"I found that teens are spending less time with their friends in person," Twenge said. "They're not hanging out with their friends as much or going to movies or going to the mall or riding around in cars or going to parties as much as they did even 10 years ago. And that's probably because they're spending so much time communicating with their friends through their phones. And that is also another really key piece of why these mental-health issues have started to go up so steeply. It's not just that they're spending time with the screens, which isn't very fulfilling and probably has negative effects, but also not doing these in-person activities, which are beneficial."