoting? How much do we really know about the power of social contagion, anyway?