Many parents are concerned about how much time their children spend on cellphones, Xboxes and other digital media. Some experts say they should be: There is a growing body of research showing an association between unhappiness and the time adolescents spend on digital media.

What is less clear is whether screen time is causing mental-health problems or if children with worse mental health spend more time with digital media.

To assess the research, we went to Jean Twenge, author of the book “iGen: Why Today’s Super-Connected Kids Are Growing Up Less Rebellious, More Tolerant, Less Happy—and Completely Unprepared for Adulthood—and What That Means for the Rest of Us”; Michael Rich, founder and director of the Center on Media and Child Health in Boston and an associate professor at Harvard Medical School; and Cara Booker, research fellow and acting graduate director at the University of Essex in the U.K., who has studied the effects of social-media use on children and adolescents.

Edited excerpts follow:

WSJ: What does the evidence tell us about the links between screen time and children’s mental health?