Researchers at Concordia University and the Centre for Research in Human Development may have resolved the cortisol paradox. In a groundbreaking study published in the journal Hormones and Behavior, they link cortisol levels not simply to behavior problems, but to the length of time individuals have experienced behavior problems.

"We studied the relationship between cortisol levels in young people with problematic behaviour such as aggression or depression, and the length of time since the onset of these behaviours," explains Paula Ruttle, lead author and PhD candidate at Concordia's Department of Psychology. "Cortisol levels were abnormally high around the time problem behaviours began, but abnormally low when they had been present for a long time."

To obtain subjects' cortisol levels, researchers analyzed saliva samples taken from 96 young people during early adolescence. They then matched cortisol levels to behavioral assessments taken in childhood and again during adolescence. Problem behaviours were classified as either "internalizing" (depression and anxiety) or "externalizing" (aggression, attentional problems).

Riding the cortisol roller coaster

Youngsters who developed depression-like symptoms or anxiety problems in adolescence had high levels of cortisol. However, those who developed symptoms earlier had abnormally low cortisol levels. The conclusion? Cortisol levels go up when individuals are first stressed by depression or anxiety, but then decline again if they experience stress for an extended period.

"It seems the body adapts to long-term stress, such as depression, by blunting its normal response," says coauthor Lisa Serbin, a psychology professor who is Ruttle's PhD supervisor and Concordia University Research Chair in Human Development.

"To take an extreme example, if someone sees a bear in the yard, that person experiences a 'flight or fight' reaction," continues Serbin, a member of the Centre for Research in Human Development. "Stress levels and therefore cortisol levels go up. However, if the same person sees bears in the yard every day for a year, the stress response is blunted. Eventually, cortisol levels become abnormally low."

Aggressive behavior in early childhood

At first glance, study results from children with aggressive behavior and attentional problems seem to contradict this theory. In this group they found that low levels of cortisol were related to aggressive behavior both during childhood and adolescence. However, the authors contend that since aggressive behavior often begins in the second year of life or earlier, subjects may have been stressed for years before entering the study, resulting in abnormally low cortisol levels.

"This blunted response makes sense from a physiological point of view," says Ruttle. "In the short term, high levels of cortisol help the body respond to stress. However, in the long term, excessive levels of cortisol are linked to a range of physical and mental health problems. So, to protect itself, the body shuts down the cortisol system -- but research shows that's not good either."

What, me worry?

Individuals with a blunted response to stress may not respond to things that would -- and should -- make other people nervous. For example, children with long-term behaviour problems perform poorly in school. Because of their blunted stress response, these youngsters may not be worried about exams, so they don't bother to prepare as much as their peers.

The study has many significant implications, according to Serbin. "This research suggests interventions should begin as soon as a behavioural problem appears," she says. "For children with severe externalizing problems, this may be very early, perhaps even when they are preschoolers or toddlers.

"We now have evidence that behavioural problems in children are linked to mental and physical health. Taking a 'wait-and-see' attitude may not be the right approach."

This research was funded by the Social Sciences and Humanities Research Council of Canada and the Canadian Institutes of Health Research.

Note: If no author is given, the source is cited instead.

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