Understanding why theory-based health behaviour interventions work (or not): A causal modelling approach

Objectives: Few intervention studies using health-related self-regulation use theories to specify the behavioural determinants subsequently targeted by relevant behaviour change techniques, and to inform fidelity and outcome measures. The objective was to develop a causal modelling approach to developing and evaluating theory-based interventions for randomised controlled trial evaluation. Methods: Causal models link determinants of behaviour through physiological and biochemical variables to health outcomes. They specify techniques to change (determinants of) behaviour, and also measures for each level of the model. The approach was developed in two RCTs of interventions to prevent Type 2 diabetes in a high-risk group (ProActive, N=365, 30-50 years) and its consequences in people with newly diagnosed diabetes (ADDITION Plus, N=500, 40-69 years). Systematic reviews, consultations, and pilot studies informed the causal models. Results: Psychological theory and evidence informed selection of (a) behavioural determinants (intervention targets), which were derived from the Theory of Planned Behaviour and Leventhal's Self-regulation Model, (b) techniques, among others based on self-regulation principles (e.g., goal setting), and (c) psychological measures. Fidelity measures include training sessions, intervention protocols, and reliable coding frames to assess use of theory-based techniques among a random sample of participants. Epidemiological evidence informed choice of target groups, target behaviours and health outcomes, and associated measures (e.g., objectively assessed energy expenditure, modelled diabetes risk). The two studies will be used to illustrate the interrelationships between choice of theory and techniques, and measures of fidelity and outcomes. Conclusions: The causal modelling approach provides a rational framework for development of theory-based health interventions, linked to implementation and outcome measures. Challenges encountered included choice of criteria to guide theory selection, and the gap between theory and practicalities of intervention delivery.