Objectives: A recent meta-analysis reported that mindfulness-based interventions (MBIs) outperform specific active control conditions but not evidence-based treatments (EBTs) across various psychiatric conditions. Given both comparison conditions represent bona fide treatments, the superiority of MBIs over other bona fide treatments is unexpected. The current study examined researcher allegiance (RA) as a potential source of bias that may explain this result. Method: All studies from the original meta-analysis that compared MBIs with bona fide psychological treatments were included. RA was independently coded using established methods. A series of meta-analyses examined the RA-outcome association and the degree to which RA may account for the effect of EBT status. Results: Sixty independent comparisons (n = 5,627) were included. MBIs outperformed bona fide treatment comparisons overall (g = 0.13), but effects were smaller with EBT comparisons. RA towards MBIs was associated with larger effects. No evidence for superiority of MBIs was found when RA was absent or balanced. Further, EBT status no longer predicted effects when controlling for RA. Conclusions: RA appears to be a potential source of bias in MBI research that should be considered when interpreting existing studies (clinical trials, meta-analyses) and planning future studies. RA may account for smaller effects when using EBT comparisons.