Children born in August who are the youngest in their school year are more likely to be diagnosed with attention deficit hyperactivity disorder, according to a new study warning natural immaturity is being medicalised.

Harvard University researchers studying data from US primary schools found that children were 30 per cent more likely to be diagnosed with ADHD if they were born in August compared to the first month of the school year, September.

Arbitrary school age cutoff dates could be contributing to this as an 11 to 12 month age gap can lead to a pronounced difference in attention span and other behaviour at younger ages, the authors said.

“Our findings suggest the possibility that large numbers of kids are being overdiagnosed and overt treated for ADHD because they happen to be relatively immature compared to their older classmates in the early years of elementary school,” said the study’s lead author Dr Timothy Layton, an assistant professor at Harvard Medical School.

However over time these differences would level out and Dr Layton and colleagues argue that doctors should be wary about the ages, relative to their peers, of children given medical referrals to diagnose ADHD.

The findings are published in The New England Journal of Medicine and are particularly relevant to the US which has some of the highest rates of ADHD diagnosis in the world.

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Experts said the UK diagnoses ADHD at one tenth the rate of the US and findings could not be directly compared, however previous studies have found evidence of age-related diagnosis differences outside the US.

Earlier this year Ofsted chief inspector Amanda Spielman warned parents were “medicating their children rather than tackling behavioural issues”, after figures showed Ritalin prescriptions doubled in a decade.

However a major study in August suggested the condition is highly treatable and hundreds of thousands more UK children should be on medication to prevent their education and life chances being hampered.

In the latest research Dr Layton and his colleagues looked at data from more than 407,000 children who started school between 2007 and 2009 and followed their medical records up to 2015.

It found 85 out of every 10,000 children born in August were diagnosed or treated for ADHD, compared to 64 per 10,000 born in September.

Overmedication is a growing issue and currently around 5 per cent of US children have a prescription for ADHD, in some states more than 15 per cent have a recorded diagnosis.

Psychiatry experts estimate around 3 per cent of the population have the condition on average around the world.

“As children grow older, small differences in age equalise and dissipate over time, but behaviourally speaking, the difference between a 6-year-old and a 7-year-old could be quite pronounced,” said Professor Anupam Jena, a health care policy expert and physician who led the study.

“A normal behaviour may appear anomalous relative to the child’s peer group.”

Failing to take account of these age differences also leads to variation in children’s academic and athletic performance with older children more likely to “excel” and be fast tracked for support, Professor Jena added.

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UK specialists cautioned against drawing comparisons with the UK as it has a radically different approach to prescribing and diagnosing ADHD, though they said the age effect could still be a factor.

“In terms of diagnosis there is a chance that this is happening here but I don’t think it’s documented [with UK studies],” Professor Philip Asherson​, a specialist in ADHD at King’s College London, told The Independent. “It’s also much less likely that has an effect on treatment decisions.”

“We treat roughly tenfold less than the US. Most studies I’ve seen suggest 1 per cent are offered treatment and very, very few are offered treatment at that age. It’s usually the point they enter secondary school, around age 10 to 12.”

Clinical guidelines in the UK also recommend ADHD is diagnosed by a specialist, unlike the US where it is usually done by GPs with an interest in child health.