Verbal suggestions about treatment effectiveness do not modulate the effectiveness of a laboratory model of EMDR therapy: Results of two preregistered studies

Background: As for many psychotherapies, there is ongoing discussion about the role of specific versus unspecific mechanisms in the effectiveness of Eye-Movement and Desensitization Reprocessing (EMDR) therapy for post-traumatic stress disorder. However, research directly examining the potential role of non-specific mechanisms in EMDR is scarce. Objectives: Here, we address the role of one non-specific factor that is often put forward, namely treatment effectiveness expectations, in a laboratory model of EMDR therapy. Method: In a lab-based study (N = 96) and an online study (N = 173), we gave participants verbal instructions to manipulate their treatment expectations. Instructions emphasized EMDR’s effectiveness or infectiveness. Then, participants were asked to recollect an unpleasant autobiographical memory with or without making eye-movements. Results: In line with previous studies, we found significant reductions of reported vividness and emotionality of negative autobiographical memories. However, this effect was not modulated by manipulated treatment efficacy expectations. It must be noted that manipulation checks indicated that it was difficult to influence treatment effectiveness expectations using verbal suggestions. Conclusions: These findings corroborate the results of two earlier reports, suggesting that the role of treatment expectations in EMDR therapy may be limited.