Risk of Adolescent Depression in Youth with ADHD: A Longitudinal Twin Investigation

Objective: In two studies, this dissertation aimed to evaluate the robustness of the association between ADHD and depression, and to elucidate its etiology. Study I examined the predictive association between childhood ADHD and adolescent depression, and also between childhood ADHD and adolescent suicidality. The study investigated methodological explanations for the co-occurrence between ADHD and depression and tested whether the association between childhood ADHD and adolescent depression is attributable to rater bias effects, or is a consequence of shared comorbidity with conduct disorder. Study I also examined psychosocial factors and evaluated whether conflict with parents, academic underperformance, and victimization by peers mediate the association between childhood ADHD and adolescent depression. Study II examined the etiology of the relationship between ADHD and depression by estimating the magnitude of the genetic and environmental contributions to the co-occurrence between the two disorders. Method: Both studies were conducted in a population-based twin sample enriched with children with disruptive disorders and consisting of 998 twins. The participants were followed up in three multi-source structured diagnostic assessments, spanning 11 to 17 years of age. Study I hypotheses were tested using generalized linear models within a complex samples framework to account for the study's recruitment strategy and allow generalization of the results to the population. Study II utilized biometric modeling and incorporated sampling weights to take into consideration the oversampling of disruptive disorders. Results: The studies yielded four primary findings. First, symptoms of ADHD at age 11 years predicted depression that developed between the ages 11 and 17 years, and the prospective relationship between childhood ADHD and adolescent depression was not attributable to rater bias effects or conduct problems. A non-significant trend was suggestive of greater risk of depression associated with ADHD in females than males. Second, the co-occurrence between ADHD and depression was governed by shared genetic factors operating either directly or indirectly via gene-environment correlations or interactions. Third, symptoms of ADHD at age 11 years predicted suicide attempts and suicidal ideation experienced through age 17, but the association with suicide attempts could be accounted for by the degree of conduct disorder symptomatology. Fourth, exposure to conflict with parents, and for girls only, bullying by peers, partly accounted for the association between childhood ADHD and adolescent depression. Conclusions: The present findings provided novel and important insights into the risk of psychopathology associated with ADHD. The results indicated that childhood ADHD was a robust predictor of adolescent depression. The etiology of the relationship was primarily genetic, although the mediation of risk is at least in part environmental. The findings highlight the need for depression screening in youth with ADHD and expansion of preventative intervention.