Rational Expectations and the Lucas Critique

How the idea that people form expectations using all available information transformed macroeconomics, and why Robert Lucas warned against relying on past patterns.

Expectations about the future shape economic decisions: how much to spend, what wages to demand, what prices to set. How people form expectations became one of the central debates in macroeconomics.

Adaptive expectations

Earlier models often assumed adaptive expectations: people form expectations based on past experience. If inflation was 5 percent last year, they expect about 5 percent this year.

Rational expectations

In 1961, economist John Muth proposed rational expectations: people use all available information, including knowledge of how the economy works and what policymakers are likely to do. Their forecasts may be wrong, but not systematically wrong.

In the 1970s, Robert Lucas, Thomas Sargent and others built this idea into macroeconomics. Lucas won the Nobel prize in 1995.

The Lucas critique

In 1976, Lucas argued that economic relationships observed in past data can change when policy changes, because people adjust their behaviour. This is the Lucas critique.

For example, the Phillips curve seemed to show a stable trade-off between inflation and unemployment. But if a government tries to exploit it by pushing inflation up, people come to expect higher inflation, and the trade-off disappears. Models based on past patterns can therefore mislead policymakers.

Implications

Credibility matters: if people believe a central bank will keep inflation low, they set wages and prices accordingly, making it easier to keep inflation low.
Policy rules: economists increasingly favoured clear, predictable policy rules over surprises.
Microfoundations: macroeconomic models were rebuilt on assumptions about individual decision-making.

Criticism

Critics, including behavioural economists, argue that real people do not form expectations as rationally as the theory assumes. Many modern models include limited information or learning.

The announced inflation target

A central bank announces it will keep inflation at 4 percent and consistently acts to achieve this. Workers and firms come to expect around 4 percent inflation and set wages and prices accordingly. Because expectations are anchored, a temporary shock, like a spike in food prices, does not spiral into persistent high inflation.

Thinking rational expectations means people are always right

Rational expectations means people use available information sensibly and do not make systematic errors, not that their forecasts are always correct.

Key takeaways

Adaptive expectations look backward; rational expectations use all available information.
Muth proposed rational expectations; Lucas and Sargent built them into macroeconomics.
The Lucas critique warns that past relationships can change when policy changes.
Credibility and predictable policy became central to modern central banking.