In order to show that ADHD is a brain condition, researchers measured the overall brain volume and the volume of 7 regions that are linked to the disorder.

Brain volume and the size of five of the brain regions were smaller in those with ADHD, including the amygdala which is responsible for emotions, the study found.

The brains of people with ADHD are smaller in five regions than the brains of people without ADHD, according to a new worldwide neuro-imaging study led by Dutch researcher Barbara Franke of the Radboud University Medical Center in Nijmegen.

Study authors revealed that they did not observe a difference in people who took drugs for ADHD, which means that the drugs don't have an effect on the brain.

"Similar differences in brain volume are also seen in other psychiatric disorders, especially major depressive disorder", adds Dr. Hoogman. ADHD is more than "just a hard child", something you hear all too often about children with ADHD'.

Given the differences in brain structure they discovered, Hoogman and colleagues suggest that ADHD should be treated like a brain disorder, and not just some label for poor parenting or hard children. Although the study, published Wednesday in the Lancet Psychiatry, included children, adolescents and adults, the scientists said the greatest differences in brain volume appeared in the brains of children.

They analyzed MRI data from 1,713 people with ADHD and 1,529 controls ranging in age from 4 to 63 years.

She says that the "unprecedented size" of their study is crucial because it helped to identify the "very small - in the range of a few percent" differences in brain region sizes.

Most often diagnosed in children, ADHD is blamed for severe and repeated bouts of inattention, hyperactivity or impulsiveness that can cause problems at school or home.

The study also took into account people who had taken medication to treat ADHD, such as methylphenidate, more commonly known as Ritalin.

Alana Blow (L) who lives with ADHD and Marfan syndrome paints alongside All Black Sam Whitelock (R) during the New Zealand All Blacks "Cure Kids" Appearance at Auckland University on June 2, 2014 in Auckland, New Zealand. The researchers therefore assume that a delay in the development of the brain is a characteristic of ADHD.

Commenting on the study from an independent perspective, Jonathan Posner of Columbia University, who works in the field of ADHD science, described these findings as an "important contribution".

He said further research was needed to determine the effects of medication on the brains of people with ADHD, and how they develop as people get older.