How does CBT work? Causal discovery modelling locates the mechanisms of change in cognitive behavioural treatment for eating disorders

Background: Cognitive behavioural therapy (CBT) is the most frequently used and recommended therapy program for mental health conditions, including for eating disorders. Despite decades of use, little is known about how change is produced in CBT across mental illnesses, and there has been no empirical validation of the causal structure of change for CBT treatment of eating disorders. Progress in optimisation of CBT and the development of alternatives particularly for non-responders, has been constrained by a lack of understanding of the mechanisms through which the therapy works. This study aimed to investigate, for the first time, mechanism by mechanism the causal direction of change across a therapeutic CBT package. Methods: Directed acyclical graphs (DAGs) were estimated using causal discovery machine learning methods applied to data from a randomised controlled trial of a digital CBT program in 114 participants with bulimia nervosa. Eating disorder symptoms and psychological distress were assessed at each treatment session across the 10-session intervention in both conditions. Treatment effects were then estimated using the inferred causal structure. Results: The inferred causal structure estimated direct effects of treatment on eating restraint (small to medium), alongside indirect effects upon preoccupation with eating, fear of weight gain, desire for weight loss, binge days, loss of control, over-exercise and fasting (small to medium). Eating restraint emerged as the first and only node that treatment directly affected, suggesting that it is the primary mechanism through which treatment produces subsequent change in other behavioural and cognitive symptoms. Conclusion: These findings provide the first empirical evidence, beyond cross-sectional or correlational analyses, for the long theorised mechanisms underlying CBT for eating disorders. Contrary to the conceptual model which posits CBT acts directly on dietary restraint and shape and weight concerns, our findings suggest treatment acts primarily through reductions in eating restraint, with downstream effects on shape- and weight-related concerns and other symptoms. To build our understanding of the processes by which effective treatments for eating disorders operate, progress personalised medicine, and optimise therapeutic outcomes, the methodology developed here can be applied broadly to examination of causal change structures in both direction and magnitude across different interventions.