A just published study by a team of researchers (which I am part of) has shown that it is the youngest children in the classroom who are most likely to be diagnosed with . The systematic review was published in the Journal of Child Psychology and . Seventeen studies covering more than 14 million children from various countries were examined. Lead author Martin Whitley commented in the Daily Mail that "It appears that across the globe some teachers are mistaking the immaturity of the youngest children in their class for ADHD." The study contributes to the central debate about ADHD and the question of medicalization: Do children diagnosed with ADHD have a brain disease?

Medicating the younger children in the classroom suggests that the medical community has mislabeled normal brain development as a pathology. If the ADHD children are simply the youngest children in the classroom, this would explain why researchers have not been able to develop an object test such as a brain scan, blood test, or genetic test to diagnose ADHD.

In spite of the fact that there are no biological markers that can be used to diagnose ADHD, official outlets can give the impression that this is the case. A recent brochure on ADHD from The National Institutes of Mental Health (NIHM) states that: “Brain-imaging studies have revealed that in youth with ADHD, the brain matures in a normal pattern but is delayed by about three years.” It continues, “More recent studies have found that the outermost layer of the brain, the cortex, shows delayed maturation overall.” It would be hard to fault the general public for concluding that brain scans can be used to identify ADHD. Not mentioned by NIMH is that the studies they refer to detected miniscule differences in a small group of children, and that the scans cannot be used in a doctor's office to determine if a child should be diagnosed with ADHD.

Birthdate is not the only source of variation when it comes to the diagnosis. The diagnosis also varies from one country to another: The CDC recently reported that one in every 10 children in the United States over the age of three has been diagnosed with ADHD, while in France, for instance, it is extremely rare. In the late 1990's, Gretchen Lefever reported that 20 percent of schoolboys in southeastern Virginia had been diagnosed and treated for ADHD. And children in Appalachia are more likely than children in California to be diagnosed—23 percent of school-aged boys in the South have received a diagnosis.

, citizenship, state of residence, and now birthdate have all been shown to play a role in who receives a diagnosis. These findings suggest that more than biology is involved.

This research was co-authored by researchers from the John Curtin Institute of Public Policy at Curtin University, University of Adelaide, including international researchers Sami Timimi from Lincolnshire Partnership NHS Foundation Trust, Jonathan Leo from Lincoln Memorial University in the U.S., Joanna Moncrieff from University College London, and Patrick Landman, a psychiatrist from France.