Can mindfulness be too much of a good thing? The value of a middle way

•Few psychological or physiological processes are universally beneficial.•Most positive phenomena reach inflection points where their effects turn negative.•Mindfulness is unlikely to be an exception to the inverted U-shape curved principle.•Some mindfulness-related processes have negative effects under certain conditions.•Research that includes the full range of possible effects would improve the efficacy of mindfulness. Previous research has found that very few, if any, psychological or physiological processes are universally beneficial. Instead, positive phenomena tend to follow a non-monotonic or inverted U-shaped trajectory where their typically positive effects eventually turn negative. This review investigates mindfulness-related processes for signs of non-monotonicity. A number of mindfulness-related processes—including, mindful attention (observing awareness, interoception), mindfulness qualities, mindful emotion regulation (prefrontal control, decentering, exposure, acceptance), and meditation practice—show signs of non-monotonicity, boundary conditions, or negative effects under certain conditions. A research agenda that investigates the possibility of mindfulness as non-monotonic may be able to provide an explanatory framework for the mix of positive, null, and negative effects that could maximize the efficacy of mindfulness-based interventions.