Researchers say they have uncovered the root cause of Attention Deficit and Hyperactivity Disorder, a condition that affects millions of children.

Those with ADHD have difficulty focusing in school and completing assignments. Parents often blame themselves, but a U.S. study in the Journal of the American Medical Association revealed the problem originates in the chemical make-up of the brain-- specifically, a lack of proteins regulating the feelings of motivation and reward. 

Researchers already knew ADHD stemmed from abnormalities in the attention and hyperactivity systems of the brain,  but now they also know about deficits in the brain's motivation and reward centers.

This new discovery explains why people with ADHD are more prone to drug abuse and obesity, the researchers added.

Scientists point to faulty dopamine transmission as the reason ADHD patients have a hard time sticking with tasks they don't find interesting or where they don't see a payoff.

"Dopamine is considered a neurotransmitter that is crucial for our ability to perceive rewards and to be motivated in our behavior," said Nora D. Volkow, M.D. of the National Institute on Drug Abuse.

Researchers used brain imaging to measure dopamine markers in 53 patients who had ADHD compared to 44 patients who did not.

"There was a lower concentration of dopamine markers in the brain of individuals with ADHD," Volkow added. "Specifically in the areas of the brain that are involved with reward and motivation."

And the lower the level, the more pronounced the ADHD.

The study results support the use of medications that elevate dopamine levels and as well as teaching methods that make tasks seem more rewarding.

*Originally aired Sept. 10, 2009