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Introduction
Major depressive disorder (MDD) is, at least in secondary and more specialized care settings with morbidity concentration, a chronic psychiatric disorder with residual symptoms, frequent relapses and estimates of recurrence ranging from 33% to 70% within several years (Pettit et al. 2006).
Previous research has shown that the causal role of severe life events is smaller in recurrences than in first-onset episodes of MDD (Monroe & Harkness, 2005; Stroud et al. 2008). This observation was described by Post (1992) as a process of behavioural 'sensitization' and electrophysiological 'kindling', suggesting that a depressive episode leaves some traces that persist after remission and recovery, and render individuals vulnerable to the onset of new episodes under the influence of only moderate or, eventually, no psychosocial stress. This idea is known as the scar hypothesis of depression (Lewinsohn et al. 1981). Scar theories assume that something, presumably encoded at the biological level, changes during an episode of depression, inducing a long-lasting change and increasing the likelihood of future episodes (Burcusa & Iacono, 2007). Scar research, however, is not confined to the neurobiological domain. Studies examining scars in depression have focused on potential scars within a broad variety of domains (psychosocial, cognitive, emotional and others) (Shea et al. 1996; Ormel et al. 2004a; Beevers et al. 2007; Burcusa & Iacono, 2007). In this paper, therefore, the term 'scar' may refer to all possible changes in cognition, emotion, behaviour or biology that develop in the aftermath of a depressive mood state and result in a stable increase in vulnerability.
Studies that address the scar hypothesis ideally assess putative factors that may become scarred before and after an episode of MDD in participants without residual symptoms. In studies that attempted to apply such a rigorous design, several potential scars were examined including depression-related cognitions (Lewinsohn et al. 1981; Barnett & Gotlib, 1988), personality (Rohde et al. 1990; Duggan et al. 1991; Shea et al. 1996; De Fruyt et al. 2006; Ormel et al. 2004b), psychosocial disability (Rohde et al. 1990; Ormel et al. 2004a), social skills (Zeiss & Lewinsohn, 1988) as well as rumination, self-esteem and negative emotionality (Beevers et al. 2007). However, in all these studies virtually...