Results of behavioral genetic and molecular genetic studies have converged to suggest
that both genetic and nongenetic factors contribute to the development of attention-deficit/hyperactivity
disorder (ADHD). We review this literature, with a particular emphasis on molecular
genetic studies. Family, twin, and adoption studies provide compelling evidence that
genes play a strong role in mediating susceptibility to ADHD. This fact is most clearly
seen in the 20 extant twin studies, which estimate the heritability of ADHD to be
.76. Molecular genetic studies suggest that the genetic architecture of ADHD is complex.
The few genome-wide scans conducted thus far are not conclusive. In contrast, the
many candidate gene studies of ADHD have produced substantial evidence implicating
several genes in the etiology of the disorder. For the eight genes for which the same
variant has been studied in three or more case-control or family-based studies, seven
show statistically significant evidence of association with ADHD on the basis of the
pooled odds ratio across studies: DRD4, DRD5, DAT, DBH, 5-HTT, HTR1B, and SNAP-25.