Mindfulness-based stress reduction triggers a long-term shift toward more positive appraisals of emotional ambiguity.

Reducing negative impacts of stress, for example through mindfulness training, benefits physical and psychological well-being and is becoming ever more crucial owing to large-scale societal uncertainties (e.g., COVID-19). Whereas extensive research has focused on mindfulness-related reductions in self-reported negativity, essentially no research has targeted task-based behavioral outcomes throughout long-term mindfulness trainings. Responses to emotionally ambiguous signals (e.g., surprised expressions), which might be appraised as either positive or negative, provide a nuanced assessment of one's emotional bias across diverse contexts, offering unique leverage for assessing the effects of mindfulness. Here, we compared the effects of short- and long-term training via Mindfulness-Based Stress Reduction on ratings of faces with a relatively clear (angry, happy) and ambiguous (surprised) valence. Ratings became more positive for ambiguity from the start (Week 1) to end of training (Week 8; p < .001), but there were no short-term effects (from a single class session). This shift toward positivity continued through an additional 8-week follow-up (Week 16; p < .001). Notably, posttraining valence bias (Week 8) was uniquely predicted by the nonreactivity facet of mindfulness (p = .01). Together, mindfulness promotes a relatively long-lasting shift toward positivity bias, which is uniquely supported by reduced emotional reactivity. (PsycInfo Database Record (c) 2022 APA, all rights reserved).