JOANNE JACOBS: Childhood’s end.

In the early 2010s, “adolescents in rich countries traded in their flip phones for smartphones and moved much more of their social lives online — particularly onto social-media platforms designed for virality and addiction,” he writes. Then, younger children “began to get access to their parents’ smartphones and, later, got their own iPads, laptops, and even smartphones during elementary school.”

Depression, anxiety, self-harm rates rose sharply in the U.S. and other developed countries, while teens reported more loneliness and friendlessness.

Academic achievement went down in the U.S. and globally beginning in the early 2010s.

The change in childhood began before the Internet, Haidt writes. In the 1990’s, American parents became more fearful, less willing to let their kids go outside and play independently. Supervised by adults, children had fewer opportunities “to make their own choices, resolve their own conflicts, and take care of one another.”

But the real slide in mental health started with the smartphone. With the iPhone (2007), the App Store (2008), and high-speed internet (50 percent of American homes in 2007), many providers of social media, video games, and porn pivoted to mobile, he writes. Childhood went online.

More from Haidt here: ‘It’s Causing Them to Drop Out of Life:’ How Phones Warped Gen Z.