Brain scans of teens with a history of aggressive bullying behavior suggest that they may actually get pleasure out of seeing someone else in pain, U.S. researchers said on Friday.

While this may come as little surprise to those who have been victimized by bullies, it is not what the researchers expected, Benjamin Lahey of the University of Chicago, who worked on the study, said in a telephone interview.

"The reason we were surprised is the prevailing view is these kids are cold and unemotional in their aggression," said Lahey, whose study appears in the journal Biological Psychology.

"This is looking like maybe they care very much," said Lahey, who worked on the study with Jean Decety, also of the University of Chicago.

The researchers compared eight boys ages 16 to 18 with aggressive conduct disorder to a group of eight adolescent boys with no unusual signs of aggression.

The boys with the conduct disorder had exhibited disruptive behavior such as starting a fight, using a weapon and stealing after confronting a victim.

They showed both groups video clips of someone inflicting pain on another person and tracked brain activity with a type of imaging called functional magnetic resonance imaging, or fMRI.

In the aggressive teens, areas of the brain linked with feeling rewarded — the amygdala and ventral striatum — became very active when they observed pain being inflicted on others.

But they showed little activity in an area of the brain involved in self-regulation — the medial prefrontal cortex and the temporoparietal junction — as was seen in the control group.

"It is entirely possible their brains are lighting in the way they are because they experience seeing pain in others as exciting and fun and pleasurable," Lahey said.

"We need to test that hypothesis more, but that is what it looks like," he said.

Lahey said the differences between the two groups were strong and striking, but cautioned that the study was small and needs to be confirmed by a larger study.