Depression is a widely prevalent, debilitating disorder that causes significant suffering for those affected. Behavioural activation (BA) is an evidence-based psychological treatment for depression. The evidence base for BA is largely grounded in individual delivery, with far less known about BA delivered in groups. Given rising demand for treatment, groups represent an attractive delivery strategy for services. Despite good rates of treatment outcomes for evidenced-based psychological treatments, considerable numbers of patients do not benefit and remain in a state of depression ‘stasis’ (i.e., their symptoms are relatively unchanged, despite receiving treatment). This thesis sought to investigate the effectiveness and efficacy of group BA, the reasons why stasis might occur and also how stasis can be reduced. First, a meta-analysis (Chapter 2) of group BA depression outcomes in trial (efficacy) and naturalistic (effectiveness) contexts is presented, to clarify the treatment effect of group-based BA interventions. Group BA is shown to be superior to controls (Hedges g effect size = 0.72) and equivalent to other active therapies (g = 0.14) at treatment completion. Chapter 3 provides an overview of depression stasis after evidence-based treatment. The chapter highlights the difficulties in identifying stasis outcomes, the extent of the problem (up to ~60% of patients treated) and the paucity of evidence about associated factors. As a result, a stasis metric is defined for use in the subsequent Chapters. An analysis of BA treatment response (Chapter 4) then investigates the effect of intervention intensity, format and duration on stasis outcomes. BA is seen to be effective at reducing depression (in 4-9 sessions) regardless of format, with larger effects seen for more intensive versions. ‘Stasis patients’ are distinguishable from ‘improvers’ after 2 sessions. Risk of a BA stasis outcome was predicted by attending fewer sessions, greater impaired functioning prior to treatment, and less severe depression. Building on these findings, Chapter 5 then tests an augmented group BA treatment to determine whether drop-out and stasis outcomes can be reduced. Whilst treatment retention remains stable, significantly fewer patients experience a stasis outcome after the augmented treatment, due to increased rates of improvement. Lastly, a mediation study (Chapter 6) evaluates whether increasing behaviour in accordance with life values (valued living) is a BA change mechanism. Discrepancies in valued-living were not related to depression severity, nor did valued-living increase as a result of group BA. Exploratory analyses showed valued living does not mediate reductions in depression symptom clusters (somatic or affective) during group BA therapy. Finally, the overall theoretical, clinical, organisational and research implications are discussed in Chapter 7, with recommendations outlined for future investigation into depression stasis and treatment.
