ADHD and the symptom dimensions inattention, impulsivity, and hyperactivity

Abstract

Jepsen, J.R.M. & Michel, M. ADHD and the symptom dimensions inattention, impulsivity, and hyperactivity, Nordic Psychology, 58, 108–135 Attention-deficit hyperactivity disorder (ADHD) is a disabling condition with onset in early childhood. ADHD often co-occurs with educational failure and poor self-esteem. This literature review includes twin studies published in English between 1996 and 2004. Our inclusion criterium was either a quantitative estimate of the heritability of ADHD, or of its inattentive and impulsive-hyperactive symptom-dimensions. This article also contains an introduction to the twin design as a method to estimate the influences from genetic and environmental sources on complex psychological traits. The methodological introduction contains a discussion of the validity of a basic assumption in the twin design referred to as “the equal environment assumption”. All together, the studies reviewed include about 38.000 children. Based on parental information, the heritability estimates are generally very high and point to a dominant genetic influence on the symptom-dimensions associated with ADHD and on ADHD defined as a category. In contrast, the non-shared environmental factors are of less influence and the shared environmental factors are of no importance. The theoretical implications of these findings are briefly discussed and support the assumption of ADHD as a developmental disorder.