Introduction: In double-blind randomized placebo-controlled trials (RCT) of therapeutic interventions, the effects of the treatment may provide feedback that undermines blinding and consequently distorts measures of the effectiveness of the intervention.
Methods and results: This possibility was confirmed in an experimental model using a dummy placebo procedure whereby participants were led to believe that they were taking part in testing a cognitive-enhancing drug. In two experiments, false feedback given about cognitive performance influenced participants' beliefs about whether they had been allocated to the active treatment or placebo. These beliefs also appeared to influence actual cognitive performance in that participants who believed they had taken the active treatment had higher accuracy in Experiment 1 and faster reaction times in Experiment 2 than those who believed they had been given a placebo. The addition of no treatment control groups in Experiment 2 showed that these effects could not be accounted for by the feedback manipulation itself, thereby supporting expectancy as a causal factor.
Discussion: These results indicate the importance of assessing participants' beliefs about their treatment allocation in real double-blind RCTs and considering if and how these may have affected the trial's outcome.