Rational Expectations and the New Classical Macroeconomics
Rational expectations theory, developed by John Muth (1961) and popularized by Robert Lucas (Nobel Prize 1995), holds that economic agents make optimal use of all available information to form their forecasts. Mankiw presents it in Chapter 14 (9th ed., "Aggregate Supply and the Short-Run Tradeoff") as one of the major intellectual revolutions in macroeconomics, having called into question the effectiveness of stabilization policies.
Last updated: 9 July 2026
Prerequisites
PrerequisitesThis explanation builds on the concepts of inflation and monetary policy introduced in "Understanding Inflation" (ch. 2, Easy level) and "Monetary Policy" (ch. 5, Intermediate level). Mastery of the Phillips curve, the quantity theory of money, and monetary transmission mechanisms is recommended before tackling this chapter.
Definition
DefinitionRational expectations refer to the hypothesis that economic agents form their forecasts by optimally using all available information, including the structure of the economic model itself.
Formally, if Xᵉ denotes the expectation of variable X and Ω the available information set, the rational expectation is: Xᵉ = E[X | Ω] — the mathematical expectation of X conditional on all available information. Agents may err individually, but they make no systematic errors.
The New Classical Macroeconomics (NCM), advanced by Lucas, Sargent, and Wallace, combines rational expectations with the assumption of permanently clearing markets to conclude that only unexpected economic policies can have a real effect on output. Any systematic and predictable policy is anticipated by agents and neutralized.
Why it matters
Rational expectations transformed the way economists conceive economic policy. Before Lucas, Keynesian models treated expectations as an exogenous parameter that policy could manipulate. The Lucas critique showed that this approach was fundamentally flawed: agents adjust their expectations in response to policies, which can neutralize the intended effects.
The main practical consequence is the central role of credibility. If the central bank is credible in its anti-inflation policy, inflation expectations remain anchored and disinflation can be achieved at lower cost in terms of unemployment. The sacrifice ratio (the GDP loss required to reduce inflation by one point) is thus directly tied to the institution's credibility.
Key points
The Lucas critique made the naive use of econometric models for policy evaluation obsolete. Any modern model must incorporate the endogenous reaction of expectations to policy changes
Central bank independence is an institutional response to the time-inconsistency problem: by delegating monetary policy to an independent agent whose aversion to inflation exceeds the government's, the inflationary bias is resolved
Inflation targeting, adopted by most central banks since the 1990s, is a direct application of the theory: by announcing a clear target and credibly committing to it, the central bank anchors expectations and stabilizes inflation at lower cost
Rational expectations does not mean agents are omniscient, nor that markets are always efficient. It means they have no systematic biases that policy can exploit
Concrete example
ExamplesThe Volcker disinflation (1979-1982) illustrates the role of expectations. Paul Volcker, Fed Chair, raised policy rates to 20% to break double-digit inflation. The cost was high (severe recession, unemployment at 10.8%) because agents did not initially believe in the Fed's resolve. Once credibility was established, inflation expectations anchored downward and inflation fell from 13.5% in 1980 to 3.2% in 1983. Conversely, disinflation in countries that adopted inflation targeting in the 1990s (Canada, New Zealand, Sweden) was accomplished at lower cost, because the target's credibility quickly anchored expectations.
Mankiw anecdote
MankiwMankiw is not a neutral expositor of NMC but one of the principal theorists of New Keynesian Economics, a current that built itself precisely as a critical response to the NMC of Lucas and Sargent. His foundational article — "Small Menu Costs and Large Business Cycles" (Quarterly Journal of Economics, vol. 100, no. 2, May 1985, pp. 529-538) — demonstrates that even minor nominal rigidities (menu costs of a few cents per product) suffice to preserve the real effectiveness of monetary policy under rational expectations. This is a direct technical rebuttal of the Sargent-Wallace Policy Ineffectiveness Proposition (1975). His tenure as Chair of the Council of Economic Advisers under George W. Bush (2003-2005) is therefore not in contradiction with his academic views but in coherent application — stabilization policies remain effective as long as they account for rational expectations rather than ignoring them. Mankiw nonetheless acknowledges with nuance in his 6th through 9th editions that NMC transformed the conception of economic policy without invalidating its use: the credibility and predictability of policies are as important as their content, which constitutes the New Keynesian synthesis of NMC's lessons.
📊 Courbe de Phillips
Market impact
MarketsRational expectations sit at the heart of how markets react to economic policy announcements. Markets instantly price in predictable information (forward guidance, Taylor rules). Only surprises — gaps between actual decisions and market expectations — trigger significant price movements. This is why markets do not react to an anticipated rate hike but react violently to an unexpected one. The central bank's credibility directly determines the level of inflation risk premia in bond markets.