H*2/-22*$#! Under the requirements of the criteria proposed by the Task Force modified by Chambless and Hollon (1998), several RCTs are presently available that provide evidence for the efficacy of PDT in specific mental disorders (Leichsenring et al., in press). There is evidence for the efficacy of PDT in depressive disorders, prolonged or complicated grief, anxiety disorders, PTSD, eating disorders, somatoform disorders, substance-related disorders, and personality disorders, including both less severe (Cluster C) and severe personality disorders (BPD). For PTSD, only one RCT exists (Brom et al., 1989). Thus, we urgently need further studies showing that PDT is effective in complex PTSDs, i.e., in patients suffering from childhood abuse. With regard to personality disorders, no RCTs exist for Cluster A personality disorders (e.g., paranoid, schizoid) and for some relevant Cluster B personality disorders (e.g., narcissistic). This is true, however, for CBT as well. In addition, further RCTs of PDT-LTPP, especially in complex mental disorders, are required.
In the studies reviewed here, PDT was either more effective than placebo therapy, supportive therapy or TAU, or no differences between PDT and CBT, or between PDT and pharmacotherapy, were found.
In a few studies, PDT was superior to a method of CBT (Milrod et al., 2007); in another study, PDT was superior to CBT in some outcome measures (Clarkin et al., 2007). However, most of the studies that found no differences in efficacy between PDT and another bona fide treatment were not sufficiently powered. As reported above, testing for non-inferiority (i.e., equivalence) requires N1 ¼ N2 ¼ 86 patients to detect an at least medium differences (effect size d ¼ 0.5) between two treatments with a sufficient power (a ¼ 0.05, twotailed test, 1-b ¼ 0.90) (Cohen, 1988). At present, only four RCT comparing PDT with a bona fide treatment fulfill this criterion (Crits-Christoph et al., 1999; Driessen et al., 2013; Knekt et al., 2008a; Leichsenring et al., 2013a). The issue of small sample size studies, however, is not specific to studies of PDT, since many studies of CBT are also not sufficiently powered (Leichsenring & Rabung, 2011). For comparisons of PDT with bona fide therapies, the between-group effect sizes were found to be small (Driessen et al., 2013; Leichsenring, 2001; Leichsenring, Salzer et al., 2011; Leichsenring et al., 2013a). Thus, it is an open question of research whether more highly powered studies would find significant differences. Furthermore, the question has to be addressed whether these (possibly small) differences are clinically relevant or significant (Jacobson & Truax, 1991). It is important, however, to realize which mental disorders lack any RCTs of PDT. This is true, for example, for dissociative disorders and for some specific forms of personality disorders (e.g., narcissistic). For PTSD, only one RCT is presently available (Brom et al., 1989). Some studies reported differences, at least in some measures, in favor of CBT. This is true, for example, for the studies on bulimia nervosa by Fairburn et al. (1986) and Garner et al. (1993), and for the studies on generalized anxiety disorder (Leichsenring et al., 2009) and social phobia (Leichsenring et al., 2013a). For the study on generalized anxiety disorder (Leichsenring et al., 2009), we discussed above whether a stronger focus on the process of worrying would possibly improve the results of PDT.
In general, future research should address the question whether the efficacy of PDT can be improved by putting a stronger focus on the specific mechanisms that maintain the psychopathology of the respective disorder. Mentalization-based therapy or TFP may serve as good examples for psychodynamic treatments that focus on the assumed processes or deficits maintaining a disorder.
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