Researchers say it's more evidence that the disorder should be considered a neurological condition.
The condition is linked with inattention, hyperactivity and strong impulses and is thought to affect one in 20 under-18s.
The brains of ADHD sufferers had a slightly smaller overall volume, and five of the seven regions-including the amygdala, which is involved in emotional responses-were smaller in people with the condition, the researchers found. Nonetheless, the findings of previous studies identified areas within the basal ganglia - the brain region responsible for controlling cognition, voluntary movement, and emotion - as being involved in ADHD, with putamen and caudate regions being specifically observed as smaller in those with the condition.
"The results from our study confirm that people with ADHD have differences in their brain structure and therefore suggest that ADHD is a disorder of the brain", lead author Martine Hoogman, Ph.D., said in a news release.
"Similar differences in brain volume are also seen in other psychiatric disorders, especially major depressive disorder", adds Dr. Hoogman.
The differences in brain size were particularly prominent in the children and less obvious in the adults with ADHD, note the authors, who suggest that their findings show that ADHD is a brain disorder characterized by delayed development in several brain regions.
Hoogman hopes that these results will provide more understanding about ADHD and disprove certain stigmas about the disorder, such as that ADHD is caused by bad parenting or simply a label given to hard children.
The study, published February 15 in The Lancet Psychiatry, refutes the notion that ADHD is the result of poor parenting, the researchers said.More news: Trump to launch major review of spy agencies amid 'illegal' leaks
They analyzed MRI data from 1,713 people with ADHD and 1,529 controls ranging in age from 4 to 63 years.
The study also took into account people who had taken medication to treat ADHD, such as methylphenidate, more commonly known as Ritalin.
A new study, the largest ever of its kind, may shed light.
"These differences are very small - in the range of a few percent - so the unprecedented size of our study was crucial to help identify these".
Furthermore, the review has shown an insignificant difference in the brain volumes.
The study will hopefully create more empathy for children with ADHD.
In a related commentary, Claudia Lugo-Candelas, Ph.D., and Jonathan E. Posner, M.D., praised the group's collaboration on such a large study. The difference between the brains of those who have ADHD and those of the ones who don't is small.