There may be some truth to the notion that bullies make other people feel bad to make themselves feel better.
A new study published in the journal Biological Psychology used fMRI scans to compare brain activity in eight unusually aggressive 16- to 18-year-old males to those of eight normal adolescent males while they watched videos of people getting hurt.
While both groups showed activity in the brain's pain centers, the brains of aggressive males, those with conduct disorder, also showed activity in the brain's pleasure centers, suggesting that they may have been enjoying what they were seeing. Normal males showed no such activity.
"It just dumbfounded us," said Benjamin Lahey, a professor of epidemiology and psychiatry at the University of Chicago.
Lahey said he expected an emotionally indifferent response to pain from subjects with conduct disorder, a mental disorder characterized by aggressive, destructive or harmful behavior towards other people and animals and can include theft, substance abuse and sexual promiscuity, according to the American Academy of Child and Adolescent Psychiatry.
Instead, fMRI scans showed a strong but highly atypical emotional response.
It turns out that the brain circuitry in people with conduct disorder is different from a neurotypical person's when it comes to pain.
In the control subjects, fMRI scans showed that the amygdala -- the part of the brain responsible for processing emotional reactions -- activated at the same time as the prefrontal cortex, which is involved in self regulation and, in this case, for holding emotions in check.
Co-activation suggested that activity in the amygdala and the prefrontal cortex is linked when shown something painful. In other words, when normal people see someone getting hurt, they respond with negative emotions.
"But in kids with conduct disorder, that connection isn't there," said Lahey, who co-authored the study with lead author Jean Decety, a professor of Psychology at the University of Chicago.
Instead, the kids with conduct disorder showed brain activity in the amygdala and the ventral striatum, which is the area of the brain associated with pleasure and rewards, which include food, sex and drug use.
Instead of responding with negative emotions, these children respond positively, suggesting they may be excited and enjoy seeing someone get hurt, Lahey said.
On top of that, subjects with conduct disorder showed no activity in the prefrontal cortex, which could have controlled those pleasurable emotions. The study results suggest that young people with conduct disorder enjoy seeing others in pain and lack the ability to control potentially inappropriate emotions.
"They're not only indifferent to the pain, they love it -- maybe," Lahey said. "They're responding to others being hurt, but in a way that's self-reinforcing."
While the study proposed an exciting new hypothesis about how young people with conduct disorder may respond to certain stimuli, experts caution that it may not be appropriate to extrapolate the results to the garden-variety bully roaming elementary and junior high school hallways.
"CD at a young age is associated with very poor psychosocial outcomes," said Dr. Paul Sagerman, assistant professor of pediatrics at Wake Forest University School of Medicine in Winston-Salem, N.C.
These outcomes include poor relationships, incarceration, depression and suicide.
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."