The 'healthy = sustainable' heuristic: Effects of health and sustainability labels on perceived sustainability and healthiness of foods

Studies show that foods perceived as healthier are often also seen as more sustainable, suggesting a 'healthy = sustainable' heuristic. However, the direction of this effect remains unclear. This study aims to investigate (1) whether perceived healthiness influences perceived sustainability or whether the reverse effect occurs and (2) whether inter-individual differences, such as health interest, moderate these effects. In an online study, participants (n = 712) were shown pictures of foods with high or low healthiness indices and asked to rate their sustainability. Conversely, they were also shown pictures of foods with high or low sustainability indices and asked to rate their healthiness. Additionally, participants' interest in health and sustainability and their belief in a relationship between the healthiness and sustainability of foods were measured. Exploratory analyses included label credibility as a control variable. Foods with a higher healthiness index were perceived as more sustainable (effect size: δ = 0.39; [CI: 0.36; 0.41]). Similarly, higher sustainability indices led to higher healthiness ratings (effect size: δ = 0.22; [CI: 0.20; 0.24]). Moderating effects of interests were small and dependent on whether label credibility was accounted for. The results support the assumption of a 'healthy = sustainable' heuristic, indicating that the effect is bidirectional. The implications for food labelling are discussed.