Bullies' brains may be wired differently, the research suggests
Bullies' brains may be hardwired to have sadistic tendencies, US imaging research suggests.
An area of the brain associated with reward lit up in scans when aggressive boys watched a video of someone inflicting pain.
Boys without a history of unusual aggression had no such response, the study in Biological Psychology found.
The aggressive teenagers also reacted more strongly to pain that was accidentally caused.
The small study of 16-18 year olds - eight with a "conduct disorder" and eight with no aggressive tendency - suggests in some boys, natural empathetic impulses may be disrupted in ways that increase aggression, the researchers said.
Those with the conduct disorder had exhibited disruptive behaviour such as starting a fight, using a weapon and stealing after confronting a victim.
Tests were done using functional MRI scans while the participants looked at video clips in which people endured pain accidentally, such as when a heavy bowl was dropped on their hands, and intentionally, such as when a person stepped on another's foot.
Aggressive adolescents showed "a specific and very strong" activation of the amygdala and ventral striatum - areas of the brain that respond to feeling rewarded - when watching pain inflicted on others, suggesting they enjoyed watching pain, the researchers said.
And unlike the control group, the boys with conduct disorder did not show activation of the parts of the area of the brain involved in self-regulation - known as the the medial prefrontal cortex and the temporoparietal junction.
Using the same type of research, study leader Jean Decety, professor in psychology and psychiatry at the University of Chicago, has previously shown that seven to 12 year olds have naturally empathy for people in pain.
This is the first study to use fMRI to study situations which would normally prompt people to be sympathetic.
"This work will help us better understand ways to work with juveniles inclined to aggression and violence."
Dr Michael Eslea, senior lecturer in psychology at the University of Central Lancashire said the research was interesting but needed to be repeated in a larger sample.
"A better understanding of the biological basis of these things is good to have but the danger is it causes people to leap to biological solutions - drugs - rather than other behavioural solutions."