While more adult criminals are being put behind bars, juvenile offenders are serving less time. This has led University of Chicago analysts Steven D. Levitt to theorize in a recent study that the falling threat of punishment for young criminals is behind a rise in youth crime.

• While the adult arrest rate for murder fell 7 percent from 1978 to 1993, the juvenile murder rate surged by 177 percent.

• At the same time, the arrest rate of youngsters for all violent crimes -- those involving the use of force -- climbed 79 percent, almost three times the rise in the adult rate.

• Meanwhile, the number of federal and state adult prisoners per violent crime committed by adults soared by 60 percent.

• But the corresponding ratio for youths in juvenile detention centers -- which paralleled that for adults in 1978 -- fell by 20 percent.

Thus, by 1993, the chances of violent young criminals being jailed were only half those of their adult counterparts.

Levitt finds sharp changes in criminal behavior in the year when youths become subject to the adult criminal-justice system -- usually when they reach 18. In states where youth incarceration rates are high and adult rates are low, violent crime rates among 18-year-olds rise 28 percent. But in states where the punitiveness of the juvenile justice system is low and that of the adult system is high, violent crime rates drop by nearly 4 percent.

Levitt credits deterrence for lowering crime and says youths "respond to incentives."