Heritability of Attention-Deficit Hyperactivity Disorder Across The Lifespan

Abstract

Abstract Attention-Deficit Hyperactivity Disorder (ADHD) is a common neurodevelopmental disorder affecting around 5-10% of children and 2.5-5% of adults worldwide. Research addressing the heritability of ADHD have yielded inconsistent findings, with studies in children suggesting higher heritability (75–90%) compared to studies in adults (30–50% )(Brikell, Kuja-Halkola, and Larsson 2015). To review the role of rater effects on the observed differences in heritability between children and adults, and to address the role of genetic and environmental influences on stability and change of ADHD over the life-span, we conducted a literature review of cross-sectional and longitudinal studies addressing the heritability of ADHD across ages, or in adulthood. We identified quantitative genetic studies published up until May 2017 by searching the NCBI s PubMed database using a combination of the keywords heritability, attention-deficit hyperactivity disorder, attention problems, ADHD, adult, genetic, self-report, twin, longitudinal and family studies. Based on the reviewed cross-sectional studies, the heritability of self-rated ADHD symptoms in adults ranged between 30%-44% and the heritability of self-rated symptoms in adolescent between 34%-54%. Cross-sectional studies including multiple informants showed that heritability estimates based on ratings from different informants (i.e., self-ratings and different parents/teachers rating each twin in a pair) consistently yield lower heritability estimates compared to studies based on ratings from a single informant. This effect appears to be independent of age. One study using cross-informant data in adults (combined parent and self-ratings) and two studies using register based clinical diagnoses in twin and family data reported the heritability of ADHD in adults to be 70–80%. The reviewed longitudinal twin studies suggest that stability in ADHD is largely due to genetic factors. In addition, longitudinal studies also provide evidence that new genetic effects for ADHD come online at different developmental stages and that these genetic factors are partly independent of those contributing to baseline symptoms. We identified one study assessing late-onset adult ADHD, which reported the heritability of both self-reported and co-informant (mother or co-twin) reported clinically relevant ADHD at age 18 to be around 35%. Together, the reviewed cross-sectional studies suggest that the reported lower heritability of ADHD in adults is likely to be at least partly explained by the switch from relying one rater (parent/teacher) in childhood, to relying on self-ratings (where each twin rates themselves) of ADHD symptoms in adulthood. When rater effects are addressed using cross-informant data or clinical diagnoses, the heritability of ADHD in adulthood appears to be comparable to the heritability of ADHD in childhood. Longitudinal studies show evidence for the role of genetic factors in both the stability and change in ADHD across ages. Emerging evidence suggest that the genes implicated in the onset of childhood ADHD are in part distinct from those associated with the developmental course of ADHD. Such findings raise new questions as to whether the heterogeneity in heritability estimates of ADHD across ages may be related to differential developmental trajectories of ADHD.