Perhaps the most important bias of meta-analyses is the expectancy effect. Cotton and Cook (1982) recommended early on that the investigators of meta-analyses explicitly state their personal view with regards to the outcome in order to acknowledge and possibly avoid the expectancy effect. At the outset of our review, we were rather critical toward the efficacy of MBT. We expressed our personal view in an earlier theoretical article (Hofmann & Asmundson, 2008) and were fully prepared to report non-significant or only small effects of MBT. We were surprised to find these effects to be rather robust and strong. Therefore, we believe that the expectancy bias was unlikely to be a significant contributor to the results, which generally support the efficacy of MBT.
