Background There is certain evidence on the efficacy of smartphone-based mental health interventions. However, the mechanisms of action remain unclear. Placebo effects contribute to the efficacy of face-to-face mental health interventions and may also be a potential mechanism of action in smartphone-based interventions. Objective This study aimed to investigate whether different types of efficacy expectancies as potential factors underlying placebo effects could be successfully induced in a smartphone-based digital placebo mental health intervention, ostensibly targeting mood and stress. Methods We conducted a randomized, controlled, single-blinded, superiority trial with a multi-arm parallel design. Participants underwent an Android smartphone-based digital placebo mental health intervention for 20 days. We induced prospective efficacy expectancies via initial instructions on the purpose of the intervention and retrospective efficacy expectancies via feedback on the success of the intervention at days 1, 4, 7, 10, and 13. A total of 132 healthy participants were randomized to a prospective expectancy–only condition (n=33), a retrospective expectancy–only condition (n=33), a combined expectancy condition (n=34), or a control condition (n=32). As the endpoint, we assessed changes in efficacy expectancies with the Credibility Expectancy Questionnaire, before the intervention and on days 1, 7, 14, and 20. For statistical analyses, we used a random effects model for the intention-to-treat sample, with intervention day as time variable and condition as two factors: prospective expectancy (yes vs no) and retrospective expectancy (yes vs no), allowed to vary over participant and intervention day. Results Credibility (β=−1.63; 95% CI −2.37 to −0.89; P<.001) and expectancy (β=−0.77; 95% CI −1.49 to −0.05; P=.04) decreased across the intervention days. For credibility and expectancy, we found significant three-way interactions: intervention day×prospective expectancy×retrospective expectancy (credibility: β=2.05; 95% CI 0.60-3.50; P=.006; expectancy: β=1.55; 95% CI 0.14-2.95; P=.03), suggesting that efficacy expectancies decreased least in the combined expectancy condition and the control condition. Conclusions To our knowledge, this is the first empirical study investigating whether efficacy expectancies can be successfully induced in a specifically designed placebo smartphone-based mental health intervention. Our findings may pave the way to diminish or exploit digital placebo effects and help to improve the efficacy of digital mental health interventions. Trial Registration Clinicaltrials.gov NCT02365220; https://clinicaltrials.gov/ct2/show/NCT02365220.
BACKGROUND Smartphone-based mental health interventions provide new ways to treat mental disorders. There is certain evidence on the efficacy of such interventions. Placebo effects represent a substantial element of the mechanisms of action of face-to-face mental health interventions. OBJECTIVE We manipulated efficacy expectancies and investigated whether time trajectories of efficacy expectancies differed between conditions across a smartphone-based digital placebo mental health intervention. METHODS We conducted a randomized, controlled, single-blinded superiority trial with a multi-arm parallel design. Participants underwent a smartphone-based digital placebo mental health intervention for 20 consecutive days. We induced prospective efficacy expectancies by manipulating initial instructions on the purpose of the intervention and retrospective efficacy expectancies by manipulating feedback on the success of the intervention at days 1, 4, 7, 10, and 13. 132 healthy participants were randomized to four conditions: prospective expectancy only (n=33), retrospective expectancy only (n=33), combined expectancy (n=34), or control (n=32). Changes in efficacy expectancies were assessed with the Credibility Expectancy Questionnaire, at the introductory session and on intervention days 1, 7, 14, and 20. We performed our analyses for the intention-to-treat sample using a random effects model, with intervention day as time variable and condition as two factors: prospective expectancy (yes vs. no), and retrospective expectancy (yes vs.no), allowed to vary over participant and intervention day. RESULTS Credibility (b = -1.63, 95%confidence interval (CI) [-2.37, -0.89], P < 0.001) and expectancy (b = -0.77, 95%CI [-1.49, -0.05], P = 0.04) decreased across intervention days. For credibility and expectancy, we found significant three-way interactions intervention day*prospective expectancy*retrospective expectancy (b = 2.05, 95%CI [0.60, 3.50], P < 0.01 resp. b = 1.55, 95%CI [0.14, 2.95] P = 0.03). Efficacy expectancies decreased least in the combined expectancy and in the control condition, most in the prospective expectancy only and the retrospective expectancy only condition. CONCLUSIONS This is the first study investigating the induction of efficacy expectancies across a placebo smartphone-based mental health intervention. Efficacy expectancies decreased throughout intervention days and differed between conditions. Our findings may pave the way for diminishing and exploiting digital placebo effects and help to improve treatment efficacy of digital mental health interventions. CLINICALTRIAL ClinicalTrials.gov Identifier: NCT02365220. Registered February 18, 2015.
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