The implementation of a nutritional policy by the recent “Plan National Nutrition Santé” (PNNS - National Nutrition and Health Programme) raises the question of the relative effectiveness of policy tools, such as price interventions and information campaigns. However, social interactions between consumers are likely to complicate policy evaluation. More precisely, suppose that a public policy has a direct effect on consumption behaviours and that, by aggregation, individual behavioural changes alter certain social norms. These norms may in turn influence individual behaviours. The overall effect of the public policy is the sum of a direct and immediate effect on individuals, and an indirect effect through social norms. With this in mind, we here present the results of a quantitative study, which identifies the impact of social norms of body shape on individual representations of ideal body and the effect of the latter on nutritional attitudes. Hence, the research asks whether collective representations of the ideal body are a parameter that must be taken into account in the economic analysis of nutritional health policies.