Rational Expectations

The rational expectations hypothesis holds that individuals, firms, and markets form predictions about future economic variables (inflation, interest rates, output) using all relevant information available, including their understanding of how the economy and government policy actually work. On average, their forecasts equal the model's true predictions, though individual forecasts may still err randomly.

The idea was first articulated by John F. Muth in his 1961 Econometrica article "Rational Expectations and the Theory of Price Movements." It was later developed into a macroeconomic framework by Robert E. Lucas Jr., Thomas J. Sargent, Neil Wallace, and Edward C. Prescott during the 1970s. Lucas received the Nobel Memorial Prize in Economic Sciences in 1995 specifically for this work; Sargent and Prescott were also later Nobel laureates.

Key implications include:

• The Lucas critique (1976): econometric models built on past behavior cannot reliably predict the effects of new policy regimes, because agents will adjust expectations when policy changes.
• Policy ineffectiveness proposition (Sargent and Wallace, 1975): systematic, anticipated monetary policy cannot affect real output, because rational agents anticipate and neutralize it through wage and price adjustments. Only unanticipated policy shocks have real effects.
• A challenge to the Keynesian Phillips curve trade-off between inflation and unemployment.

Rational expectations became the methodological foundation for New Classical macroeconomics and is embedded in modern DSGE (dynamic stochastic general equilibrium) models used by central banks including the Federal Reserve, ECB, and IMF.

Critics argue the assumption is unrealistic: it requires agents to know the "true" model of the economy, and behavioral economists (notably Daniel Kahneman and Richard Thaler) have documented systematic forecasting biases. Alternatives include adaptive expectations, bounded rationality (Herbert Simon), and learning models developed by George Evans and Seppo Honkapohja.