Theory-based promotion of diet and commuting behavior change to reduce carbon footprint: Randomized parallel trial of the GROW app

Following a low-meat diet, and engaging in active commuting, can significantly reduce individual carbon footprint while providing health benefits. Behavior change interventions targeting these behaviors often fail due to a focus on motivational aspects, neglecting volitional processes crucial for translating intention into behavior. Based on the health action process approach, this double-blind randomized parallel trial tested the efficacy of the GROW app in reducing diet- and commuting-related carbon footprint. We compared an app version targeting motivational and volitional determinants (volitional group) with a version targeting motivation only (motivational group). Participants (N = 226) logged diet- and commuting-related behaviors daily for five weeks; psychological determinants were measured weekly. Multilevel models showed an overall reduction in diet-related carbon footprint during the study (B = -0.1, 95% CI [-0.02, -0.01]); however, this decrease did not differ between intervention groups. No change was observed in commuting-related or combined (diet + commuting) carbon footprints. Action planning and action control emerged as the strongest determinants of lower diet-related carbon footprint. The findings show preliminary indication of the efficacy of digital behavior change interventions for reducing diet-related carbon footprint. Structural interventions might be more suitable to change commuting-related carbon footprint.