One of the most contentious debates in child development in the United States is what's leading to the sharp rise in cases of attention-deficit hyperactivity disorder (ADHD). More than one in 10 children ages 4 to 17 are estimated to have the diagnosis, and scientists are looking at the role things such as genetics, environmental exposures and low birth weight might play.

An increasingly vocal group of educators has expressed concern that it has more to do with the increasing emphasis on academics in kindergarten in many districts and that children who have trouble sitting still and are more wiggly than their peers are being prematurely slapped with the label.

A new paper published in the Journal of Pediatrics provides some evidence to support that theory.

The study involved examining data from 378,881 children ages 4 to 17 from 1997 to 2011 in Taiwan and found that ADHD was significantly more likely to be diagnosed in the youngest children in a grade. Taiwan, like many U.S. school districts, has a cut-off date of Aug. 31 for school enrollment. The researchers found that boys and girls born in August were much more likely to receive an ADHD diagnosis than their counterparts born in September who were 11 months older.

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Boys born in August had the highest rate of diagnosis, at 4.5 per cent, while girls born in September had the lowest rate, at 0.5 per cent. The August boys were more likely to be medicated.

The trend held for those newly diagnosed in preschool and elementary school, but not for those who were given the label as adolescents. Author Mu-Hong Chen and colleagues wrote that this "may imply that increasing age and maturity lessens the impact of birth month on ADHD diagnoses."

Diagnosing ADHD is never black and white. Unlike some conditions for which there is a blood test or imaging test, no biomarker for ADHD exists. So doctors must sort through things such as parent interviews, teacher forms and checklists to determine whether a child is more hyper or inattentive than the norm. That inevitably involves comparing children to their classroom peers and may put the younger ones who are more immature at a disadvantage. In the study, the researchers noted the "subjectivity in ADHD diagnosis and treatment."

The findings from the Taiwan study are similar to studies conducted in Western countries including the United States, Canada, Spain and Sweden.

"Our findings emphasize the importance of considering the age of a child within a grade when diagnosing ADHD and prescribing medication for treating ADHD," the researchers wrote.