And while the findings can't conclude whether brain abnormality is a cause of ADHD or the result of it, Posner says they suggest the behavioural problems in children with ADHD are actually neurological.
With funding from the National Institutes of Health, researchers from the global ENIGMA ADHD Working Group embarked on what they said is the largest study performed on brain differences in people with and without ADHD.
He notes that the study makes an important contribution by "providing robust evidence to support the notion of ADHD as a brain disorder with substantial effects on the volumes of subcortical nuclei".
"These differences are very small.so the unprecedented size of our study was crucial to help identify these", said Hoogman, the study's lead author.
Posner wasn't involved in the study, though he did publish a commentary in the same edition of Lancet Psychiatry based on the research.
Hoogman points to other conditions where brain size differences are commonplace, such as major depressive disorder (MDD). They found that the brains of children with the condition were slightly smaller in five regions, including those that control emotions, voluntary movement and understanding.
Attention Deficit Hyperactive Disorder (ADHD) results, in essence, in sudden bursts of hyperactivity, coupled with inattention and a frequent inability to focus. They found that two regions in the ganglia, the caudate and putamen, tend to be smaller in people with ADHD.
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From the MRI scans, the team could assess overall brain volume as well as the size of seven regions of the brain that previous studies have linked to ADHD.
The regions which had a reduced volume were the amygdala, the hippocampus, the nucleus accumbens, the putamen and the caudate nucleus.
The first author, geneticist Martine Hoogman of Radboud University in the Netherlands, said the amygdala "is a structure that is not so well known to be implicated in ADHD". The amygdala plays a role in regulating emotion; the nucleus accumbens is associated with reward processing; and the hippocampus is associated with motivation and emotional problems.
Despite the large numbers of participants of all ages, the study was not created to investigate how ADHD might develop over a person's lifetime.
Hoogman said: "We hope that this will help to reduce stigma that ADHD is "just a label" for hard children, or caused by poor parenting".
Dr. Jonathan Posner, associate professor of psychiatry at Columbia University in NY, was not involved in the study. Additionally, research was expected to decide the impact of medication on the brains of the individuals with ADHD, and how they developed as individual get older.
Another recent study in ADHD dates back to May a year ago, and posits that children with the disorder have some symptoms also found with rare forms of cancer.