Differences in the brain rather than parental influence or poor diet causes attention-deficit hyperactivity disorder (ADHD), according to university study.
Children with ADHD have been found to have twice as many rare copy-number variants, compared with those without the condition. The variants are small segments of DNA that have been duplicated or are simply missing.
Researchers said the DNA segments overlapped with those linked with schizophrenia and autism, the most significant of which was found at a section of chromosome 16.
This chromosome has already been linked to several psychiatric disorders and affects several genes, such as one known to influence brain development, the Cardiff University team said.
The study is the first to offer evidence of a genetic cause of ADHD.
Anita Thapar, Professor at the university's department of psychological medicine and neurology, said: "We hope that these findings will help overcome the stigma associated with ADHD."
One in 50 children in the UK are believed to have ADHD, which makes them fidgety, restless, impulsive and easily distracted.