Het kruipt onder de huid. Hoe temperament en boosheid verband houden met zelfverwonding.

Non-suicidal self-injury (NSSI) refers to the deliberate and direct destruction of one's own body tissue without suicidal intent, such as cutting or burning oneself (International Society for the Study of Self-Injury, 2024). Lifetime prevalence of NSSI among emerging adults is 13.4% (Swannell et al., 2014). According to Nock's (2009) integrated model, NSSI is triggered by both intra- and interpersonal factors. However, the role of reactive and regulative temperament and anger as intrapersonal risk factors of NSSI have been understudied. Therefore, this doctoral dissertation aims to understand "How temperamental dimensions and anger are related to non-suicidal self-injurious thoughts and behaviors". The first objective of this dissertation was to enhance the assessment of reactive temperament. In the first study, we investigated the psychometric properties of the Behavioral Inhibition System and Behavioral Activation System Scales (BIS/BAS Scales; Carver & White, 1994) in older adults. The findings showed that the BIS/BAS Scales were a valid and reliable instrument to assess temperament among older adults. In a second study, we examined the psychometric features of the Dutch Reinforcement Sensitivity Theory of Personality Questionnaire (RST-PQ; Corr & Cooper, 2016) among adults. We found that the original factor structure of the RST-PQ did not fit the data well. We developed a brief version of the RST-PQ, which showed good reliability and validity, creating new opportunities to assess reactive temperament in psychopathology. The second objective encompassed between-person differences between NSSI and aggression, as well as between the recency and severity spectrum of NSSI. In a third study, we investigated the differential relations between NSSI and aggressive behavior. NSSI was associated with high BIS (anxiety) and low BAS-Impulsivity, reflecting a depressive temperamental profile and supporting the emotion-regulation function of NSSI. Conversely, aggressive behavior was associated with low BIS and high BAS-Impulsivity. The fourth study examined NSSI across a spectrum of engagement, from no history to recent and severe NSSI. Depressive symptoms and NSSI versatility (i.e., number of NSSI methods) were key predictors of more recent and severe NSSI, confirming the need to tailor interventions that focus on emotion regulation strategies to reduce depressive symptoms and diminish the number of NSSI behaviors. The third objective was to examine NSSI in daily life. The fifth study used Ecological Momentary Assessment (EMA) to investigate anger at self and others among emerging adults with past-year NSSI. The findings showed that, in individuals with a history of NSSI, increased anger at self predicted NSSI thoughts and decreased perceived self-efficacy to resist NSSI. Anger at self also predicted NSSI, but this association faded when controlling for prior NSSI thoughts, emphasizing the need for interventions targeting anger reduction and adaptive emotion regulation. In sum, the research identified temperamental dimensions and anger as two important intrapersonal vulnerability factors of NSSI, endorsing the added value of interventions that focus on emotion regulation to prevent NSSI. The findings support a stepped-care approach, in which individuals' needs are addressed according to increasing levels of NSSI severity (Westers & Plener, 2020).