What is "rational expectations"

In economics, "rational expectations" are model-consistent expectations, in that agents inside the model on average assume the model's predictions are valid. Rational expectations ensure internal consistency in aggregate stochastic models. To obtain consistency within a model, the predictions of the future value of economically relevant variables are optimal given the decision-makers' information set and model structure. The rational expectations assumption is used especially in many contemporary macroeconomic models. Rational expectations does not imply individual rationality and should not be confused with rational choice theory, which is used extensively in, among others, game theory.

Since most macroeconomic models today study decisions over many periods, the expectations of workers, consumers and firms about future economic conditions are an essential part of the model. How to model these expectations has long been controversial, and it is well known that the macroeconomic predictions of the model may differ depending on the assumptions made about expectations (see Cobweb model). To assume rational expectations is to assume that agents' expectations may be wrong, but are correct on average over time. In other words, although the future is not fully predictable, agents' expectations are assumed not to be systematically biased and use all relevant information in forming expectations of economic variables.

This way of modeling expectations was originally proposed by John F. Muth (1961) and later became influential when it was used by Robert Lucas, Jr. and others. Modeling expectations is crucial in all models which study how a large number of individuals, firms and organizations make choices under uncertainty. For example, negotiations between workers and firms will be influenced by the expected level of inflation, and the value of a share of stock is dependent on the expected future income from that stock.

Deirdre McCloskey emphasized that "rational expectations" is an expression of intellectual modesty: "Muth's notion was that the professors [of economics], even if correct in their model of man, could do no better in predicting than could the hog farmer or steelmaker or insurance company. The notion is one of intellectual modesty. The common sense is "rationality": therefore Muth called the argument "rational expectations"."