An associate professor of sociology at Iowa State University, where he is also director of the criminal justice program, DeLisi worked the third shift in a county jail in Colorado while he was a graduate student. That experience led to his first book, Career Criminals in Society (Sage Publications, 2005).

He's now co-author of a new edition of the popular juvenile delinquency textbook Delinquency in Society -- joining Robert Regoli, a professor of sociology at the University of Colorado; and John Hewitt, professor of criminal justice at Grand Valley State University, on the book. DeLisi and ISU sociology lecturer Pete Conis also co-edited another book that was published in October titled Violent Offenders. That book features contributions from an international array of experts on violent criminals -- homicide offenders, sex offenders and psychopaths.

The books draw on new interdisciplinary scientific insights on career criminals -- getting contributions from academics in such fields as pediatrics and neuroscience, as well as more traditional sociology. That provides a more comprehensive look at those who turn to crime.

"Historically, when people study juvenile delinquency, they studied 15-,16- and 17-year-olds," DeLisi said. "And now, they're studying people from birth to death -- and linking both pro-social and anti-social behaviors that happen in adolescence, adulthood and even very early in childhood. And so I think it's very exciting to be a criminologist right now because it's just so much more scientifically rigorous and sophisticated than it used to be."

DeLisi says there are two types of juvenile delinquents, defined as those offenders under age 18. About 90 percent are influenced by peers in schools and neighborhoods to commit low-level crime like underage drinking and vandalism. The other group exhibits more pronounced delinquency by committing more serious and regular criminal offenses. That group has drawn the most interest from today's interdisciplinary researchers.

"So if we were looking at a chronic delinquent, the same kind of sociological factors that influence him or her in adolescence are there, but there are also personality factors from psychology that are very important -- like anti-social personalities -- and even biological factors that interact with the environment to produce that behavior," he said.

Even if an individual possesses a genetic risk factor, DeLisi has found that doesn't guarantee a life of crime.

"If you live in a very pro-social neighborhood, have good parents, even if you have a genetic risk factor, you're not going to be involved in crime," he said. "You're insulated because of your socialization. But if you have some of these genetic risk factors and you grew up in a very abusive background, then that's when we see this sort of nature/nurture interaction that produces the misconduct.

"So what we're able to do now in the study of juvenile delinquency is answer that question, 'Why is it that you can have 100 people who had the same family background -- whether it was good or bad -- and yet only some of those kids go on to have delinquent behavior?" he continued. "And the answer is that those are kids who had genetic risk factors that interacted with those family backgrounds to produce the anti-social conduct."

DeLisi reports about one in three juveniles has police contact before the age of 18. Between six and 10 percent have police contact five or more times. These "chronic" juvenile delinquents may be on the way to becoming the violent offenders DeLisi studied for his second book.

It provides a snapshot of violent offenders. He wrote chapters on the importance of violent offenders to criminology, the socialization of violent offenders, and psychopathy and its relationship and importance to violent offenders -- a chapter he calls Still Psychopathic After All These Years.

"The point of using that title is that in almost any serious behavioral outcome, psychopathy's very important," DeLisi said. "So whether you're talking about serial killing, sexual offending, prison misconduct, etc., the offenders who are psychopathic are worse than those who are not."

But while there are some common traits to psychopaths, DeLisi says that some of those same traits might also be common in successful leaders.

Traits of a psychopathic personality include:

* Cold
* Impersonal
* Impulsive
* Lack of concern for others
* Without conscience

DeLisi has found psychopaths separate themselves from others by their lack of empathy and conscience -- usually measured by the amount of callous and non-emotional traits they possess.

"There's a difference between someone who's tough-minded and can make difficult decisions, compared with someone who ignores a person who is lying hurt, or would take delight in hurting other people, or would be unphased by fear," he said. "So the depth of callous and unemotional traits, to me, separates criminals from non-criminals."

And DeLisi has spent his academic life studying and documenting the difference between the two.