Divorce spreads through a social network like a contagion, suggests one 2013 paper — messy, expensive, complicated contagion. People who had divorced friends were 75 percent more likely to become divorced themselves. And even if it was a friend of a friend doing the divorcing, the people this study tracked were still 33 percent more likely to end their own marriages. The data comes from a survey of more than 5,000 adult residents of Framingham, Massachusetts, started by doctors in 1948 who intended to track lifestyle factors that may cause heart disease. But researchers have used the survey in recent years to track social contagion — the way emotions and habits spread throughout a social network. Happiness, obesity, and the decision to quit smoking are all varying degrees of “contagious,” according to research on this data set, and so is, apparently, divorce.