Approximately 10-20% of children and adolescents experience socio-emotional difficulties severe enough to merit a diagnosis of a mental health disorder. More than 40% of these youth develop at least one other mental illness throughout their life. A developmental perspective that investigates the interrelations between different domains of socio-emotional development from early life to adulthood can offer important insights into why socio-emotional difficulties commonly co-occur. Using state-of-the-art longitudinal statistical techniques, this project aims to significantly advance our understanding of the mechanisms underlying co-occurring socio-emotional difficulties. It will illuminate the roles of both genetics and psychosocial factors in linking different socio-emotional difficulties together using robust study designs that consider a range of confounders and providing the best available evidence yet on the causal mechanisms underlying the co-occurrence of socio-emotional difficulties. As well as informing preventions and interventions, this will contribute to better, more comprehensive theories of the developmental roots of mental health.