According to the Mayo Clinic, more and more scientists are agreeing that changes in the brain may be a biggest cause of ADD / ADHD. They also agree that the environment may contribute to or worsen behavior.
Scientists have discovered some important differences in the brains of people with ADHD as compared to those without the disorder. They found that the brain chemicals called neurotransmitters don't work properly in people with ADHD. Also, there seems to be less activity in the areas of the brain that control activity and attention in people with ADHD. They also discovered that children with ADHD have up to 4 percent smaller brain volumes than do children without ADHD.
Most of us already know that ADHD tends to run in families. About 25 percent of children with ADHD have at least one relative with the disorder.
Women who smoke while pregnant have a higher risk of that particular baby having ADHD. Drug and alcohol abuse during pregnancy is suspected to reduce activity of the nerve cells that make neurotransmitters. Pregnant women who are exposed to the chemical, polychlorinated biphenyls (PCB's), is also more likely to have children with symptoms of ADHD.
Preschool children exposed to lead and PCB's are at increased risk of developmental and behavioral problems. Exposure to lead, which is found mainly in the paint and pipes of older buildings, has been linked to disruptive and even violent behavior and to a short attention span. The paint can flake and eventually become tiny pieces, mixed in with the playground soil.