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People with ADHD, attention-deficit/hyperactivity disorder, have different brain structures than people without ADHD, according to an international team of researchers that conducted the largest brain imaging study of its kind.

Of the seven brain regions they studied, five—including the amygdala, which is responsible for regulating emotions—were found to be smaller in those with ADHD compared with those in a control group. The other regions with decreased volume were the caudate nucleus (which plays a role in goal-directed action), the putamen (linked to learning and responding to stimuli), the nucleus acumbens (rewards and motivation), and the hippocampus (which helps form memories).

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Scientists find people with ADHD have smaller brain regions—including the amygdala, which is responsible for regulating emotions.

The amygdala had the greatest volume reduction, which the researchers thought was telling because of the extent of emotional problems that come with the disorder. Here’s Amy Ellis Nutt, reporting for The Washington Post:

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. … We do know from other functional studies of the amygdala that it is involved in emotion regulation and recognizing emotional stimuli. But it is also involved in the process of [inhibiting] a response. Both cognitive processes are characteristic of ADHD, so it does make sense to have found this structure to be implicated in ADHD.”

The research was conducted by a worldwide consortium called ENIGMA (Enhancing Neuro Imaging Genetics through Meta Analysis). More than 3,200 children, adolescents, and adults in nine countries underwent MRI brain scans. Nearly half of them had been diagnosed with ADHD, while the remaining participants served as controls.

In the past, small sample sizes stymied ADHD research, making many results unreliable. But now, thanks to this study, scientists believe they have conclusive evidence that brain differences and ADHD are not related to medication or other psychiatric disorders people with ADHD may also have. It also provides further evidence that ADHD is not a problem of motivation or parenting, as it is often portrayed.

ENIGMA scientists hope their study will also help the general public better understand the disorder and hopefully reduce stigma.