New research has found the youngest children in West Australian primary school classes are twice as likely as their oldest classmates to receive medication for Attention Deficit Hyperactivity Disorder (ADHD).

Published in the Medical Journal of Australia, the research analysed data for 311,384 WA schoolchildren, of whom 5,937 received at least one government subsidised ADHD prescription in 2013. The proportion of boys receiving medication (2.9 per cent) was much higher than that of girls (0.8 per cent).

Among children aged 6–10 years, those born in June (the last month of the recommended school-year intake) were about twice as likely (boys 1.93 times, girls 2.11 times) to have received ADHD medication as those born in the first intake month (the previous July).

For children aged 11–15 years, the effect was smaller, but still significant. Similar patterns were found when comparing children born in the first three months (July, August September) and the last three months (April, May, June) of the WA school year intake.

The ADHD late birth date effect was fist demonstrated in four large scale studies conducted in the US, Canada and Taiwan. The prescribing rate for children in the WA study was 1.9 per cent, slightly larger than that reported in the Taiwanese study (1.6 per cent). The late birth date effects identified in WA and Taiwan were of similar strength to those in the three North American studies, where the reported prescribing rates were much higher (4.5 per cent, 5.8 per cent and 3.6 per cent).

We need further research on the ADHD late birth date effect in other Australian states, which unlike WA, allow greater flexibility for parents in deciding when their child starts school. It could be that allowing parents to decide when their child is ready for school prevents misdiagnosis. Alternatively, the greater age range within a class that occurs when there is increased flexibility could exacerbate the late birth date effect.

Why does birth date affect ADHD diagnosis?

A likely cause of the late birth date effect is that some teachers compare the maturity of their students without due regard to their relative age, resulting in higher rates of diagnosis among younger class members. Of course, teachers don't diagnose ADHD; that can only be done in most Australian states by a paediatrician or child psychiatrist.

But research has demonstrated in many cases that teachers are the first to suggest a child may have ADHD. Even when they don't encourage parents to have their child assessed for ADHD, teachers still play a central role in the diagnostic process by providing information about a child's behaviour compared to "age appropriate standards".

Questioning ADHD as a diagnosis

The late birth date effect is not the only factor creating unease about ADHD. Multiple studies, including the WA study, have established boys are three to four times more likely to be medicated for ADHD. If, as is routinely claimed, ADHD is a neurobiological disorder, a child's birthdate or gender should have no bearing on their chances of being diagnosed.

Other risk factors for receiving medication for ADHD include race, class, postcode and clinician, teacher and parental attitudes; none of which have anything to do with a child's neurobiology.

In addition, sleep deprivation, bullying, abuse, trauma, poor nutrition, toxins, dehydration, hearing and eyesight problems, giftedness (boredom), intellectual disadvantage (frustration) and a host of other factors can cause the impulsive, inattentive and hyperactive behaviours central to the diagnosis of ADHD.

Another common criticism of ADHD as a pathological condition is that the diagnostic criteria "medicalise" normal - if somewhat annoying - childhood behaviours. Critics contend teacher and parent reports of children "often" fidgeting, losing toys and pencils, playing loudly, interrupting, forgetting, climbing or talking excessively, being disorganised and easily distracted, failing to remain seated, and being on the go (as if driven by a motor) should not be construed as evidence of a psychiatric disorder best treated with amphetamines.

Proponents counter that stimulant medication for ADHD children is like "insulin for a diabetic" or "eyeglasses for the mind". There is no doubt low dose stimulants often make rowdy children more compliant. However, a 2010 WA Health Department study found ADHD diagnosed children who had used stimulants were 10.5 times more likely to fail academically than children diagnosed with ADHD but never medicated.

As evidenced by rapidly increasing child ADHD prescribing rates in Australia and internationally, ADHD proponents seem to be winning the very public and ongoing ADHD debate. But history has taught us that as societal values change, definitions of mental illness change. It wasn't long ago that the inventors of ADHD as a diagnostic entity, the American Psychiatric Association, classified homosexuality as a disease treatable with electric shock and other forms of aversion therapy.

Perhaps in the future playing loudly, talking and climbing excessively, fidgeting and disliking homework will no longer be regarded as evidence of a psychiatric disorder, best treated with amphetamines and similar drugs.

Martin Paul Whitely is visiting fellow at Murdoch University and Suzanne Robinson is associate professor of Health Policy and Management at Curtin University.

Originally published in The Conversation.