Since Rosenzweig's "Dodo Bird Verdict" in 1936, the "common" versus "specific" factors debate has continued to polarize the field of psychotherapy. Treatment expectancy is an important but often overlooked common factor. The current study investigated the role of treatment expectancy in the Treatment of Adolescents with Depression Study (TADS). Four-hundred three adolescents (M _age=14.62, SD=1.56) filled out the Treatment Expectancy for Adolescents (TEA) measure prior to treatment randomization to one of four treatments: fluoxetine (FLX), cognitive behavior therapy (CBT), their combination (COMB), and placebo (PBO). Adolescents randomized to CBT or COMB also filled out the CBT Rationale Acceptance and Expectation for Improvement (C-RAEI) form during their second session of CBT.

Before finding out their treatment assignments, adolescents endorsed higher treatment expectancies for COMB than CBT and medication only. Family income levels below $75,000 and higher levels of depression severity, hopelessness, and suicidality were associated with lower expectations for improvement with CBT. The presence of a comorbid anxiety disorder diagnosis was associated with lower expectations for medication without CBT. Separate random coefficients and logistic regression models identified treatment expectancy as a predictor of outcome for three primary outcome measures in TADS, irrespective of treatment assignment. Severity of depression moderated this relationship; mild to moderately depressed adolescents appeared to be more sensitive to the effects of treatment expectancy than marked to severely depressed adolescents. The opposite results were found for the self-rated outcome measure in TADS based on the C-RAIE. For marked to severely depressed adolescents assigned to CBT or COMB, acceptance of treatment rationale and expectancy for improvement were associated with treatment response.

These results suggest that treatment expectancy is an important common factor of treatment for mild to moderately depressed adolescents prior to treatment initiation, although it may be especially important for initially skeptical, marked to severely depressed adolescents to "buy in" to treatment after treatment initiation. Treatment effects were still found after controlling for the effects of treatment expectancy on outcome. It seems that both the "common" factor of treatment expectancy and the "specific" factor of treatment assignment contributed to outcome in TADS.