ETIOLOGIES

ADHD has very strong biological contributions to its occurrence. While precise causes have not yet been identified, there is little question that heredity/genetics makes the largest contribution to the expression of the disorder in the population. The heritability of ADHD averages approximately 80 percent, meaning that genetic factors account for 80 percent of the differences among individuals in this set of behavioral traits. For comparison, consider that this figure rivals that for the role of genetics in human height. Several genes associated with the disorder have been identified and undoubtedly more will be so given that ADHD represents a set of complex behavioral traits and so a single gene is unlikely to account for the disorder. In instances where heredity does not seem to be a factor, difficulties during pregnancy, prenatal exposure to alcohol and tobacco smoke, prematurity of delivery and significantly low birth weight, excessively high body lead levels, as well as post-natal injury to the prefrontal regions of the brain have all been found to contribute to the risk for the disorder in varying degrees. Research has not supported popularly held views that ADHD arises from excessive sugar intake, food additives, excessive viewing of television, or poor child management by parents. Some drugs used to treat seizure disorders in children may increase symptoms of ADHD in those children as side effects of these drugs but these effects are reversible.