Both anecdotally and in research, anxiety and depression among young people—often associated with self-harm—have risen sharply over the last decade. There seems little doubt that Gen Z is suffering in real ways. But there is not a consensus on the cause or causes, nor how to address them. The social psychologist Jonathan Haidt believes that enough evidence has accumulated to convict a suspect. Smartphones and social media, Haidt says, have caused a “great rewiring” in those born after 1995. The argument has hit a nerve: his new book, “The Anxious Generation,” was No. 1 on the New York *Times* hardcover nonfiction best-seller list. Speaking with David Remnick, Haidt is quick to differentiate social-media apps—with their constant stream of notifications, and their emphasis on performance—from technology writ large; mental health was not affected, he says, for millennials, who grew up earlier in the evolution of the Internet. Haidt, who earlier wrote about an excessive emphasis on safety in the book “The Coddling of the American Mind,” feels that our priorities when it comes to child safety are exactly wrong. “We’re overprotecting in [the real world], and I’m saying, lighten up, let your kids out! And we’re underprotecting in another, and I’m saying, don’t let your kids spend nine hours a day on the Internet talking with strange men. It’s just not a good idea.” To social scientists who have asserted that the evidence Haidt marshals does not prove a causative link between social media and depression, “I keep asking for alternative theories,” he says. “You don’t think it’s the smartphones and social media—what is it? . . . You can give me whatever theory you want about trends in American society, but nobody can explain why it happened so suddenly in 2012 and 2013—not just here but in Canada, the U.K., Australia, New Zealand, Northern Europe. I’m waiting,” he adds sarcastically, “for someone to find a chemical.” The good news, Haidt says, is there are achievable ways to limit the harm.

Note: In his conversation with David Remnick, Jonathan Haidt misstated some information about a working paper that studies unhappiness across nations. The authors are David G. Blanchflower, Alex Bryson, and Xiaowei Xu, and it includes data on thirty-four countries.