Heioo: New encyclopedia entry on Rational Expectations

2026-10-06T05:44:02Z

New encyclopedia entry on Rational Expectations

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'''Rational expectations''' is the hypothesis that economic agents form expectations using all available information and the true structure of the economy, so that their expectations are, on average, correct. It was introduced by John Muth and applied to macroeconomics by Robert Lucas and others, transforming the analysis of policy.

== The Hypothesis ==
Under rational expectations, agents' subjective expectations coincide with the objective conditional expectations implied by the economic model. Agents do not make systematic errors, and any errors that occur are random and unpredictable. Expectations incorporate all relevant information, including the structure of the economy and the policies in place.

The hypothesis does not require that every individual be perfectly informed or that expectations always be correct. It requires that errors be unsystematic, so that agents do not persistently over- or under-predict. In this sense, rational expectations is a statement about the average behaviour of expectations, not about any particular individual.

== Implications for Policy ==
Rational expectations has profound implications for policy. If agents anticipate policy, then systematic policy cannot affect real variables, because agents adjust their behaviour in anticipation. Only unanticipated policy changes have real effects, and these are, by construction, unpredictable.

The policy ineffectiveness proposition formalises this result for monetary policy. It implies that the central bank cannot systematically reduce unemployment below its natural rate and that the principal role of policy is to provide a predictable environment. The proposition has been influential in shaping the case for rules and for credibility in policy.

== The Lucas Critique ==
The Lucas critique holds that the parameters of econometric models are not stable when policy changes, because they reflect the behaviour of agents who adapt to the policy environment. Models estimated under one policy regime cannot be used to evaluate the effects of a different regime, since the expectations and behaviour of agents would change.

The critique undermined the use of large-scale macroeconometric models for policy evaluation and motivated the development of models with explicit microfoundations in which agents' behaviour is derived from preferences and constraints. It remains a central methodological principle in macroeconomics.

== Evidence and Criticism ==
Evidence on rational expectations is mixed. Survey measures of expectations show systematic deviations from full rationality, including persistent biases and underreaction to news. These findings have motivated models of expectations that incorporate learning, sticky information, or behavioural biases.

Critics argue that full rationality is implausible and that models incorporating it fail to match important features of the data. Defenders argue that rational expectations is a useful benchmark and that departures from it should be modelled explicitly rather than assumed. The debate continues, and modern macroeconomics often incorporates elements of both rational and behavioural expectations.

== Adaptive versus Rational Expectations ==
Rational expectations contrasts with adaptive expectations, in which expectations adjust gradually in response to past forecast errors. Under adaptive expectations, agents make systematic errors when the variable being forecast exhibits trends or when the structure of the economy changes, and these errors can be exploited by policy.

Rational expectations eliminates systematic errors, since agents use all available information, including knowledge of policy. The transition from adaptive to rational expectations represented a fundamental shift in the analysis of expectations and in the assessment of policy, and it underpins the modern approach to expectations in macroeconomics.

== Learning and Imperfect Information ==
Full rational expectations requires that agents know the true structure of the economy, an assumption that is strong and often implausible. Models of learning relax this assumption, allowing agents to form expectations by estimating the structure over time. In these models, expectations converge toward rational expectations as information accumulates.

Learning models help explain the dynamics of expectations and the response of the economy to structural change. They also clarify the conditions under which expectations converge and the circumstances in which they may not, and they inform the analysis of the costs of regime changes and of the credibility of policy.

== Sticky Information ==
The sticky information model holds that agents update their information and expectations infrequently, because of the costs of acquiring and processing information. At any time, only a fraction of agents have current information, while others act on outdated expectations, producing gradual adjustment of aggregate expectations.

The model helps explain the persistence of inflation and the dynamics of the Phillips curve, and it provides a microfoundation for the observed sluggishness of expectations. It complements other models of imperfect information and contributes to the analysis of the transmission of shocks and of the effects of policy.

== Expectations in Financial Markets ==
Expectations are central to financial markets, where prices reflect the market's assessment of future payoffs. Evidence suggests that expectations deviate from full rationality in systematic ways, including extrapolative behaviour, overreaction, and underreaction to news.

These departures from rationality motivate behavioural finance, which incorporates psychological findings into the analysis of markets. The behaviour of expectations in financial markets has implications for asset pricing, for the stability of the financial system, and for the conduct of policy, and it remains an active area of research.

== See also ==
* [[New Classical Economics]]
* [[Macroeconomics]]
* [[Monetary economics]]

== References ==

<ref name="r1">Muth, J. Rational expectations and the theory of price movements.</ref>
<ref name="r2">Lucas, R. Econometric policy evaluation: a critique.</ref>
<ref name="r3">Research on expectations formation and the Lucas critique.</ref>

<references />

[[Category:Macroeconomics]]
[[Category:Econometrics]]
[[Category:Monetary economics]]

Rational Expectations - Revision history

Heioo: New encyclopedia entry on Rational Expectations