Do ideas matter? North’s analysis is that actors’ mental perceptions guide their strategic choices and thus shape the institutional environment. This latter point seems particularly relevant regarding the participation of economic organizations in the political making process. Nevertheless the empirical description of this role has not been clearly established: Few works try to make the impact of actors’ mental perceptions on institutional change operational (Dinar and Saleth, 2004). Our aim is to contribute to this empirical description, by leading a comparative study of two farm sectors in Costa Rica, based on a dynamic approach of mental perceptions in relation with the institutional change occurring during the liberalisation process. To do so, we carry out a statistical analysis of mental perceptions through a textual analysis of actors’ perceptions of the political making process. Thus, we provide an empirical confirmation that mental models are influenced by specific institutional environments and lead to different political strategies. Then, we show that when an exogenous change occurs in the institutional environment, previous mental models can persist and lead to inefficient behaviours. More generally, our study empirically emphasizes the interest of taking into account agents’ mental perceptions for a better understanding of institutional and economic dynamics.