Understanding exactly why these trends are on the rise is always a challenge, says Twenge, since researchers can only point out correlations, not causes. But, she says, since the trends are "pretty large in a fairly short period of time, that helps us narrow what the likely cause might be."

She thinks the rise in smartphone and social media use is a significant factor. By 2012, smartphones had become widespread, she says, and it's around that same time that social media began to dominate young people's lives. For example, in 2009 about half of high school seniors visited social media sites every day. That's climbed to about 85 percent today, with Instagram and Snapchat replacing Facebook as the main "go to social media site," she says.

It's not just the phone or social media itself, says Twenge. It's the amount of time teens and young adults spend with it. As Twenge found in earlier research, the more time they spend, the greater the risk of depressive symptoms. Twenge says it's known from a body of research that in-person social contact is good for mental health. She questions whether spending that same amount of time on Instagram and Snapchat is just as beneficial, and says "it seems clear the answer is 'No.' "