NEW YORK, Jan 13 (Reuters Health) -- Low levels of a stress hormone may be an underlying cause of serious, persistent aggression in young boys with behavioral problems, the results of a new study suggest. Among 7- to 12-year-old boys with behavioral problems, those who had low levels of the hormone cortisol were three times more aggressive and three times more likely to be considered the meanest kids by their classmates than boys with higher levels of the hormone.

"Kids become aggressive for all kinds of reasons, but there seems to be an association between starting (aggressive behavior) early and staying aggressive... that seems to be associated with low cortisol," the study's lead author, Dr. Keith McBurnett, of the University of Chicago, Illinois, told Reuters Health in an interview. While many children and teens go through a rebellious stage, there is a group of children whose aggressive behavior starts early and continues into adulthood, according to McBurnett.

"These are kids who have a basic difference in their temperaments," he said. According to the results of the study, these children with severe aggression tend to have low levels of cortisol, McBurnett noted. In the study, the investigators gathered data on 38 boys who showed aggressive problem behavior, such as being cruel to other people or animals, initiating fights, stealing from others or forcing others into sexual activity.

Each year for 4 years, the boys underwent examinations in which they were evaluated for signs of aggressiveness. During the second and fourth exams, levels of cortisol in saliva were measured.

Besides the professional evaluations and interviews with parents and teachers, the children were also judged by their peers. During the first 2 years, their classmates were asked to rate the behavior of all students, including naming the nicest, the meanest and the shyest student, as well as the one who got into the most fights. At the end of 4 years, there was a clear relationship between low cortisol levels and aggressive behavior, the authors report in the January issue of the Archives of General Psychiatry.

About one-third of boys with low cortisol levels were voted the meanest kid in class, compared to just one-tenth of the boys with higher cortisol levels. In addition, boys who had low cortisol levels were more likely to become aggressive at a younger age. Exactly how cortisol relates to behavior is uncertain, but researchers suggest that low levels of the hormone may affect how some aggressive children respond to stress.

According to McBurnett's team, it is unclear what causes the stress-control system to go awry -- genetics, some sort of stress in the womb or during infancy, poor parenting, deprivation or some other factor may be responsible.

Archives of General Psychiatry 2000;57.