Everyone knows that violence in America rises on a surge of testosterone when boys become teenagers, peaks in their early 20s and then declines.

The Thinkers
This monthly series will highlight people from Western Pennsylvania who are on the forefront of new ideas in their fields.

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Pam Panchak, Post-Gazette Daniel S. Nagin Age: 57 Position:Teresa and H. John Heinz III professor of public policy and statistics, Carnegie Mellon University. Education: Bachelor's in administrative and managerial sciences, master's in industrial administration, Carnegie Mellon University, 1971; Ph.D., urban and public affairs, Carnegie Mellon University, 1976. Previous positions: Visiting scholar, University of Cambridge, 2003; professor of management, Carnegie Mellon, 1986-98; deputy secretary for fiscal policy and analysis, Pennsylvania Department of Revenue, 1981-86. Professional honors: Life member, Clare Hall College, University of Cambridge; fellow, American Association for the Advancement of Science; fellow, American Society of Criminology; North Eastern State Tax Officials award for excellence in tax administration, 1985. Publications: "Group-Based Modeling of Development," Harvard University Press, 2005; "Deterrence and Incapacitation: Estimating the Effects of Criminal Sanctions on Crime Rates," co-author, National Academy of Sciences, 1978; More than 90 articles in scientific journals. The Series Click here to view other installments in this continuing series.

But everyone is wrong, says Daniel Nagin, a criminologist and professor at Carnegie Mellon University. Violence actually peaks much earlier.

"If you shift your attention away from criminal violence and instead think about physical aggression -- using force or the threat of force to get what you want -- who do you think are the most physically aggressive people in the world?

"It's 2-to-3-year-olds, and there's a lot of evidence to support this."

Most people don't know this because toddlers can't cause much harm and because the vast majority of children soon learn to control their physical aggression, which declines steadily for the rest of their lives, Dr. Nagin said.

Working with fellow researcher Richard Tremblay of the University of Montreal, Dr. Nagin has been able to show that when boys enter school, they tend to separate into four groups on a physical aggression scale.

About 80 percent fall into two middle groups that start out moderately aggressive and then become less so. Another 16 percent hardly ever use violence.

But 4 percent are highly aggressive and remain that way. This group accounts for much of the extreme violence and criminal delinquency that shows up in adolescence.

Why do most children become less belligerent as they grow older?

"One is their parents and the circumstances they grow up in and having proper caregivers," Dr. Nagin said. "But it's also the case that human beings are social creatures and we learn that in most of our everyday living, physical aggression is not a winning strategy, because kids who are physically violent are shunned."

When he and Dr. Tremblay teased apart the influences on children's lives, they found only two factors strongly linked to chronic violence -- having a mother with little education or a mother who had her first child when she was a teenager.

Having a mother who had her first child as a teen, he said, "is probably a marker for something deeper. I think it's that when you have a problem child, that's when good parenting skills are so important, and I think these women disproportionately lack the parenting skills to deal with a difficult child."

Several studies demonstrate how vital those skills can be, he said, especially the work of David Olds at the University of Colorado.

Dr. Olds has shown that when nurses mentored low-income pregnant women and stayed with them for the first two years of their children's lives, teaching them about healthy living, effective parenting and how to make good choices for their own futures, it made a huge difference in reducing delinquency, addiction and other destructive behavior when the children became older.

Dr. Nagin is highly skeptical of studies showing a link between violence in the media and aggressive behavior because almost all children become less abusive after leaving toddlerhood.

The young men most likely to commit violence "are also likely to be the group who are most attracted" to violent video games, TV and movies, he said.

"Separating cause from effect in these circumstances is devilishly difficult."

He also noted that "millions of people play violent video games and watch violent movies and don't become violent themselves."

A life of crime

Dr. Nagin never planned on becoming a criminologist.

After growing up in Churchill and getting his bachelor's and master's degrees at Carnegie Mellon, he planned on doing health research. That's what landed him in New York City in the early 1970s, working for the New York Academy of Sciences.

One day while he and a friend were boarding the subway, they were held up at gunpoint by a man who later was caught trying to rob an undercover police officer.

It would be a good story to say that being at the wrong end of a gun propelled him into criminal science, but that's not how it happened.

Instead, his path was shaped by a meeting he had one day with a mentor, renowned Carnegie Mellon criminologist Alfred Blumstein. Dr. Nagin wondered out loud why there was such a strong link between crime and young men, and Dr. Blumstein challenged him to find out.

Nearly 30 years later, he's still unraveling the answers.

For boys who remain violent and replenish the ranks of America's most dangerous criminals, there still are endless debates over how much they can be deterred by police, courts and prisons.

Dr. Nagin's work has shown that the relationship between crime and punishment is complex, and his findings don't provide easy bromides for liberals or conservatives.

On the one hand, he thinks putting more people in prison has helped to lower the crime rate by taking criminals out of circulation and making them more wary of breaking the law.

On the other hand, he joins many other scientists in condemning the huge increase in mandatory sentences for drug offenders, especially federal court sentences for crack cocaine possession.

"I think those policies are indefensible," Dr. Nagin said. "First of all, they're unjust. And there is no evidence that these policies have been effective in reducing the amount of drug use in our society."

He also is no fan of "three strikes" laws that send people to prison for life after they've committed three felonies.

"I don't think these laws make a lot of sense because the prison population is aging because of these much longer sentences, and that means our prisons are going to be increasingly filled with old people" at a cost of $30,000 to $50,000 a year each, he said.

Then there is the always contentious issue of capital punishment.

Recently, there has been a groundswell of studies arguing that capital punishment is a deterrent because the states that have carried out the most executions have shown the biggest drops in homicide rates, he said. But those were followed by studies showing statistical flaws in these analyses, leaving the issue up in the air.

"Capital punishment may deter homicides," Dr. Nagin concluded, "but I don't think any of the evidence out there purporting to prove deterrence is at all convincing."

What is badly needed, he said, is a major national research effort to study the relationship between crime, punishment and prevention in the same way the country studies the effectiveness of new medical treatments.

Without such a project, he said, "it's too easy to always ratchet things up and simply increase prison sentences, and the net result of that is we wind up having prison populations that we can't afford to maintain."

As he moves into his fourth decade of criminology research, Dr. Nagin hopes to focus more on how to keep young people off the pathway toward crime. After years of trying to define the nature of crime, he'd like to figure out how to prevent it.