Rational expectations and the Cournot-Theocharis problem

In dynamic models in economics, often “rational expectations” areassumed. These are meant to show that the agents can correctly foresee the result of their own and the other agents' actions. In this paper, it is shown that this cannot happen in a simple oligopoly model with a linear demand function and constant marginal costs. “Naive expectations,” that is, where each agent assumes the other agents to retain their previous period action, are shown to result in a 2-period cycle. However, adapting to the observed periodicity always doubles the actual resulting periodicity. In general, it is impossible for the agents to learn any periodicity exceptthe trivial case of a fixed point. This makes the whole idea of “rational expectations” untenable in Cournot oligopoly models.