Global study finds youngest in class more likely to be diagnosed with ADHD

A new global study led by Curtin University researchers has found that children who are the youngest in their classroom are more likely to be diagnosed with Attention Deficit Hyperactivity Disorder (ADHD) than their older classmates.

The research, published in the Journal of Child Psychology and Psychiatry, systematically reviewed studies that examine the relationship between a child’s age relative to their classmates and their chances of being diagnosed with, or medicated for, ADHD.

Seventeen studies covering more than 14 million children – five in the USA, two in Spain and one each in Canada, Finland, Germany, Netherlands, Iceland, Israel, Norway, Sweden, Taiwan and Australia – found it was more common for the youngest children in a classroom to be diagnosed with ADHD and medicated.

Lead author Dr Martin Whitely, Research Fellow at the John Curtin Institute of Public Policy based at Curtin University, said there are no biological markers or physical tests for ADHD and the diagnosis is based in large part on teacher reports of a child’s behaviour.

“It appears that across the globe some teachers are mistaking the immaturity of the youngest children in their class for ADHD. Although teachers don’t diagnose it, they are often the first to suggest a child may have ADHD,” Dr Whitely said.

“Our research shows that the ADHD late-birthday effect occurs in both high prescribing countries, like the USA, Canada and Iceland, and in countries where ADHD is far less common, like Finland, Sweden and Taiwan. Our findings challenge the notion that misdiagnosis only happens in countries where there is a high rate of prescriptions for ADHD.”

Dr Whitely explained that only two studies, both in Denmark, demonstrated a weak or non-existent late birth-date effect, with the majority of late-born Danish boys held back a year, which makes it unclear whether this prevents or just disguises the effect.

“Further research could help us determine whether allowing parents to decide if their child is ready to begin school helps reduce this and other late birthday effects. It could be that it helps the delayed-entry child but other children suffer,” Dr Whitely said.

Co-author Professor Jon Jureidini, a Child and Adolescent Psychiatrist from the University of Adelaide, said the findings highlighted the importance of teachers, doctors and parents being aware of the impact of relative age and giving the youngest children in class the extra time they need to mature.

“Mistaking perfectly normal age-related immaturity for ADHD is just one of many problems with the label. Children who are sleep deprived, bullied, have suffered abuse or have a host of other problems, often get labelled ADHD,” Professor Jureidini said.

“Not only does this result in them getting potentially harmful drugs they don’t need, but their real problems don’t get identified and addressed.”

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

The research titled, ‘ADHD late birthdate effect common in both high and low prescribing international jurisdictions: Systematic review,’ can be found online here.