You may recall the glint of joy in that bully's eyes way back when. Yes, he probably enjoyed it.

A new study used brain scans to show that unusually aggressive youth may actually enjoy inflicting pain on others.

Past research has shown that up to half of sixth graders say they've been bullied. Other studies have suggested too much TV can contribute to making bullies out of kids, but there seems also to be a pure biological component: Finger length is known to be a predictor of male aggression. Bullying doesn't end with childhood. Research finds bullying to be common in the workplace.

In the new work, researchers studied eight boys age 16 to 18 who had started fights, used weapons or stolen something after confronting a victim. A control group of boys had no such history of aggression.

The youth were shown 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.

The results suggest natural empathy responses can be distorted in ways that increase aggression.

"Aggressive adolescents showed a specific and very strong activation of the amygdala and ventral striatum (an area that responds to feeling rewarded) when watching pain inflicted on others, which suggested that they enjoyed watching pain," said Jean Decety, Professor in Psychology and Psychiatry at the University of Chicago. Unlike the control group, the youth with conduct disorder did not activate the area of the brain involved in self-regulation (the medial prefrontal cortex and the temporoparietal junction).

The control group reacted similarly to younger children in a study released earlier this year, in which Decety and his colleagues used brain scans to show 7- to 12-year-olds are naturally empathetic toward people in pain.

The new findings are detailed in the current issue of the journal Biological Psychology. The National Science Foundation supported the work.