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Numerous studies have shown that childhood maltreatment is an important risk factor for the development of various mental disorders, including depression (Green et al., 2010; Nelson et al., 2017). Depressed patients who were exposed to childhood maltreatment also show a more severe course of the disease and a higher treatment-resistance (Nelson et al., 2017). Therefore, it is of high relevance to understand the connections and mechanisms between childhood maltreatment and depression to be able to support this patient group more effectively in the long term. However, little is known about the differential effects of different types of child abuse and neglect on mental disorders in adulthood. Therefore, in this dissertation, the effects of the five specific types of childhood maltreatment - emotional abuse, emotional neglect, sexual abuse, physical abuse, and physical neglect - are considered separately (aim 1). This is particularly relevant since the focus of previous research was mainly on sexual and physical abuse (Stoltenborgh et al., 2015). However, current research indicates that emotional maltreatment is particularly prevalent and could have at least comparable consequences in the long run (e.g. Nelson et al., 2017; Ross et al., 2019). The underlying mechanisms of the effect of childhood maltreatment on the development and maintenance of depressive symptoms are also poorly understood. Therefore, based on clinical theories, attachment style, social cognition, interpersonal problems, emotion regulation, and self-compassion are investigated as potential mechanisms in the present dissertation (aim 2). Knowledge of potential psychological mechanisms is particularly relevant as they represent important starting points for prevention programs and therapies. The theory of the Cognitive Behavioral Analysis System of Psychotherapy (CBASP), a psychotherapy approach specifically tailored to patients with a persistent depressive disorder (PDD), also assumes that patients with PDD were particularly often exposed to childhood maltreatment and, as a result, exhibit changes in social cognition and their interpersonal behavior (McCullough, 2003). Since the assumptions of the CBASP approach are largely based on clinical experience, these hypotheses need to be further empirically tested. Therefore, the third aim of this work is to investigate differences between patients with PDD and patients with episodic depression (ED). These three research aims are addressed by four studies that form the cumulative dissertation. Data of studies I and II was drawn from the subproject 1 of the DFG Research Group 2107. Data of study III was collected in a psychiatric inpatient clinic, a psychosomatic inpatient clinic, and in a psychotherapeutic outpatient clinic. Study IV was conducted as part of the CBASPersonalized project to evaluate a six-week inpatient treatment program in a psychosomatic inpatient clinic. Study I compared the prevalence of childhood maltreatment types in patients with depression (n = 604), schizophrenia (n = 107), bipolar disorder (n = 103), and in healthy control participants (n = 715). Emotional abuse, emotional neglect, and physical neglect were the most frequently reported types in all groups. The three patient groups did not differ from each other in any childhood maltreatment type but reported all types more frequently than the healthy control participants. The subgroup of patients with PDD (n = 65) reported all types of childhood maltreatment even more often than other patients. In total, about 15% of the healthy control subjects, about 57% of the patients with schizophrenia, bipolar disorder, or depression, and about 75% of the subgroup of patients with PDD were affected by childhood maltreatment. Anxiety and depression symptoms in the pooled patient sample were predicted by emotional abuse and emotional neglect. Study II examined the attachment style as a possible mediator of the effect of childhood maltreatment on depression severity in patients with depression (n = 580). It was shown that attachment mediated the effect of emotional maltreatment on depression severity. There was a specific indirect effect of emotional abuse on depression severity through anxious attachment and of emotional neglect on depression severity through avoidant attachment. The hypothesized model of a sequential mediation of childhood maltreatment via insecure attachment and reduced social support on depression severity could also be supported. Study III compared patients with ED (n = 38) and PDD (n = 34) as well as healthy control participants (n = 39) in different facets of social cognition and interpersonal problems. In addition, it was investigated whether these differences are rooted in experiences of childhood maltreatment. Patients with PDD reported more frequent experiences of childhood maltreatment and higher empathic distress than the comparison groups. Both patient groups also reported more interpersonal problems. The effect of childhood maltreatment on depression severity was mediated through interpersonal problems and empathic distress. In study IV it was tested if interpersonal problems, empathic distress, emotional regulation, and self-compassion mediate the relationship between childhood maltreatment and depression severity in patients with PDD (N = 96). Emotional abuse predicted lower self-compassion and stronger inter-personal problems. The assumption of a sequential indirect path of emotional abuse via reduced self-compassion and increased emotional regulation difficulties to depression severity could be supported. In addition, interpersonal problems also mediated the effect. Changes in interpersonal problems, emotion regulation, and self-compassion were correlated with changes in depression severity over treatment. Some limitations should be considered when interpreting the reported results. In particular, the cross-sectional designs of studies I to III should be mentioned, which do not allow conclusions about causality. Furthermore, the assessment of childhood maltreatment with retrospective self-report in all four studies is discussed as a limitation. The high prevalence of childhood maltreatment in the different patient groups – and especially among patients with PDD – emphasizes the enormous importance of preventing childhood maltreatment in order to reduce suffering over the entire life span. The results also show that emotional abuse and emotional neglect – maltreatment types that are often overlooked in practice and little studied in research – appear to have a particularly strong impact on depression and various psychological skills. Fostering interpersonal skills, self-compassion, and emotion regulation seem to be promising targets in the treatment of patients with persistent depressive disorder and experiences of childhood maltreatment.