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.