The genetic disorder causes hyperactivity in children who risk exclusion from school and sometimes even trigger threats of industrial action by teachers.
The study will rekindle the "nature or nurture" debate over disruptive behaviour, says a British educational psychologist working with Attention DeficitHyperactivity Disorder (ADHD) children.
The university researchers, in California and Toronto, have discovered a link between ADHD in children and an abnormality in a gene already known to cause impulsive adult behaviour.
The findings have been welcomed by educational psychologist Lisa Blakemore-Brown, who works with the Learning Assessment Centre in Pagham, West Sussex. However, she believes the research will be treated sceptically by those who blame poor parenting.
The American research, published in the journal Molecular Psychiatry, says a gene controlling transmission of dopamine, one of several molecular messengers vital for normal brain functioning, is more than twice as likely to be damaged in children with ADHD than those without.
There is likely to be at least one sufferer in every British classroom. In America, around 2 million or 1 in 20 schoolchildren are diagnosed with the disorder and treated with a controversial stimulant, Ritalin. American commentators see this as a significant first step in understanding the genetics of ADHD.
The discovery that a dopamine gene may be at the heart of some children's disruptive behaviour fits research into Ritalin. It corrects a deficiency in brain cell messenger transmission and helps 70 per cent of ADHD sufferers, but no one knows why. Mrs Blakemore-Brown says the research fits known patterns: it has been "blindingly obvious" that when families of affected children are studied "we find a number of relatives, father, grandfathers and so on all presenting the behavioural traits".
She says identifying a genetic site could lead to better-targeted medication without the side effects which have led to Ritalin being dubbed the "chemical cosh".
The researchers say that their sample - 78 children - is small and they want others to replicate their findings. But they also expect further defective genes to be found.
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