The exact cause of ADHD is still unknown. Several theories about possible causes have been examined and dismissed including eating refined sugar, playing video games, and watching too much television. Another theory is that a poor home environment or bad parenting may cause ADHD, but researchers are accumulating more and more evidence that ADHD does not stem from the home environment, but from biological differences within the brain.

Studies on the brains of people with ADHD show subtle differences in the structure and activity of certain brain areas. For example, the activity in a brain area called the prefrontal cortex which is involved in impulse control seems to be abnormal in people with ADHD. There also appears to be a difference in the activity of certain chemical messengers (neurotransmitters) called dopamine and norepinephrine in the brains of in people with ADHD. Additionally, there appears to be less activity in the areas of the brain that control activity and attention in people with ADHD. Finally, brain scans show that children with ADHD have 3-4 percent smaller brain volumes than children without ADHD.

But the question still remains, what creates these differences in the brains of people with ADHD? Researchers are examining several possibilities including genetics and brain injuries. Some children who have a history of brain injury may show some signs of behavior similar to that of ADHD, but not all children with ADHD have had a brain injury. ADHD tends to run in families so researchers are examining the possibility that genetics may influence the development of ADHD. Other factors that have been associated with ADHD and continue to be studied include fetal exposure to drugs and toxins (including cigarettes) and childhood exposure to certain environmental toxins such as PCBs (polychlorinated biphenyls) found in contaminated meat, fish, and dairy products.