(Es completamente irreal pensar la posibilidad de que todos los agentes (representados por el agente teórico único) prevean todos los estados contingentes, porque el cambio de un solo estado contingente origina el cambio de los demás, lógicamente.) pero, aparte de esto, un cambio de un estado A otro B sin fricciones requiere que todos los agentes tengan planes iguales o consistentes unos con otros, porque si son diferentes, al rectificar ante el shock necesariamente lo harán incoherentemente, con lo que se habrá roto la condición de equilibrio con que sólo uno de ellos yerre: su error llevará a los demás a falsas expectativas. Será más peobablbe que en vez de una convergencia hacia el equilibrio se desencadene una riada de selecciones erróneas y movimientos acumulativos llamados ciclos.For an intertemporal equilibrium to exist, there must be a complete set of markets for all future periods and contingent states of the world, or, alternatively, there must be correct expectations shared by all agents about all future prices and the probability that each contingent future state of the world will be realized. By the way, If you think about it for a moment, the notion that probabilities can be assigned to every contingent future state of the world is mind-bogglingly unrealistic, because the number of contingent states must rapidly become uncountable, because every single contingency itself gives rise to further potential contingencies, and so on and on and on. But forget about that little complication. What intertemporal equilibrium requires is that all expectations of all individuals be in agreement – or at least not be inconsistent, some agents possibly having an incomplete set of expectations about future prices and future states of the world. If individuals differ in their expectations, so that their planned future purchases and sales are based on what they expect future prices to be when the time comes for those transactions to be carried out, then individuals will not be able to execute their plans as intended when at least one of them finds that actual prices are different from what they had been expected to be.
Además de esto, ya no tiene sentido hablar de shock, porque la simple posibilidad de que las expectativas sean diferentes y originen decisiones erróneas o alejadas de la convergencia, es el verdadero shock original. Si las expectativas no se realizan como se esperaba, entonces no son racionales.What this means is that expectations can be rational only when everyone has identical expectations. If people have divergent expectations, then the expectations of at least some people will necessarily be disappointed — the expectations of both people with differing expectations cannot be simultaneously realized — and those individuals whose expectations have been disappointed will have to revise their plans. But that means that the expectations of those people who were correct were also not rational, because the prices that they expected were not equilibrium prices. So unless all agents have the same expectations about the future, the expectations of no one are rational. Rational expectations are a fixed point, and that fixed point cannot be attained unless everyone shares those expectations.
Beyond that little problem, Mason raises the further problem that, in a rational-expectations equilibrium, it makes no sense to speak of a shock, because the only possible meaning of “shock” in the context of a full intertemporal (aka rational-expectations) equilibrium is a failure of expectations to be realized. But if expectations are not realized, expectations were not rational. So the whole New Classical modeling strategy of identifying shocks to a system in rational-expectations equilibrium, and “predicting” the responses to these shocks as if they had been anticipated is self-contradictory and incoherent.
I think I disagree with some of this David.
I can have rational expectations, and update my expectation rationally when new information comes in.
My rational expectation can be different from your rational expectation, if I have different information from you.
In both cases the expectations will be probabilistic; I won’t (rationally) hold any belief with certainty, if I know I don’t have full information.
Game theorists don’t bother calling it “rational expectations”. They talk about Bayes-Nash (or something) equilibrium.