Narcissism--an inflated view of the self--is everywhere. Public figures say it's what makes them stray from their wives. Parents teach it by dressing children in T-shirts that say "Princess." Teenagers and young adults hone it on Facebook, and celebrity newsmakers have elevated it to an art form. And it's what's making people depressed, lonely, and buried under piles of debt.

Jean Twenge's influential first book, Generation Me, spurred a national debate with its depiction of the challenges twenty- and thirty-somethings face in today's world--and the fallout these issues create for...