Thought-shape fusion in residential eating disorder treatment: Cognitive defusion as a mediator between thought-action fusion and treatment outcome.

An understudied cognitive bias within eating disorder (ED) psychopathology is Thought-Shape Fusion (TSF), which involves irrational beliefs about the likelihood and moral implications of feared outcomes related to shape, weight, and food. This phenomenon has received less attention within the context of ED treatment, with little known about potential processes of change to address TSF and ultimately promote ED recovery. We propose cognitive defusion as a process of change, a metacognitive process that emphasizes observing thoughts objectively rather than appraising thoughts as absolute truth. We explored whether cognitive defusion, that is, reductions in body image-related cognitive fusion, mediated the relationship between trait-level TSF and treatment outcomes in a transdiagnostic ED sample of adult and adolescent females (N = 130) presenting to residential care. We found that reductions in body image-related cognitive fusion mediates the association between trait-level TSF at baseline and ED severity at discharge. However, when the sample was separated into adolescent and adult subgroups, these results only remained significant for adolescents. These findings underscore the relevance of targeting cognitive defusion as a potential mechanism to address the impact of trait levels of TSF cognitions on ED psychopathology.