"I think most scientists in the field already know that the brains of people with ADHD show differences, but I now hope to have shown convincing evidence ...that will reach the general public", said Ms. Hoogman, "and show that it has [a basis in the brain] just like other psychiatric disorders".
"By the time they become adults, the differences in their brains are not significant anymore", she said.
Finding that the amygdala, the brain's emotional regulator, had the greatest volume reduction in ADHD was particularly important to the researchers because of the ubiquity of emotional problems in the disorder.
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. These results will encourage the scientific community to classify ADHD as a true disturbance, and not simply as a difficulty.
"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", she said.
Its ADHD project was four times the size of the previously largest study and was conducted at 23 locations in nine countries by 80 researchers, primarily psychiatrists and neuroscientists.
Attention deficit disorder ( ADHD ) is often associated with hyperactivity.
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Dr Hoogman said that similar differences in brain volume can be seen in patients with other psychiatric conditions such as major depressive disorder.
"Many people doubt not only the existence of the disorder, but also its biological basis", he says. The scans showed that overall brain volume is smaller for people with ADHD, according to Radboud University. These brain regions were the hippocampus, amygdala, nucleus accumbens, putamen, and caudate nucleus. Drugs for treating ADHD, such as Ritalin have been reprimanded for symptoms including weight loss or gain, damage liver, and suicidal thoughts.
"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". The hippocampus constitutes the area where our memories form, while the nucleus accumbens is the area responsible for processing motivation and reward.
The researchers concluded that a delay in brain development is typical of the condition, hoping the analysis would stop people associating it with "difficult children" or bad parenting.
This study, due to its much larger size, can make that link, says Arthur Caye, a psychiatry PhD student at the Hospital de Clínicas de Porto Alegre in Brazil who has published research on ADHD, and who was not involved in the study.
Typically, people with ADHD have poor attention skills and could be hyperactive.
ADHD causes inattention, hyperactivity and impulsivity - and at least one in 20 children in Canada are estimated to have the disorder. 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.