The new institutional economics has recently developed the concept of mental model, defined as the way agents perceive their position in their environment, to analyze agents' behaviours. Nevertheless, the empirical evidence that mental models contribute to explain behaviors has not been made explicit. In this paper, we analyze the role of farm organisation leaders' mental model in a policy negotiation process, in a context of liberalisation in Costa Rica. We characterize mental models through a statistical textual analysis of leaders' discourses, and we measure mental models' relative weight in the negotiation outcome, building upon a partial least square approach. We find that mental models influence organizations' behaviours and outcomes, with a similar level of reliability than the organisation's financial resource, technical capacity, and human resource.