The previous two decades have seen an exponential increase in the number of published scientific investigations on the efficacy of mindfulness-based stress reduction (MBSR) training to improve function in a wide range of physical and psychological processes. The resulting body of work provides strong evidence that MBSR has salubrious effects. Yet, when compared directly to groups with training that matches MBSR in factors common to most legitimate interventions, such as learning new skills, expectation of benefit, social engagement and support, and attention from expert instructors, both groups tend to improve to a similar extent. This raises the question of whether there are benefits that are specific to training in mindfulness and if so, why are we not detecting them? Here, we discuss the factors that contribute to the general lack of differentiation between MBSR and active control groups, including the specificity of outcome measures and experimental design, random assignment, inclusion/exclusion criteria, and the time course and trajectory of change. In addition, we offer recommendations to address these factors in future research.
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