The cause of ADHD has not been fully defined. One theory springs from observations in functional brain imagining studies between those with and without symptoms. However, other authorities point out that similar variations have been shown in studies of the structure of the brain of affected and non-affected individuals. Animal studies have demonstrated differences in the chemistry of brain transmitters involved with judgment, impulse control, alertness, planning, and mental flexibility.
A genetic predisposition has been demonstrated in (identical) twin and sibling studies. If one identical twin is diagnosed with ADHD, there is at 92% probability of diagnosis with the twin sibling. When comparing nonidentical twin sibling subjects, the probability falls to 33%. (Overall population incidence is 8%-10% in the U.S., as described above.)
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