What's Wrong With the Teenage Mind?

How does the boy who can thoughtfully explain the reasons never to drink and drive end up in a drunken crash? Why does the girl who knows all about birth control find herself pregnant by a boy she doesn't even like? What happened to the gifted, imaginative child who excelled through high school but then dropped out of college, drifted from job to job and now lives in his parents' basement?

Adolescence has always been troubled, but for reasons that are somewhat mysterious, puberty is now kicking in at an earlier and earlier age. A leading theory points to changes in energy balance as children eat more and move less.

here to read the rest of The Saturday Essay for the Wall Street Journal by Alison Gopnik, a professor of psychology at the University of California, Berkeley, and the author, most recently, of "The Philosophical Baby: What Children's Minds Tell Us About Truth, Love and the Meaning of Life."