Up close and personal: investigating the development of psychopathology using intensive longitudinal data

Many people experience mental health problems at some point in their lives. A possible approach to improve our knowledge concerning the development of psychiatric symptoms might be to more closely observe the experiences people have. This can be done using daily diary designs, which is a method in which people answer questions about their mood, behavior and thoughts, one or more times a day for several weeks or months. With these measurements it is also possible to investigate how different thoughts, behavior and mood are related to each other for the person who completes the diary measurements. The hypothesis is that if symptoms easily trigger other symptoms and continue to activate each other, people have a greater risk of developing or experiencing persistent psychiatric symptoms. If this hypothesis is correct, it means that we can study the relations between symptoms to increase our understanding of how psychiatric symptoms and disorders develop. The studies in this thesis showed that the long-duration daily diary designs necessary to test this hypothesis are as feasible and acceptable as short-duration diary studies. However, the studies in this thesis also indicated that we may have been too optimistic about the potential of diary measurements to contribute to our understanding of how psychiatric symptoms develop in individuals. We found no indications for the idea that persons for whom symptoms are more interrelated over time experience more persistent symptoms, nor for the idea that specific associations between symptoms are indicative of differences between persons in the type of psychiatric disorders they experience. We saw many differences between people in symptom patterns, but could not connect these to other meaningful differences in functioning. The findings in this thesis mainly suggest that diary data adequately reflect individuals’ current condition, rather than their past or future vulnerability. In short, there is still a lot of work to be done if we want to better understand how psychiatric symptoms develop in specific people. This dissertation has shown that even with the availability of diary data, the challenges to use this data optimally in order to better understand the development of psychopathology are numerous.