A new study in the Lancet provides the first direct evidence that genetic abnormalities are responsible for attention-deficit hyperactivity disorder, ADHD.

Although previous research had shown that ADHD can be inherited, no specific genes have been identified before that seem to underlie this condition. But that doesn't mean that someone with these particular genetic markers will have ADHD, experts say.

"This tells us that there is a biological marker that tells us that this person is susceptible to develop this disease," said Dr. Robert Marion, chief of genetics and developmental medicine at the Children’s Hospital at Montefiore and Albert Einstein College of Medicine, who was not involved in the study.

Researchers led by Dr. Nigel Williams at the Cardiff University School of Medicine in Wales fully analyzed data from 366 children with ADHD and 1,047 who did not have ADHD.

The Lancet study finds that children with ADHD have more large, rare copy-number variants than children without the condition. "Copy-number variants" are pieces of DNA that are either missing or extraneous in the chromosomes. Other studies have suggested that these variants may be associated with schizophrenia, autism, epilepsy, and other neurodevelopmental disorders.

Specifically, this study suggests a shared biological basis between ADHD and autism because of the shared genetic characteristics found in this study. Researchers also found genetic overlaps with schizophrenia.

""Eventually we’re going to be able to do testing that will identify susceptibility to ADHD or other conditions," Marion said.

But that test won't be available any time soon, especially because there appear to be a large number of variations in DNA that predispose a person to ADHD, Marion said.

The results suggest that ADHD is "not purely a social construct," the study authors wrote. "It’s a real hard and true disorder," Marion added.

On the other hand, this doesn't rule out environmental factors, Marion said. It may be that specific environments determine whether a person with these genetic variations goes on to develop ADHD, autism, schizophrenia, or nothing at all. However, scientists do not yet know precisely what those factors are.

In the future, as more becomes known about the genetics of ADHD, interventions may be tailored toward specific children before they begin to have serious problems in school, Marion said.

The study's "results are exciting, but how these findings will be clinically translated is still speculative," writes J. Peter Burbach, of the University Medical Center Utrecht in the Netherlands, in a Comment in the journal. More research should be done to further examine the associations between these genes and their outcomes.