Many people have ADHD, but where did it come from? A new study suggests that it might have been from of delayed maturation of the brain. Brains of people with ADHD have usually smaller structure or part for emotions.

Brain volume of people with ADHD or attention deficit hyperactivity disorder is usually smaller, CNN reported. When brain scans were reviewed, key brain regions, in particular the amygdala, which is in charge of controling the emotions, are found to be a size smaller. The size difference is big compared to people without ADHD, especially with children, according to a study published Wednesday in the medical journal The Lancet Psychiatry.

Studies suggested that there is more than one in every 20 people who have ADHD. ADHD is a neurodevelopmental disorder. It can be the reason as to why people are inattentive, impulsive and or hyperactive. Also, most symptoms are found out when child is turning into an adult, more or less two thirds.

According to Washington Post, there were seven regions in the brain that was studied or ADHD. Five of them are affected. The five are the caudate nucleus (related to goal-directed action), the putamen (connected in learning and responding to stimuli), the nucleus accumbens (linked to rewards and motivation) and the hippocampus (where memories start).

Studies about ADHD are usually having small samples. However, with this one, it has 1,713 people with a diagnosis of ADHD and 1,529 people without a diagnosis, ranging in age from 4 to 63 years old.

The amygdala is not really known to be linked with ADHD. What scientists know is that it is related to 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," geneticist Martine Hoogman of Radboud University in the Netherlands said.