Two studies have linked ADHD with a deficiency in dopamine in the brain. This may be one reason for higher risk for substance abuse in people with ADHD as they attempt to self-medicate their brains. (Amen, Daniel, MD. The Brain in the News, Amen Clinic Newsletter. August, 2007.)
The incidence of ADHD is about 10% of all school children and is found very much more in boys than in girls. The reason is because boys have a higher incidence of right brain dominance than girls do. The male hormone Testosterone boosts the right hemisphere and Estrogen the female hormone boosts left hemisphere. It presents as either a learning problem (left brain immaturity) or behavior problem (right brain excess), or both. (Levin, W. J. Dr. ATTENTIONAL DEFICIT HYPERACTIVITY DISORDER. 1999.)
Refer to Dysfunctions of the Brain for additional information.