There have been increasingly loud public warnings that social media is harming teenagers’ mental health — most recently from the U.S. surgeon general — adding to many parents’ fears about what all the time spent on phones is doing to their children’s brains. Although many scientists share the concern, there is little research to prove that social media is harmful — or to indicate which sites, apps or features are problematic. There isn’t even a shared definition of what social media is. It leave