Attention-deficit hyperactivity disorder is a genetic condition, research suggested today.
The behaviour of children with the disorder can be explained by differences in the brain rather than parenting skills or diet, according to the study by scientists at Cardiff University.
The team found rare copy number variants - where small segments of DNA are duplicated or missing - were twice as common in children with ADHD than those without the condition.
According to the research, published in the Lancet, there was overlap between the affected parts of the DNA and those associated with autism and schizophrenia.
The most significant overlap was found at a particular region on chromosome 16, which has previously been implicated in schizophrenia and other psychiatric disorders and spans a number of genes including one known to play a role in the development of the brain, the team found.
Controversy has surrounded the cause of ADHD because, although children with the disorder are statistically more likely to also have a parent with the condition, there was no direct evidence to show it was genetic.
"We hope that these findings will help overcome the stigma associated with ADHD," said Professor Anita Thapar, from the Department of Psychological Medicine and Neurology.
"Too often, people dismiss ADHD as being down to bad parenting or poor diet. As a clinician, it was clear to me that this was unlikely to be the case.
"Now we can say with confidence that ADHD is a genetic disease and that the brains of children with this condition develop differently to those of other children."
ADHD affects around one in 50 children in the UK, making them fidgety, restless, impulsive and easily distracted, often causing them problems at home and school.
There is no cure, although medication and behavioural therapy can help reduce symptoms.
The researchers analysed the genomes of 366 children diagnosed with ADHD against more than 1,000 control samples to search for variations in their genetic make-up.
They found 57 large, rare copy number variants in the ADHD sufferers compared with 78 among the 1,047 controls.
Nigel Williams, also from the Department of Psychological Medicine and Neurology, said: "Children with ADHD have a significantly higher rate of missing or duplicated DNA segments compared to other children and we have seen a clear genetic link between these segments and other brain disorders.
"These findings give us tantalising clues to the changes that can lead to ADHD."
John Williams, head of neuroscience and mental health at the Wellcome Trust, which was one of the funders of the research, said: "These findings are testament to the perseverance of Professor Thapar and colleagues to prove the often unfashionable theory that ADHD is a brain disorder with genetic links.
"Using leading-edge technology, they have begun to shed light on the causes of what is a complex and often distressing disorder for both the children and their families."