There is now a lot of evidence for the effectiveness of cognitive behaviour therapy (CBT) as a treatment for major depression but there is still some uncertainty as to how it actually works. The cognitive mediation model says that it works by changing people's dysfunctional attitudes; maladaptive, inflexible and extreme assumptions by which the self or the world is judged. Dysfunctional attitudes are more prevalent in depressed people and decline with treatment but the complication model holds that it is depression that produces dysfunctional attitudes and not vice versa. Other theories are that depression and dysfunctional attitudes contribute to one another or that there is a third, underlying factor, common to both. A study of 130 people with major depression in Canada divided them into three groups. One group received CBT, another group received interpersonal therapy and a third group received pharmacotherapy. A comparison of CBT with interpersonal therapy showed that a reduction in dysfunctional attitudes explained the effectiveness of CBT. However when CBT was compared to pharmacotherapy the complication model was found to fit the facts better.
Quilty, L.C., McBride, C. and Bagby, R.M. - Evidence for the cognitive mediational model of cognitive behavioural therapy for depression Psychological Medicine November 2008, 38(11), 1521-1530