Think a smartphone will make your teen happier? Think again. A new study from San Diego State University concludes “Phone-addicted teens are unhappy.”

SDSU psychology professor Jean Twenge led the study, released Monday. She’s also the author of iGen: Why Today’s Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy — And Unprepared for Adulthood. The 2017 book explores modern youth’s relationship with technology. In September The Atlantic published an excerpt.

The new study confirms what Twenge observed in her book. As smartphone use increased, teen happiness decreased. Specifically, young people’s life satisfaction, self-esteem and happiness plummeted after 2012. That’s the first year smartphone use topped 50% in America.

SDSU’s study examined data from the Monitoring the Future longitudinal study. The survey asked over 1 million kids in 8th, 10th and 12th grades about their screen time. Teens who spent more time looking at smartphones, tablets and computers reported less happiness. And those who spent more time playing sports, reading or interacting in person? They reported more happiness. 

Twenge admits “this study can’t show causation.” But “several other studies have shown that more social media use leads to unhappiness, but unhappiness does not lead to more social media use.”

Twenge maintains that moderate screen time is okay. Once it rises above an hour per day, teens’ happiness begins to drop. She suggests limiting “digital media” use to two hours per day, and spending more time exercising and with friends.

“The advent of the smartphone is the most plausible explanation for the sudden decrease in teens’ psychological well-being,” Twenge said.