"When Dr. Twenge noticed a dramatic increase in the teenage suicide rate around 2012 she put two and two together.
"That's when teens started spending a lot more of their time online and texting and social media; and a lot less time hanging out with their friends in person," she said.
They're the generation that grew up with iPhones.
Experts say it's the lack of face-to-face, social interaction that's to blame for changes in teenage behavior.
Some of the changes are positive.
"Teens are physically safer than ever, so they're safer drivers. They are less likely to drink alcohol and the homicide rate among young people has reached all-time lows," said Dr. Twenge.
And some of the changes are negative.
"Teens who spend more time online and texting and on social media are more likely to be unhappy and depressed," Twenge said.
"Young people are getting to their first job without as much experience with independence. And some are finding it hard to make decisions on their own without their parents," she said.
But there is a solution. Simply limit your child's screen time to less than two hours a day. And yes, there's an app for that."