The poet and critic Randall Jarrell once said that a poet is someone who “manages, in a lifetime of standing out in thunderstorms, to be struck by lightning five or six times.” Jonathan Haidt, in his career as a wide-ranging social psychologist, has been flagrantly right about at least one big thing. His book “The Anxious Generation: How the Great Rewiring of Childhood Is Causing an Epidemic of Mental Illness” was a best-seller when it was published two years ago and its influence has hardly waned. As both a researcher and as a popularizer, Haidt argues that social-media companies have unleashed what is essentially a “mind-altering” danger in our lives—particularly in the lives of kids.