In separate cases three years ago, Michael Harris and Walter Biggs, both of Los Angeles, were arrested and charged with robbery. Both Harris and Biggs (whose real names are not used here) were convicted and sentenced to custody. Harris, 16, served 2 1/2 years; Biggs, 25, however, did only 18 months.

It wasn't that Harris was the more dangerous criminal, nor was the difference merely one of the vagaries of the justice system. Harris did more time because, as a juvenile, he was sentenced to the California Youth Authority, while Biggs was sent to a state prison.

"That's not uncommon," one Los Angeles juvenile parole officer said. "Juveniles sent to the Youth Authority normally serve longer sentences than adults in state prison."

Such is the rougher, tougher world of juvenile justice.

It is a world where some states lock up larger percentages of children than adults, where youngsters who are traditionally denied jury trials may serve longer sentences than their adult counterparts, where youths are often housed in overcrowded facilities and sometimes punished in ways that would be unthinkable in state prisons.

All because people believed something that wasn't true.

Beginning about 1975, crimes by children in the United States began a steady decline, but the public perception was that juvenile delinquency was going through the roof.

So the United States began to lock up its wayward youngsters at unprecedented rates, eventually transforming the way the nation deals with them.

Once, juvenile crime brought determined attempts to rehabilitate kids, even though that sometimes required keeping them in custody.

Today, virtually all authorities on juvenile justice agree, custody has become largely an end in itself. The pendulum has swung away from rehabilitation and toward punishment.

Recent juvenile crime statistics make it increasingly clear that it has not worked.

Starting in the mid-1970s, the experts say, America decided to get tough with its juvenile delinquents and to separate them from the rest of society, even though Justice Department figures showed juvenile crime declining. From 1978 to 1988, while the per capita rate of crime among youths dropped by 19%, their lockup rate increased by nearly 50%.

"The public got tough-minded, and the elected officials got tough-minded for them," said Fred Jordan, director of probation for San Francisco County. "We had a series of attorneys general who talked about the 'juvenile crime wave' even though the numbers weren't going up."