In other words, dangerous criminals don't turn violent. They just stay that way.
These findings have been replicated in multiple large studies by several researchers on several continents.
"It's highly reliable," said Brad J. Bushman, a psychology professor at Ohio State University and an expert on child violence, who noted that toddlers use physical aggression even more than people in violent youth gangs do. "Thank God toddlers don't carry weapons."
The son of a professional football player, Dr. Tremblay played football himself and was fascinated with its regulated version of extreme physical aggression. After college he did social work in a prison and saw firsthand how seldom such programs changed violent criminals. By the time the violent child gets big, it's often too late.
So he trained his focus earlier and earlier, and learned that the younger the children, the more they whacked each other. With adolescents, physically aggressive acts can be counted in incidents per month; with toddlers, he said, "you count the number per hour."
In most children, though, this is as bad as it gets. The rate of violence peaks at 24 months, declines steadily through adolescence and plunges in early adulthood. But as Dr. Tremblay and Daniel S. Nagin, a criminologist at Carnegie Mellon University, found in a pivotal 1999 study, a troublesome few do not follow this pattern.
The study tracked behavior in 1,037 mostly disadvantaged Quebec schoolboys from kindergarten through age 18. The boys fell into four distinct trajectories of physical aggression.
The most peaceable 20 percent, a "no problem" group, showed little physical aggression at any age; two larger groups showed moderate and high rates of aggression as preschoolers. In these three groups violence fell through childhood and adolescence, and dropped to almost nothing when the boys reached their 20s.
A fourth group, about 5 percent, peaked higher during toddlerhood and declined far more slowly. Their curve was more plateau than hill.
As they moved into late adolescence and young adulthood, their aggression grew ever more dangerous, and it tailed off late. At age 17 they were four times as physically aggressive as the moderate group and committed 14 times as many criminal infractions. It's these chronically violent individuals, Dr. Tremblay says, who are responsible for most violent crime.
(These numbers are all for boys and young men; girls' physical aggression declines in arcs similar to those of boys, but at sharply lower levels.)
Image: Angry boy, via Shutterstock