My name is legion for we are many: lessons learned from linking and splitting psychiatric disorders

If an individual has a psychiatric disorder, this individual has a high chance of having another psychiatric disorder. This is called comorbidity. The aim of my dissertation is to increase our knowledge about the comorbidity of psychiatric disorders.
The first part of my dissertation focuses on ADHD and shows that the prevalence of ADHD gradually decreases throughout the lifespan, and that adults with ADHD indeed often have other psychiatric problems. These problems persist or even increase during adulthood, while they decrease in individuals without ADHD. Although all psychiatric problems can co-occur, ADHD is most strongly related to aggressive behaviour problems and autism spectrum problems, both within individuals and within families.
The second part of my dissertation demonstrates that comorbidity arises due to shared genetic liability and shared environmental risk factors among psychiatric problems. Most environmental risk factors increase the likelihood of any psychiatric disorder, particularly in the presence of high genetic liability. In the case of moderate genetic liability, a small number of risk factors are specifically linked to internalizing problems, externalizing problems, or developmental disorders.
Despite new classification systems attempting to incorporate comorbidity and shared causes of psychiatric disorders, my dissertation concludes that these do not provide better alternatives to the currently most used classification system, the DSM. Future research should therefore focus on improving the DSM. To achieve this, researchers should go beyond the current disorders in the DSM and link psychiatric disorders based on comorbidity and split them based on cause.