In his book The Anxious Generation: How the Great Rewiring of Childhood Is Causing An Epidemic of Mental Illness, Jonathan Haidt tells the story of what happened to Gen Z (born 1995). They became “the first generation in history to go through puberty with a portal in their pockets that called them away from the people nearby and into an alternative universe that was exciting, addictive, unstable and . . . unsuitable for children and adolescents.”

In 2011, 23% of teens had a smartphone. By 2016, 79% of teens owned a smartphone, as did 28% of children eight to 12. Soon teens were reporting they spent an average of almost seven hours a day on screens. “One out of every four teens said that they were online ‘almost constantly,’” Mr. Haidt writes.

Girls moved their social lives onto social media. Boys burrowed into immersive video games, Reddit, YouTube, and pornography.

Suddenly children “spent far less time playing with, talking to, touching, or even making eye contact with their friends and families.” They withdrew from “embodied social behaviors” essential for successful human development. It left them not noticing the world.

Signs of a mental-health crisis quickly emerged. Rates of mental illness among the young went up dramatically in many Western countries between 2010 and 2015. Between 2010 and 2024 major depression among teens went up 145% among girls, 161% among boys. There was a rise in disorders related to anxiety as well. Mr. Haidt looked at changes that weren’t self-reported—studies charting emergency psychiatric care and admissions. They too were up. “The rate of self-harm for . . . young adolescent girls nearly tripled from 2010 to 2020.”