Lost in fragmentation. Een ontwikkelings- en klinisch perspectief op beperkingen in zelf-functioneren als centraal kenmerk van persoonlijkheidspathologie.

This dissertation investigated the central role of impairments in self-functioning within personality pathology. Long recognized in clinical theory, impairments in self-functioning have been formally emphasized as a core feature of personality disorders (PDs) in the most recent dimensional models within psychiatric classification systems. In these models, the (in)ability to integrate aspects of the self into a cohesive and stable identity reflects the degree to which personality functioning is impaired. The emergence of these dimensional models focusing on impairments in self-functioning opens new avenues for research and clinical application, which this dissertation explored across three parts. Part I of this dissertation focused on measurement. A brief screener for impairments in self-functioning (the Five Item Screening Scale for Personality Disorder; FISSPD) was psychometrically evaluated in community adolescents/adults, and a clinical sample of adults. This screening tool showed promise for use in settings requiring a quick evaluation of the severity of personality pathology, as well as in specific populations such as adolescents. Part II adopted a developmental perspective, as we examined the longitudinal development of impairments in self-functioning throughout adolescence (age 12-19) in a community sample, and investigated real-time fluctuations and associations between aspects of impairments in self-functioning in a clinical sample of adolescents and emerging adults. Furthermore, we investigated long- and short-term associations between impairments in self-functioning and psychopathology, highlighting their transdiagnostic significance. In Part III, we bridged research and clinical practice by exploring how impairments in self-functioning align with the theoretical model underlying schema therapy, a widely used treatment for PDs. We did so by discussing existing literature on how aspects of schema theory relate to the dimensional PD models, and by an empirical investigation of the associations between impairments in self-functioning, personal identity, and schema modes. Collectively, the findings presented in this dissertation offer novel and valuable insights into impairments in self-functioning from both a developmental and clinical perspective. These findings contribute to both research and clinical practice. The dissertation concludes with recommendations for future research and clinical applications.