Some researchers emphasize the potential role of social media exposure and use of smartphones. There is some evidence that girls, who have shown greater rates of increase in depression than boys, experience more cyberbullying because of their greater use of mobile phones and texting. But most studies of digital technology and mental health are correlational and can’t prove causation.

Drugs and alcohol are always a popular culprit, but in this case they are an unlikely explanation, as the studies cited above controlled for drug use. In addition, there is no evidence of a significant increase in the use of drugs or alcohol in young people during the study period.

It is legitimate (though controversial) to ask whether the Food and Drug Administration’s “black box” warning for antidepressants back in 2004 — which said the drugs could cause suicidal thoughts and actions in some children and teenagers — discouraged the use of these drugs and unwittingly helped fuel the rise in teen depression. Within two years of the F.D.A. advisory, antidepressant use dropped by 31 percent in teens and 24 percent in young adults. Although antidepressant use recovered somewhat after 2008, it has remained below levels that would have been expected based on prescribing patterns before the warnings appeared.