The phrase “generation gap” became popular in the late 1960s, as baby boomers were coming of age. To hear social psychologist Jonathan Haidt tell it, today’s generation gap has widened into a chasm. “We have a whole generation that’s doing terribly,” he says in an interview at his professorial office, book-lined and hushed, at New York University’s Stern School of Business. He calls it a “national crisis.”

At 59, Mr. Haidt is a young boomer, and he isn’t talking about millennials, some of whom are in their 40s by now. Rather, he has in mind the younger cohort, Generation Z, usually defined as those born between 1997 and 2012. “When you look at Americans born after 1995,” Mr. Haidt says, “what you find is that they have extraordinarily high rates of anxiety, depression, self-harm, suicide and fragility.” There has “never been a generation this depressed, anxious and fragile.”