Good behaviors — like quitting smoking or staying slender or being happy — pass from friend to friend almost as if they were contagious viruses. The Framingham participants, the data suggested, influenced one another’s health just by socializing. And the same was true of bad behaviors — clusters of friends appeared to “infect” each other with obesity, unhappiness and smoking.FOR DECADES, SOCIOLOGISTS and philosophers have suspected that behaviors can be “contagious.” In the 1930s, the Austrian sociologist Jacob Moreno began to draw sociograms, little maps of who knew whom in friendship or workplace circles, and he discovered that the shape of social connection varied widely from person to person. Some were sociometric “stars,” picked by many others as a friend, while others were “isolates,” virtually friendless. In the 1940s and 1950s, social scientists began to analyze how the shape of a social network could affect people’s behavior; others examined the way information, gossip and opinion flowed through that network. One pioneer was Paul Lazarsfeld, a sociologist at Columbia University, who analyzed how a commercial product became popular; he argued it was a two-step process, in which highly connected people first absorbed the mass-media ads for a product and then mentioned the product to their many friends. (This concept later bloomed in the 1990s and in this decade with the rage for “buzz marketing” — the attempt to identify thought-leaders who would spread the word about a new product virally.) Lazarsfeld also studied how political opinions flowed through friendship circles; he would ask a group of friends to identify the most influential members of their group, then map out how a political view or support for a candidate spread through and around those individuals. By the 1980s and 1990s, alarmed by the dangers of smoking among young Americans, health care workers began to do the same work on groupings of teenagers to discover exactly how each individual was influenced to pick up the habit. The language of contagion is part of pop culture today, thanks in part to the influence of Malcolm Gladwell’s best-selling book “The Tipping Point.” It’s now common to speak of social changes as epidemics (like the “obesity epidemic”) and to talk about “superconnectors” who are so promiscuously well linked that they exert an outsize influence in society, ushering trends into existence almost single-handedly. Yet the truth is, scientists have never successfully demonstrated that this is really how the world works. None of the case studies directly observed the contagion process in action. They were reverse-engineered later, with sociologists or marketers conducting interviews to try to reconstruct who told whom about what — which meant that people were potentially misrecalling how they were influenced or whom they influenced. And these studies focused on small groups of people, a few dozen or a few hundred at most, which meant they didn’t necessarily indicate much about how a contagious notion spread — if indeed it did — among the broad public. Were superconnectors truly important? How many times did someone need to be exposed to a trend or behavior before they “caught” it? Certainly, scientists knew that a person could influence an immediate peer — but could that influence spread further? Despite our pop-cultural faith in social contagion, no one really knew how it worked.