And conduct disorder is not very common, affecting 1 to 4 percent of 9- to 17-year-olds in the United States, according to the U.S. Department of Health and Human Services. The disorder is also far more common among boys than girls.
Sagerman, who was not associated with the study, pointed out that bullying typically peaks in young adolescence, between the sixth and eighth grades, and may serve to impress peers.
Using aggressiveness to gain something tangible such as social approval versus personal reward is an important distinction between a regular bully and someone with conduct disorder. Indeed, aggressiveness can be channeled constructively to allow a neurotypical person to excel later in life, in business or athletics, for example.
Although the study is not large enough to draw firm conclusions, it does lead to some new hypotheses and questions.
"The question is what's the chicken and what's the egg?" Sagerman said.
Is bullying a learned behavior, creating pathways in the brain that lead to conduct disorder, or is conduct disorder inherent and results in aggressive behavior?
"If parts of the brain are stimulated by an act of pain, can you make that jump and say maybe they'll go out and replicate that pain ... because that stimulates their brain? That, theoretically, makes sense," Sagerman said.
Though it's too expensive at this point, Sagerman pointed out that fMRI could be a useful technique to detect if certain areas in the brain are active when young children begin to show aggressive behavior.
For example, if a young child were to develop a habit of hitting other children, and a brain scan showed no activity in the prefrontal cortex when he or she saw someone in pain but the ventral striatum showed activity, that could indicate the child may develop conduct disorder in the future.
Early intervention and therapy may help reprogram the brain circuitry in a way that could help prevent conduct disorder, or at least keep it under control.
But if this is indeed possible, researchers have yet to devise the best therapies to deal with conduct disorder. And they could have their work cut out for them.
"If we confirm this," Chicago's Lahey said, "people who develop these therapies are going to be scratching their heads for a while."