Genetic and environmental contributions to the inattention and impulsivity-hyperactivity associated with ADHD in 576 twin boys, aged 11 and 12 years, participants in the Minnesota Twin Family Study, were estimated by teacher ratings and maternal interviews in a report from the Department of Psychology and the Institute of Human Genetics, University of Minnesota, Minneapolis. Univariate analyses of the teacher and maternal reports confirmed the importance of genetic factors in the mediation of both inattention and hyperactivity-impulsivity subtypes of ADHD. For both traits, the genetic factor was considerably higher for maternal reports, which suggests a rater bias. Environmental factors, shared or nonshared among family members, had lesser contributions to the etiology of ADHD, and shared environment affected only the teacher-rated inattention dimension. 
COMMENT. Twin studies involving only males suggest that genetic factors are important in the cause of both inattention and hyperactivity-impulsivity subtypes of ADHD. Environmental experiences may exert a lesser effect on the development of these traits. The mode of inheritance of ADHD is undetermined, and results in affected girls, who characteristically demonstrate a greater degree of inattention, may be different. The inclusion of multiple informants, including psychologists and physicians, both neurologists and psychiatrists, would tend to lessen the influence of bias in studies involving ratings.
The extent of the genetic influence in the cause of ADHD varies according to the informant source . Environmental organic factors in etiology may receive greater emphasis and recognition among patients attending for pediatric neurology evaluation.
ADHD is a continuum, not a discrete entity, according to a large twin study, involving almost 2000 families recruited from the Australian MRC Twin Registry, and reported from the Prince of Wales Hospital, Randwick, NSW Australia . ADHD may be explained as an inherited trait with liability and expression throughout the population, a deviance from an acceptable norm, and not restricted to an arbitrary number of symptoms or DSM criteria. The need for treatment including medication is relative, and dependent on multiple factors.