by Robert J. Samuelson

About the author: Robert J. Samuelson is a Newsweek contributing editor and a columnist for the Washington Post.

Never before have so many people been so successfully treated for mental illness. Depending on how you read the numbers, between 6 and 11 percent of Americans annually receive some form of medical therapy for psychological problems. In 1950, less than 1 percent did. New drugs provide relief for victims of schizophrenia and manic depression who were once untreatable. Prozac, the best-known antidepressant, was introduced only in 1986. In 1995, worldwide sales totaled $2 billion.

It’s this huge progress that ultimately underlies the proposal to require insurance companies to provide a “parity” of benefits between physical and mental illness. Though this is a bad idea that would drive up health...