The first direct evidence of a genetic link to attention-deficit hyperactivity disorder has been found, a study says.
Scientists from Cardiff University, writing in The Lancet, said the disorder was a brain problem like autism - not due to bad parenting.
They analysed stretches of DNA from 366 children who had been diagnosed with the disorder.
But other experts agued ADHD was caused by a mixture of genetic and environmental factors.
At least 2% of children in the UK are thought to have attention-deficit hyperactivity disorder (ADHD).
Affected children are restless and impulsive. They may also have destructive tendencies, and experience serious problems at school and within family life.
The researchers compared genetic samples from ADHD children, with DNA from 1,047 people without the condition.
They found that 15% of the ADHD group had large and rare variations in their DNA - compared with 7% in the control group.
Professor Anita Thapar said: "We found that, compared with the control group, the children with ADHD have a much higher rate of chunks of DNA that are either duplicated or missing.
"This is really exciting - because it gives us the first direct genetic link to ADHD.
"We have looked at lots of potential risk factors in the environment - such as parenting or what happens before birth - but there isn't the evidence to say they're linked to ADHD.
"There's a lot of public misunderstanding about ADHD. Some people say it's not a real disorder, or that it's the result of bad parenting.
"Finding this direct link should address the issue of stigma."
The researchers stressed that there is no single gene behind ADHD, and the work is at too early a stage to lead to any test for the disorder.
But they hope the study will help unravel the biological basis of ADHD. This could eventually lead to new treatments.
The work was largely funded by the Wellcome Trust, with extra support from the Medical Research Council.
The chief executive of a charity and support group ADDIS, Andrea Bilbow, said: "We are very excited. We've always known there was a genetic link - through studies and anecdotally.
"This paper will help us deal more confidently with the sceptics, who are always so eager to blame parents or teachers. It shows there is a definite genetic anomaly in children with ADHD."
But the study has been criticised by some experts.
Professor Tim Kendall, a director of the National Collaborating Centre for Mental Health, said a number of factors caused ADHD and blaming it purely on genetics could mean incorrect treatments.
"I'm pretty sure these studies are not going to produce undoubtable evidence that ADHD is caused solely genetically.
"I am saying it's a mixture of genetic and environmental factors, and the important thing is that we don't end up thinking this is a biological problem which is only subject to biological treatments like Ritalin."
Oliver James, a clinical child psychologist and broadcaster, cited studies which looked at the effect of anxiety among pregnant women, and disturbed early relations between mothers and their babies.
He said: "Only 57 out of the 366 children with ADHD had the genetic variant supposed to be a cause of the illness.
"That would suggest that other factors are the main cause in the vast majority of cases.
"Genes hardly explain at all why some kids have ADHD and not others."