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Background: Patient outcome expectancy - the belief that treatment will lead to an improvement in symptoms – is linked to favourable therapeutic outcomes in major depressive disorder (MDD). The present study extends this literature by investigating the temporal dynamics of expectancy, and by exploring whether expectancy during treatment is linked to differential outcomes across treatment modalities, for both optimistic versus pessimistic expectancy. Methods: A total of 104 patients with MDD were randomized to receive either cognitive behavioral therapy (CBT) or pharmacotherapy for 16 weeks. Outcome expectancy was measured throughout treatment using the Depression Change Expectancy Scale (DCES). Depression severity was measured using both the Hamilton Depression Rating Scale and Beck Depression Inventory-II. Results: Latent growth curve models supported improvement in expectancy across both treatments. Cross-lagged panel models revealed that both higher optimistic and lower pessimistic expectancy at mid-treatment predicted greater treatment response in pharmacotherapy. For CBT, the associative patterns between expectancy and depression differed as a function of expectancy type; higher optimistic expectancy at pre-treatment and lower pessimistic expectancy at mid-treatment predicted greater treatment response. Limitations: The sample size limited statistical power and the complexity of models that could be explored. Conclusions: Results suggest that outcome expectancy improved during treatment for depression. Whether outcome expectancy represents a specific mechanism for the reduction of depression warrants further investigation.

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