Evidence for the cognitive mediational model of cognitive behavioural therapy for depression

Background. Although empirical support for the efficacy of cognitive behavioural therapy (CBT) as a treatment for major depressive disorder (MDD) is well established, its mechanism of action is uncertain. In this investigation, we examined evidence for the cognitive mediational model in a randomized control trial involving CBT, interpersonal therapy (IPT) and pharmacotherapy (PHT) in patients with MDD. Method. One hundred and thirty participants diagnosed with MDD were treated with CBT, IPT or PHT. Participants completed the Hamilton Depression Rating Scale, Beck Depression Inventory - II and Dysfunctional Attitudes Scale prior to and following treatment. Results. The cognitive mediational model, in which dysfunctional attitudes are proposed to mediate depressive symptom reduction in response to treatment, provided a good fit to the data when contrasting CBT v. IPT, with results supporting a mediational role for dysfunctional attitude change in depressive symptom reduction. The complication model, in which dysfunctional attitudes are proposed to be a consequence of depressive symptom reduction, provided a good fit to the data when contrasting CBT v. PHT, with results supporting a mediational role for depressive symptom reduction in dysfunctional attitude change. Conclusions. There was no evidence for a mediational role for dysfunctional attitude change in IPT. Changes in dysfunctional attitudes accompanied both CBT and PHT; however, empirical evidence suggests that the role of attitudes in treatment outcome may differ between these two treatments.