Serious mental illnesses (depressive disorder, bipolar disoder, psychotic disorder) are among the leading causes of disability. On top of the burden that mental illness itself poses, individuals affected by it are often also at higher odds of substance use (smoking, alcohol, cannabis) and cardiovascular disease. Together, these so-called co-morbidities of mental illness drive the low life expectancy in individuals with mental illness compared to the general population. It is unclear however, what the causal nature is of these relationships. It may be that symptoms of mental illness increase the risk of substance use and cardiovascular disease, but there is also evidence that these co-morbidities can increase the risk of developing mental ill-health. In this project we strive to elucidate the causal nature of the relationship between mental illness and its co-morbidities, by investigating how symptoms of mental illness and (precursors of) co-morbidities develop and associate to each other from early adolescence up to early adulthood.