Genetic factors in the expression of attention-deficit hyperactivity disorder.

ABSTRACT Although the definition of attention-deficit hyperactivity disorder has changed to some degree over time, there is evidence that genetic factors are important for the concurrent expression of inattention, impulsivity, and hyperactivity. The evidence regarding the hypothesis that genetic factors are important for the expression of this syndrome, and of elements of this syndrome, is reviewed. While genetic factors appear to be important, the specific genetic mode of transmission is not known. Currently available studies on this syndrome, based on research over the last 20 years, focus on varying phenotypic presentations. Further clarification of the definition of the phenotype, both in children and adults, is still needed, so that future research can proceed toward a more complete understanding of the genetic mechanisms.