Synergy in messaging: A multi-component approach reduces food Waste in German cafeterias

Efforts to reduce food waste in workplace cafeterias are increasing, but the effect of informational interventions remains unclear. We tested three one-week messaging campaigns—prompt messaging, social norm messaging, and a combined message with co-created site-specific tailoring—across four German cafeterias. Analyses of nearly 40,000 tray observations and surveys from 2361 diners showed that only the combined intervention significantly reduced plate waste; neither prompt nor norm messages alone had a meaningful impact. Tailoring messages to individual sites provided no additional benefit, indicating that structural factors—particularly portion size—dominate waste outcomes. Diners also tended to underreport small leftovers, highlighting cognitive biases in self-reporting. By combining objective observations with survey data, our findings emphasise the need for synergistic, multi-component approaches that pair portion control and menu design with engaging, coordinated strategies to substantially cut food waste.