lnternalizing problems in childhood and adolescence: predictive, etiological, and neurodevelopmental aspects

Sammanfattning: Internalizing disorders such as anxiety and depressive disorders are common mental disorders in childhood and adolescence. They are characterized by disordered emotions and moods, causing emotional distress and impairment. The overarching aim of the thesis is to investigate the relationship between internalizing problems and adverse outcomes in late adolescence/early adulthood, and between internalizing problems at different ages (i.e., childhood vs. adolescence), while taking various neurodevelopmental disorders (NDDs) into account. All studies are based on the Child and Adolescent Twin Study in Sweden, which uses self- and/or parent-reported survey data, clinical diagnoses from the Swedish National Patient Register, and educational data from the National School Register. Study I examines the relationship between anxiety at age 15 and psychiatric outcomes in late adolescence/early adulthood. The results show that adolescent anxiety predicted a variety of psychiatric outcomes, even when adjusting for NDDs. Study II describes the relationship between internalizing problems at age 15 and educational achievement in late adolescence/ early adulthood in a genetically sensitive design. Internalizing problems were found to be negatively associated with educational achievement. Additive genetics accounted for the largest part of the covariation between internalizing problems and educational outcomes, with roughly one half of unique genetic effects being due to internalizing problems and one half due to NDDs. Study III assesses the extent to which internalizing symptoms at age 15 can be explained by symptoms of anxiety, depression, and NDDs at age 9. The results showed that, overall, a small proportion of internalizing symptoms in adolescence can be explained by anxiety, depression, and NDDs in childhood. Study IV examines different occurrences of NDDs in individuals with anxiety/depression at age 9 versus age 15. Compared to individuals without anxiety/depression, those with childhood-onset anxiety/depression had a substantially higher prevalence of NDD screening diagnoses than did individuals with adolescent-onset. In conclusion, internalizing problems in adolescence can predict a plethora of psychiatric outcomes and educational underachievement, and they share etiological influences with the latter, independently of NDDs. Childhood and adolescent internalizing problems possibly represent different constructs and are differentially related to NDDs. Future studies investigating treatment approaches and the etiology of internalizing problems may benefit from taking age-of-onset into account.