DURHAM, N.C. -- It's a long-simmering debate in juvenile justice: Do young offenders become worse because of their experience with the justice system, or are they somehow different than people who don't have their first criminal conviction until later in life?

"There seems to be a lot of evidence that people who are convicted early are more heavily involved in crime," says postdoctoral researcher Amber Beckley at Duke University, who has a new study out on the topic that appears online in the Journal of Criminal Justice.

Using data from a study that has tracked nearly 1,000 New Zealanders from birth to age 38, Beckley looked at patterns that would distinguish youthful offenders from what she calls "adult-onset offenders."

Of the 931 study participants, 138 males began criminal activity as juveniles. The adult-onset group consisted of 66 males. Across the entire cohort, in fact, 42 percent of the men have some sort of conviction, ranging from shoplifting and DUI to property crimes and assaults.

Using this unusually rich source of data, the study was able to look at childhood history compared with adult behavior. Beckley, who is in the department of psychology and neuroscience at Duke, said the adult-onset group had a history of anti-social behavior back to childhood, but reported committing relatively fewer crimes.

The researchers looked at several possible reasons for adult-onset criminal behavior.

This group reported committing more crimes than folks who had never been convicted, but fewer crimes than people who had been in trouble as juveniles.

Contrary to some hypotheses, adult-onset offenders in this study were not found to come from significantly wealthier socioeconomic backgrounds, nor were they any more intelligent than those who were caught younger. They were more likely than non-offenders to have schizophrenia or bipolar disorder and to be dependent on alcohol, but they were no more likely to be unemployed.

Beckley said her findings have some clear implications also for the mental health component of adult-onset criminal behavior. "It should be addressed in sentencing, because it's not now and most incarcerations aren't exactly therapeutic."

"I don't think the court system has a large role in the juvenile offender's trajectory," Beckley said. The New Zealand subjects who were juvenile offenders self-reported that they were doing more and worse crimes at a young age, "before they even got caught."

If there were any recommendations out of this study, Beckley said, hers would be to focus more on juvenile offenders while perhaps being a bit more lenient on first-time adult offenders. "We should continue to devote resources to juvenile justice because two-thirds of the criminal population first commit crimes in adolescence."