London ? Don’t blame attention-deficit hyperactivity disorder on poor parenting or excess candy. A new study suggests it’s all about genes.

Hyperactive children have a larger proportion of small DNA segments that are either duplicated or missing, according to a study published online Thursday by the medical journal The Lancet.

The findings may help remove the stigma attached to the disorder, said Anita Thapar of Cardiff University School of Genetics, the study’s lead author.

“Some people say ‘it’s not a real disorder’ or ‘it’s the result of bad parenting’ and parents and children can encounter much stigma because of this,” Thapar said at a news conference in London on Wednesday. “So finding this direct genetic link to ADHD should help clear this misunderstanding.”

Thapar and her colleagues compared 366 children, ages 5 to 17, who had been diagnosed with ADHD with 1,047 who hadn’t. Children with other diagnosed psychiatric illnesses weren’t included in the study. Kids with ADHD were almost twice as likely to have stretches of large, rare, chromosomal deletions and duplications know as copy number variants, the researchers found.

The genetic abnormalities were found in the same positions as in patients with other neuro-developmental disorders like autism and schizophrenia, suggesting a common biological basis for the diseases, the researchers said. The children in the study who were intellectually disabled had more copy number variants. There was no overall difference between girls and boys, the researchers said.

The research hasn’t pinpointed a single gene, or group of genes, at the root of the disorder, according to Thapar. The findings probably won’t affect treatment guidelines, she said.

Patients with ADHD have difficulties with attention, increased activity and impulsivity, according to the American Academy of Child & Adolescent Psychiatry. Between 3 and 7 percent of school-aged children and about 4 percent of adults have ADHD, the group estimates.