Jean Twenge had never seen anything like it. The San Diego State University professor in personality psychology had been studying generational differences for 25 years, since she was a 22-year-old Ph.D. student at the University of Michigan. Her research had generated more than 120 scientific publications. In 2012, when the proportion of Americans who own smartphones surpassed 50 percent, she noticed abrupt changes in teen behavior and emotional states. “In these big national surveys of teens that I keep an eye on, there started to be some pretty sudden changes in the ways teens use their time and how they said they were feeling in their mental health,” she said.