Genetics of childhood disorders: XXI. ADHD, part 5: a behavioral genetic perspective.

We believe that a valid classification is an essential step in science. In medicine, and hence in psychiatry, classification is diagnosis. —Robins and Guze, 1970, p. 107 As conceptualized in DSM-IV, attention-deficit/hyperactivity disorder (ADHD) is a common syndrome of childhood characterized by problems of inattention and/or hyperactive-impulsive behavior that is frequently associated with comorbid psychiatric conditions and educational or vocational failure. The prevalence of ADHD is estimated to be between 3% and 9% in children and several-fold higher in boys. These prevalence estimates and recent prescription patterns for stimulant medications have led to controversies in the lay press regarding possible overdiagnosis and overtreatment of ADHD. The results of modern twin studies suggest that much of the controversy surrounding the supposed epidemic of ADHD is secondary to fundamental misconceptualizations about its nature. Notwithstanding the tremendous advances in treatment and research that have been made possible by the adoption of a standard diagnostic nomenclature, the DSM approach is categorical. In contrast, the results of several population-based twin studies from around the world are most compatible with ADHD representing the extreme of one or more continua of problems with inattention and hyperactivity-impulsivity (for example, Levy et al., 1997). If correct, ADHD is akin to common medical conditions such as hypertension or diabetes in that the criteria for defining who is ill represent an arbitrary cutoff along a continuum of blood pressure or serum glucose level, respectively. The definition of a case, then, becomes entwined with the efficacy of our treatment approaches, whether pharmacological or behavioral. The conceptualization of ADHD as a quantitative trait is in keeping with the noncategorical tradition such as represented by the Attention Problems (AP) scale of Achenbach (1977). Whether viewed as a discrete disorder or as a quantitative trait, estimates of the heritability of ADHD are uniformly high (reviewed by Faraone and Biederman, 1998; Todd, 2000).