Lead author Theodore Beauchaine of the University of Washington said the study reaffirmed that boys ages 8 to 12 diagnosed with conduct and oppositional defiance disorders have lower heart rates and sweated less while at rest and playing a video game for money compared to boys without conduct disorders.

However, girls of the same age exhibited the same physiological responses whether they did or did not have conduct and oppositional defiance disorders.

The researchers took physiological measurements such as heart rate, blood flow and the workings of muscles and glands of 110 boys and 65 girls while they played a computerized game. About half of the boys and girls met the criteria for conduct and/or oppositional defiant order, the others did not.

"It was not unusual for some children to make $50 playing this game, which is a considerable amount of money for kids of these ages," Beauchaine said in a statement. "Normal boys get pretty excited while they play, but boys with conduct problems don't. However, we found no differences in the way the groups of girls responded."

The findings are published in Journal of the American Academy of Child & Adolescent Psychiatry.