TORONTO - Kids who are younger than their classmates are more likely to be diagnosed with or treated for attention deficit hyperactivity disorder, two new U.S. studies have found.
The studies appeared online, and will be in an upcoming print issue of the Journal of Health Economics. One of the studies by researchers at North Carolina State University, the University of Notre Dame and the University of Minnesota found large discrepancies in diagnosis and treatment rates based on small differences in dates of birth.
Children who were born just before the kindergarten eligibility date were compared to those close to their age but born just after the eligibility date. They had a 25 per cent higher rate of ADHD diagnosis and treatment, the study found.
There were no biological or medical reasons for the different ADHD diagnosis rates, said co-author Melinda Morrill, a research assistant professor of economics at North Carolina State.
"It appears that the youngest in the class who are likely less mature, or are exhibiting immature behaviour, that these children are being mistakenly diagnosed as having ADHD," she said from Jackson Hole, Wyo., where she was attending a conference.
Three separate data sources covering tens of thousands of children aged seven to 17 were used, she said.
The pattern was easy to see, she indicated, but one limitation of the data is that it's unclear whether the young children are being over-diagnosed or if the children who are relatively old are being under-diagnosed.
"We think that there's differential diagnosis based on something that doesn't have to do with underlying biological or medical reasons," Morrill said.
She noted that ADHD is a mental health disorder that's recognized by behaviours like inattention, hyperactivity and lack of focus -- and these are behaviours that all children exhibit at some point.
"According to National Institute of Mental Health, when a doctor is evaluating whether or not a child has ADHD, he asks the parents, 'Does this happen more often than the child's peers -- is it normal behaviour or is it more severe?"' she said.
"Because of that comparison that's built right into the description of ADHD, right into the diagnosis, we think that there's a real role for the child's peer group to matter in who gets diagnosed or not."
The other study of data on nearly 12,000 children by a Michigan State University economist had similar findings and is a "nice complement," Morrill said.
It found that "the youngest child in the class is 60 per cent more likely to be diagnosed with ADHD than the oldest child in the class."
The study was conducted by economist Todd Elder, and suggested that nearly one million children in the United States are potentially misdiagnosed with attention deficit hyperactivity disorder simply because they are the youngest and most immature in their kindergarten classes.
"If a child is behaving poorly, if he's inattentive, if he can't sit still, it may simply be because he's five and the other kids are six," Elder said in a release.
He said these kids are significantly more likely than their older classmates to receive prescribed stimulants to modify their behaviour, and this is worrisome because of the unknown impacts of long-term stimulant use on children's health.
Morrill said it's her hope that people recognize that this pattern of diagnosis and treatment is going on.
"And so when considering whether or not to give a diagnosis or to medicate a child, to take this into account, to think about whether the child is being compared to children of the same age and maturity level, or is the child being compared to children that are a year older and more mature and more able to sit still and pay attention."
