Back in 2017, psychologist Jean Twenge set off a firestorm in the field of psychology.

Twenge studies generational trends at San Diego State University. When she looked at mental health metrics for teenagers around 2012, what she saw shocked her. "In all my analyses of generational data — some reaching back to the 1930s — I had never seen anything like it," Twenge wrote in the Atlantic in 2017.

Twenge warned of a mental health crisis on the horizon. Rates of depression, anxiety and loneliness were rising. And she had a hypothesis for the cause: smartphones and all the social media that comes along with them. "Smartphones were used by the majority of Americans around 2012, and that's the same time loneliness increases. That's very suspicious," Twenge told NPR in 2017.