Kids who are the youngest in their class are significantly more likely to be diagnosed and treated for ADHD, a new Canadian study has found.

A number of branches of research have shown that younger kids in a class often struggle keeping up with their older peers – particularly in the earliest grades.

Malcolm Gladwell famously explained the "relative-age effect" in his book "Outliers" noting that kids born later in the year are less likely than slightly older kids to be picked for hockey teams.

And in the classroom, kids born in November and December often appear to take longer to learn to read and write when compared to kids born almost a full year ahead of them.

Researchers from the University of British Columbia wanted to find out whether this "relative age effect" might affect diagnoses of attention-deficit/hyperactivity disorder, as well.

They looked at more than 900,000 children in British Columbia who were between 6 and 12 years old at any point between December, 1997, and November, 2008.

They found that kids born in December were 39 per cent more likely to be diagnosed with ADHD, and 48 per cent more likely to be treated with ADHD medication compared to kids born in January.

Although boys are usually about three times more likely to be diagnosed with ADHD, the age effect seen in the study was most pronounced among girls. Girls born in December were 70 per cent more likely to be given a diagnosis of ADHD compared to girls born in January.

Lead author Richard Morrow, a researcher in pharmaceutical policy at the University of British Columbia, believes that the relative age of kids is influencing whether they are diagnosed and treated for ADHD.

He notes that while the study looked only at kids in B.C., he's confident the results would be similar across Canada, since the schools systems are similar.

Morrow says his study suggests younger children are being inappropriately labelled and treated for ADHD, all because they act less mature.

"The lack of maturity of some children is sometimes being mistaken as a neurobehavioural disorder like ADHD, and that is raising questions about overdiagnosis and overtreatment," he told CTV News.

Morrow notes that a child born in December would be in the same class as one born much earlier in the year. While the difference in ages is only a matter of months, in elementary school, that can be a big difference.

"Some of the children in the class can be up to a year younger than other children. That can be quite a difference in maturity, in terms of children being able to sit still or follow through on tasks. We can expect their behaviour is different," he said.

Morrow points out there can be lots of problems of misdiagnosing kids with ADHD. The kids can be prescribed medications they don't need -- meds that carry side effects of their own, such as sleep disruption, heart risks and slower growth rates.

As well, kids who have been labelled ADHD may be treated differently by teachers and parents, which could lead to negative self-perception and social issues.

Morrow says doctors assessing kids for ADHD should take into account their relative ages, and put more emphasis on a child's behaviour outside of school, in multiple environments, as well as in the classroom.

Mary Lynn Trotter, mother of an eight-year-old girl born in November who was correctly diagnosed with ADHD, says classroom behaviour should not be the only indicator.

"I worry that people might get the wrong idea of ADHD, worry that it's a label attached to children in the classroom," she told CTV News.

"I seriously hope that teachers don't make those determinations in the classroom. If they see things of concern, they (should) inform parents."

The study was funded by a Catalyst Post-Market Drug Safety and Effectiveness grant from the Canadian Institutes of Health Research and a grant from the British Columbia Ministry of Health.

With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip