ADHD is a brain disorder and should not be used as a convenient label for difficult children or poor parenting, the first major physical study of the condition has concluded.
Researchers analysed the brain volumes of more than 3,200 people and noticed that those of patients with ADHD (attention-deficit hyperactivity disorder) were underdeveloped in five key regions.
Areas governing emotion and motivation were found to be smaller than in the general population, regardless of whether the participants were taking brain medication.
The scientists behind the study, which is published in the Lancet, say their findings prove for the first time that the common condition has a physical cause.
Approximately one in 20 children under the age of 18 - about 400,000 - are affected by the disorder, which is characterised by impulsive behaviour, inattention and hyperactivity.
Two thirds of children affected continue to experience symptoms in adulthood.
Last year a separate study found the condition was being vastly over-diagnosed and often used as a slapdash term for a collection of behavioural problems, as well as simple immaturity.
Researchers have suspected for some time that “real” ADHD patients have different brain shapes to normal people of the same age, however previous studies have been to small to prove the hypothesis.
Dr Martine Hoogman, who led the new research at Radboud University in Nijmegen, said: “The results from our study confirm that people with ADHD have differences in their brain structure and therefore suggest that ADHD is a disorder of the brain.
“We hope that this will help to reduce stigma that ADHD is just a label for difficult children or caused by poor parenting.
"This is definitely not the case, and we hope that this work will contribute to a better understanding of the disorder.”
The international team of researchers measured the differences in the brain structure of 1,713 people with a diagnosis of ADHD and 1,529 without, all aged between four and 63-years-old.
All 3,242 people had an MRI scan to measure their overall brain volume, as well as the size in seven regions thought to be linked to ADHD.
Among the regions found to be underdeveloped in the case of ADHD patients was the hippocampus, which may contribute to the disorder through its role regulating emotion and motivation, the Lancet study said.
Dr Hoogman said that similar differences in brain volume can be seen in patients with other psychiatric conditions such as major depressive disorder.
“These differences are very small – in the range of a few percent – so the unprecedented size of our study was crucial to help identify these,” she said.
Prescriptions for drugs such as Ritalin for children diagnosed with ADHD are thought to have doubled in the last decade, despite concerns they can cause adverse reactions like weight loss, liver toxicity and suicidal thoughts.
The causes of ADHD are not known, however the condition has been shown to run in families, while premature birth or born to a mother who smoked or abused drugs or alcohol during pregnancy have all been suggested as having a role.
The Lancet authors said the next stage of research will be to design a study that can track ADHD sufferers as they grow up to understand how their brains develop.
Dr Wendy Burn, president-elect at the Royal College of Psychiatrists and co-chair of the College’s Gatsby Wellcome Neuroscience Project said: “Neither children nor their parents should be blamed for the diagnosis of ADHD. A child suffering from this common and often distressing form of mental illness cannot simply be classed as ‘difficult’ - their behaviour is affected by a disorder of brain function.
“It is vital that neuroscience research is applied in clinical practice so that doctors, particularly psychiatrists, are trained to understand how increased knowledge of brain development can significantly improve the care of people suffering from ADHD and other mental illnesses.”