|dc.description.abstract||Approximately 10 to 20% of children and adolescents suffer from a mental health condition. In addition, more than 40% of these children are likely to develop at least one additional mental illness concurrently or later in life.
A variety of different theories has tried to explain the high co-occurrence of mental health difficulties. Among these, developmental cascade models hypothesise that difficulties in one domain have directional effects on other mental health domains potentially through mediating factors such as peer problems. Previous research investigating such cascade models has been limited in that these models were mostly applied to the analysis of pairwise comorbidities even though most mental health domains have previously been found to be interconnected. Additionally, previous studies have not adequately operationalised the within-person processes implied by cascade models as methods commonly used (such as the cross-lagged panel model) confound within- and between-person effects.
This thesis aimed to gain a comprehensive understanding of the interrelations between multiple mental health domains across childhood and adolescence using methods suitable for disaggregating within- from between-person effects. In addition to domain level analyses, this thesis further intended to provide more detailed insights into the symptom level relations between different mental health problems in order to identify symptoms that might bridge different mental health disorders. Another aim of this thesis was to evaluate potential mediating factors in the developmental relations between different mental health domains. Finally, this thesis explored whether a range of phenotypic and polygenic risk factors increase the risk for developing co-occurring mental health problems. The data used in this thesis came from three longitudinal cohort studies: The Millennium Cohort Study, the Avon Longitudinal Study of Parents and Children and the Zurich Project on the Social Development from Childhood to Adulthood.
Results of the empirical studies suggest that all domains of socio-emotional development, that is conduct problems, hyperactivity/inattention, emotional problems, peer problems and prosociality are highly interlinked over development. In addition, findings of symptom level analyses between symptoms of ADHD and internalising problems as well as between symptoms of ADHD and aggressive behaviours indicated that specific symptoms might be particularly relevant in activating symptoms from another mental health domain, thus showing the value of moving beyond domain level analyses. Investigations of potential mediating factors in developmental relations between ADHD symptoms and internalising and externalising problems found no support for parental involvement, peer problems or academic achievement mediating their developmental relations. Finally, analyses of antecedents of co-occurring mental health problems showed that internalising and externalising problems likely share a similar set of underlying phenotypic and polygenic risk factors.
Overall, the results of this thesis highlight that there is a clear need to develop more integrative theories of comorbidity as all investigated mental health domains were connected to each other across childhood and adolescence. Such theories should not only acknowledge the complex interplay between different mental health problems but also acknowledge the many biological, sociological, and psychological factors that may affect how different mental health problems relate to each other within an individual.||en