Phenotypic and measurement influences on heritability estimates in childhood ADHD

Abstract

Twin studies described a strongly heritable component of attention-deficit/hyperactivity disorder (ADHD) in children and adolescents. However, findings varied considerably between studies. In addition, ADHD presents with a high rate of comorbid disorders and associated psychopathology. Therefore, this literature review reports findings from population-based twin studies regarding the influence of subtypes, assessment instruments, rater effects, sex differences, and comorbidity rates on ADHD heritability estimates. In addition, genetic effects on the persistence of ADHD are discussed. By reviewing relevant factors influencing heritability estimates more homogeneous subtypes relevant for molecular genetic studies can be elicited. A systematic search of population-based twin studies in ADHD was performed, using the databases PubMed and PsycInfo. Results of family studies were added in case insufficient or contradictory findings were obtained in twin studies. Heritability estimates were strongly influenced by rater effects and assessment instruments. Inattentive and hyperactiveimpulsive symptoms were likely influenced by common as well as specific genetic risk factors. Besides persistent ADHD, ADHD accompanied by symptoms of conduct or antisocial personality disorder might be another strongly genetically determined subtype, however, family environmental risk factors have also been established for this pattern of comorbidity.

How French media have portrayed ADHD to the lay public and to social workers

“…Notice that the inference raised in this quotation is scientifically wrong: a high heritability does not prove a strong genetic causation because heritability studies cannot disentangle pure genetic effects from gene–environment interactions (Freitag, Rohde, Lempp, & Romanos, 2010; Visscher, Hill, & Wray, 2008). Moreover, estimates of ADHD heritability were strongly influenced by assessment instruments and rating scale (Freitag et al, 2010).…”

“…Moreover, estimates of ADHD heritability were strongly influenced by assessment instruments and rating scale (Freitag et al, 2010). For example, whereas most twin studies using rating scales reported high heritability estimates (60–80%) (Freitag et al, 2010), objective measurements of inattention and impulsivity led to lower estimates (30–36%) (Freitag et al, 2010; Heiser et al, 2006). …”

ADHD: a critical update for educational professionals

“…For example, the higher co-occurrence of ADHD between monozygotic twins (who share 100% of their genes) compared to their dizygotic counterparts (who share 50% of their genes) cannot rule out the influence of environment, as homozygote twins are often treated more similarly and more often have a physical and psychological closeness than their heterozygotic counterparts (Furman, 2008). Moreover, these studies still depend on observational tools to assess both parent and child behaviour, and the more sophisticated these tools are (and less prone to rater bias), the lower the estimated genetic effect (Freitag, Rohde, Lempp, & Romanos, 2010). Research into the co-occurrence of ADHD in families suffers from extreme difficulty to separate genetic influences from environmental factors (Furman, 2008) that typically run in families such as poverty, parenting style and divorce (Hjern et al, 2010).…”

Academic textbooks on ADHD genetics: balanced or biased?

“…For instance, “the assumption that genetic and environmental influences are independent” (Johnson, Penke, & Spinath, 2011, p. 258) does not hold when genes and environments correlate and interact with each other. Furthermore, the “equal environment assumption”, positing that fraternal and identical twin pairs have similar environments, is often scrutinized, for instance because “twins develop in different types of placental environments” (Freitag, Rohde, Lempp, & Romanos, 2010, p. 314).…”