'California is God's best moment," says Joel Kotkin. "It's the best place in the world to live." Or at least it used to be.

Mr. Kotkin, one of the nation's premier demographers, left his native New York City in 1971 to enroll at the University of California, Berkeley. The state was a far-out paradise for hipsters who had grown up listening to the Mamas & the Papas' iconic "California Dreamin'" and the Beach Boys' "California Girls." But it also attracted young, ambitious people...