The study raises new concerns that some children who are simply immature (compared to their classroom peers) may be misdiagnosed and unnecessarily treated for ADHD, which is characterized by poor attention and impulse control.
Researchers followed more than 900,000 children living in British Columbia, Canada, where the cutoff for entry into kindergarten or first grade is Dec. 31 -- meaning that children born in December would be close to a year younger than classmates born in January.
Compared to children whose birthdays were in January, boys born in December were about 30% more likely to be diagnosed with ADHD. Girls born in December were 70% more likely to be diagnosed with ADHD.
Treatment also varied. Boys born in December were 41% more likely to be treated and girls 77% more likely to be treated with prescription medications for ADHD than their counterparts born in January.
Several earlier studies have also found a higher incidence of ADHD among the youngest children in classrooms, suggesting that less mature children may be inappropriately labeled and treated for ADHD.
"It certainly appears that in some cases lack of maturity is being misinterpreted as ADHD, and it raises alarms about over-diagnosis," says researcher Richard L. Morrow, of the University of British Columbia.
Although boys are 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% more likely to have a diagnosis of ADHD, compared to girls born in January.
Psychiatrist and co-researcher Jane Garland, MD, says parents, teachers, and clinicians need to be aware that immaturity may mimic symptoms of ADHD, adding that more emphasis on behavior outside the school setting may be needed for children who are younger than their classmates.
The study appears in the March issue of the Canadian Medical Association Journal.
Is ADHD Overdiagnosed?
It has long been known that younger children tend to struggle more, academically, than their older classmates, says psychologist Thomas Power, PhD, who directs the Center for Management of ADHD at The Children’s Hospital of Philadelphia.