At face value it sounds too arbitrary to be possible.
A child's birthdate is a powerful predictor of whether they will be medicated for ADHD, an Australian study suggests.
In reality, the findings show children who are younger and less mature than their classmates are being misdiagnosed and prescribed ADHD drugs, its lead author argues.
The new research published Monday wades into a fierce decades-long debate over ADHD diagnosis and whether unruly children are being medicated inappropriately.
The analysis of more than 300,000 Western Australian school children found the youngest in the class were significantly more likely to have been given drugs to treat ADHD compared to their older classmates.
Children aged six to 10 years old who were born in June (the last month of the recommended school year intake) were roughly twice as likely to have been prescribed ADHD medication on the PBS than children born in the previous July, the oldest in the class.
The effect was less marked but still significant among children aged 11-15, found the study published in the Medical Journal of Australia.
International studies found similar results. A population study of almost 1 million Canadian children found boys who were the youngest in their school year were 30 per cent more likely to be diagnosed with the condition, and 41 per cent more likely to be medicated.
The problem was the diagnostic criteria for ADHD was too vague, said lead researcher Martin Whitely, former WA politician and critic of ADHD prescribing among children.
"[The criteria] are sloppy and imprecise and so we get these crazy distortions," said Dr Whitley, whose PhD thesis investigated ADHD policy in Australia.
"To diagnose kids with ADHD we rely on subjective things like whether they're distracted easily, playing too loudly, fidgeting in their seat, disliking or avoiding homework, climbing excessively, and interrupting. Of course we're going to see problems with diagnostic reliability," he said.
ADHD was the most common mental disorder in children and adolescents (7.4 per cent), followed by anxiety disorders (6.9 per cent), major depressive disorder (2.8 per cent) and conduct disorder (2.1 per cent), found the Department of Health's 2015 Mental Health of Children and Adolescents report.
The chief architect of the fourth edition of the psychiatric bible, the Diagnostic and Statistical Manual (DSM-IV) Dr Allen Frances famously disparaged his own work, saying the DSM-IV had unwittingly triggering an ADHD epidemic by lowering the threshold for diagnosis after the review process was inundated with drug company marketing.
We need to throw away the "meaningless label" of ADHD altogether, Dr Whitely said.
"It's a non-explanation that stops us looking at what's really going on with these kids," he said.
"Sometimes they just need to grow up, sometimes it's trauma, bullying, eyesight, sleep deprivation, poor nutrition, inappropriate teaching styles, they're too bright and bored in class or they're struggling and disadvantaged.
"There were thousands of possible reasons, but when you throw a silly label at them, and amphetamines, that's child neglect," he said.
The number of people treated with ADHD drugs on the PBS has risen by 31 per cent over five years, a 2015 Department of Health report showed.
Dr Whitley said a big risk factor for an ADHD diagnosis was which doctor's door a child walks through.
"There are some high-profile prescribers, in NSW in particular, where you are pretty much guaranteed to walk out with a script. Other doctors are far more cautious and would have a completely different approach to those kids," he said.
Royal Australian and New Zealand College of Psychiatrists chair of child and adolescent psychiatry Dr Nick Kowalenko said the argument that immaturity may be a contributing factor to ADHD diagnosis and treatment warranted investigation.
Yet while there was the occasional "rogue prescriber", the bulk of Australian clinicians involved in treating children with behavioural problems did so with great care, Dr Kowalenko said.
"There has been a lot of work done to ensure the quality of diagnosis by practitioners, particularly for pediatricians, who are the group most likely to see these children," Dr Kowalenko said.
The prescribing rate of ADHD medication in Australia was roughly consistent with the prevalence of ADHD, Dr Kowalenko said.
"It's really important that parents are informed and can ask prescribers the questions that need to be asked to ensure their child receives all the support they might need," he said
Questions including what a comprehensive treatment for ADHD entails beyond medication, a trial of any medication that is reviewed, input from the child's school and teachers, the risks and benefits of prescribing medication, Dr Kowalenko said.
"The benefits need to clearly outweigh the risks," he said.