ERMES is an interdisciplinary research project that mobilizes disciplines belonging to the humanities and social sciences (anthropology, sociology, education sciences), behavioral sciences (decision sciences, psychology, economics) and computer sciences (knowledge representation, machine learning). By developing a new interdisciplinary method mainly based on the combination of ethnographic, econometric, experimental and modeling approaches, ERMES aims to characterize the mechanisms of transmission of food messages between peers and the effects of peer education programs on eating behaviors. The project will compare such education actions with more commonly deployed approaches to food education (based on teacher intervention). Moreover ERMES proposes to explore the food messages that children receive and relay (especially those related to their health or environmental consequences) and to better understand which profiles of children are most likely to be influenced by these messages. The study will take place in 14 elementary schools (24 fourth grade classes) in the city of Paris (socially and economically contrasted) and the department of Seine-Saint-Denis (socially and economically contrasted). WP1 will consist of an ethnographic study in four fourth grade classes (complemented by quantitative data from questionnaires that will be used in WP2) to better understand the underlying mechanisms, in particular the role of group dynamics in children's dietary behaviors and discourses. WP2 proposes to study the short and medium term effects of food messages issued either by peers or by teachers on the knowledge, attitudes and behaviors of the recipient children, with a randomized controlled trial conducted in 500 pupils. WP3 aims at presenting the results of WP1 and WP2 to different circles of experts and civil society actors in order to formulate proposals for guidelines for public action in the field of food education. The work carried out in this WP3 will also contribute more broadly to the study of behavior change and to public action on health-related behavior change. The originality of the project lies in the study of the role and influence of friendship networks on the circulation of food messages among children and their consequences on food discourse and behavior. It will be based on a mapping of the groups of friends in each class, notably by means of a questionnaire that has never been used in France. ERMES comes from a multi-actor research project supported since 2020 by the ARS Île-de-France, bringing together scientists, a local authority in Seine-Saint-Denis, school and extracurricular professionals, as well as associative actors who deal with health promotion among young people (MODALITEA project). This framework will ensure effective dissemination of research results and, in particular, improve health education programs and specify the conditions that will make these programs more effective.