Environmental and Genetic Influences in Attention Deficit Hyperactivity Disorder (ADHD) and its Comorbidities

Research in past decades has demonstrated the persistence of attention deficit hyperactivity disorder (ADHD) into adulthood, but many questions regarding prevalence, causes, and comorbidities of ADHD in adults remain to be investigated. Previous research focusing on childhood ADHD identified high heritability. Genetic and environmental influences on ADHD symptoms in adults and their association with comorbid conditions are not fully understood. The overall aim of this thesis was to study adult ADHD symptoms in the population and investigate associations with substance use disorders (SUD) and binge eating. In all four papers, we used population-based self-report data from twins aged 20–46 years from the Swedish Twin Registry. We used twin methods to explore the role of genetic and environmental factors underlying ADHD symptoms and their comorbidities. Study I examined the phenotypic association between ADHD and various forms of SUD. ADHD in adults was strongly associated with alcohol abuse and alcohol dependence, illicit drug use and regular nicotine use, with no differences between ADHD subtypes and no apparent substance preference. In Studies II and IV, we used bivariate twin models to examine the role of genetic and environmental factors in the association of adult ADHD symptoms with alcohol dependence (II) and with binge eating (IV). For ADHD symptoms and alcohol dependence, 64% of the overlap was explained by common genetic factors. The remaining variance was accounted for by environmental factors specific for each twin, with no sex differences for the overlap. Similarly, 91% of the association between ADHD symptoms and binge-eating behaviour was explained by common genetic factors. In Study III, using a within-twin pair analysis, we demonstrated that although most of the association between adult ADHD symptoms and self-reported childhood maltreatment (an environmental risk factor for ADHD) was explained by familial (genetic and environmental) confounding, our results were also consistent with a causal interpretation. In conclusion, adult ADHD symptoms are associated with SUD and binge-eating behaviour. We replicated findings from adolescent studies regarding shared genetic risk factors for alcohol dependence and ADHD symptoms in adults. For binge eating, we showed for the first time that shared genetic factors mainly explained the association with ADHD symptoms. Alterations in mesolimbic reward processing as well as the frontal, executive and inhibitory systems have been described for ADHD, alcohol dependence and binge-eating behaviour, possibly suggesting common genetic and neurobiological factors for all three conditions. Results that support a causal hypothesis regarding the association between childhood maltreatment and ADHD symptoms in adults need follow-up in longitudinal clinical samples that can examine neurobiological underpinnings of environmental effects. Clinically, the results of this thesis support that ADHD in adults be considered and addressed in adults with SUD or binge-eating behaviour. Given the common genetic risk factors and the role of the early childhood environment, family interventions should be considered for these populations