This thesis investigates personality functioning and maladaptive traits as indicators and treatment targets in various, mostly clinical and adult populations. While personality functioning derives from object relations and mentalization theories, both theories of etiology and treatment within a psychodynamic paradigm, maladaptive traits derive from a lexical or multivariate paradigm within personality psychology. This thesis sought to answer questions on these concepts' assessment, etiological claims, and clinical utility. To these aims, five empirical studies are included. The first study investigated the psychodynamic theory that early childhood adversities and personality functioning interact in predicting adult psychopathology. Using a large representative sample, the study found that these factors explained up to 91% of the variance in symptoms of depression, anxiety, and somatization. In addition, previous analyses based on this data corroborate that personality functioning may be assessed reliably using a short questionnaire. The second study developed the PID5BF+ questionnaire to assess maladaptive traits as per DSM-5 and ICD-11. This measurement, validated on large clinical and non-clinical samples, successfully distinguishes between various psychopathologies, including differentiating internalizing disorders and borderline personality disorder. A slightly refined version was tested in 13 languages across 16,327 participants, proved reliable, and is now globally used in mental health studies. The third study investigated the lexical approach of maladaptive traits together with the etiologically informed approach of personality functioning, using data from mental health app users. It found that aspects of personality functioning, as outlined in object relations and mentalization theory, are central to psychopathology and explain large proportions of higher order variance in a hierarchical taxonomy of psychopathology. Specifically, interpersonal aspects of personality functioning were linked to externalizing disorders, while self-related aspects were associated with internalizing disorders. The study also demonstrated that personality functioning accounts for a significant portion of longitudinal variance in affective well-being and psychosocial functioning. Additionally, it highlighted that while personality functioning captures broad psychopathology aspects, maladaptive traits are more useful in capturing variance that is specific to particular disorders.
Study 4 explored if maladaptive traits are malleable through clinical psychological interventions, comparing a guided online intervention with face-to-face/blended psychotherapy in small clinical samples. While both treatments effectively reduced mental health symptoms, their impacts on maladaptive traits differed. The online intervention primarily influenced negative affectivity, consistent with its focus on targeting neuroticism. In contrast, the face-to-face therapy resulted in significant changes across all maladaptive trait domains and the overall personality dysfunction score. Notably, in the face-to-face setting, the reduction of personality dysfunction was a more robust predictor of post-treatment symptom reduction than in the online treatment. Study 5, a large randomized controlled trial, investigated the effects of a self-guided internet-based intervention, employing multiple imputation and random forest lee-bound methods to handle missing data. Precision of these methods depends on baseline variables that predict future outcomes and behaviors of the participants. Personality functioning and maladaptive traits were significant predictors for trial dropout, indicating that individuals with greater personality dysfunction tend to disengage more from self-guided digital mental health interventions, suggesting a need for more personalized support for this population in such interventions. Taken together, these studies establish that personality functioning is instrumental in capturing broad aspects of psychopathology, while maladaptive traits are more specific to individual psychopathologies. The findings suggest the potential for expanding the dual assessment approach of severity and style in the diagnostics of personality disorder according to ICD-11 to general psychopathology. Additionally, these studies show that personality dysfunction and maladaptive traits can change through clinical interventions and are useful in handling missing data in clinical trials. This thesis underscores the emerging relevance of these concepts, rooted in early psychological theories, for modern mental health research and practice.