Family, twin, and adoption studies provide strong support for the idea that genes influence the etiology of ADHD. Family studies find the parents and siblings of ADHD children to have a five-fold increase in the risk for ADHD. Children of ADHD adults have a ten-fold increase in risk, which has led to the idea that persistent cases of ADHD may have a stronger genetic component. Consistent with a genetic theory of ADHD, second-degree relatives (such as cousins) are at increased risk for the disorder but their risk is lower than that seen in first-degree relatives.
Family studies have provided evidence for the genetic heterogeneity of ADHD. Studies that systematically assess other psychiatric disorders suggest that ADHD and major depression often occur together in families; that ADHD children with conduct and/or bipolar disorders might be a distinct familial subtype of ADHD; and that ADHD is familially independent from anxiety disorders and learning disabilities. It may therefore be appropriate to divide ADH children into those with and those without conduct and bipolar disorders, thus forming more familially homogeneous subgroups. In contrast, major depression may be a nonspecific manifestation of different ADHD subforms.
Several twin studies have provided evidence of genetic influence on hyperactive and inattentive symptoms. An early study found the heritabil-ity of hyperactivity to be 64 percent. A study of ADHD in twins who also had reading disabilities reported the heritability of attention-related behaviors to be 98 percent. All twin studies considered together suggest that the heritability of ADHD is about 70 percent, which makes it one of the most heritable of psychiatric disorders.
Adoption studies also implicate genes in the etiology of ADHD. Two early studies found that the adoptive relatives of hyperactive children were less likely to be hyperactive or have associated conditions than the biological relatives. Biological relatives of hyperactive children also performed more poorly on standardized measures of attention than did adoptive relatives. A study using the contemporary definition of ADHD found that biological, not adoptive, relationships account for the transmission of ADHD.
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