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Monetary theory in macroeconomics is structured by rival schools with distinct commitments about money's role. The classical foundation is the Quantity Theory of Money, spanning from David Hume to Irving Fisher, which asserts long-run money neutrality and a direct link between money supply and price levels, enshrining the classical dichotomy. This tradition viewed money as a veil over real economic activity, emphasizing its medium-of-exchange function and the stability of velocity in the equation of exchange.
The Keynesian revolution, led by John Maynard Keynes, overturned this orthodoxy with Keynesian Monetary Theory, which highlighted short-run non-neutrality due to sticky prices and wages. Central was the liquidity preference theory, where money demand hinges on interest rates and speculative motives, allowing monetary policy to influence aggregate demand and output through the interest rate channel. This framework justified active discretionary policy to manage economic cycles and dominated mid-20th-century macroeconomics.
Monetarism, championed by Milton Friedman, revived and refined the Quantity Theory into a modern empirical school. It stressed the paramount importance of money supply growth for nominal income in the long run, while acknowledging short-run real effects via expectational errors. Monetarists advocated for rule-based monetary policy to ensure stability, contesting Keynesian discretion and emphasizing the natural rate hypothesis. This school's focus on monetary aggregates and inflation control shaped policy debates in the 1970s.
The New Classical Macroeconomics, propelled by Robert Lucas, introduced rational expectations and the Lucas critique, asserting that anticipated monetary policy is neutral and systematic activism is ineffective. This school embraced market clearing and real business cycle elements, often marginalizing monetary shocks in favor of real factors. In response, New Keynesian Economics developed microfoundations with nominal rigidities like menu costs and wage staggering to explain persistent monetary non-neutrality, integrating rational expectations to justify stabilization policy within dynamic stochastic general equilibrium models.
Alternative strands include Post-Keynesian Monetary Theory, which emphasizes endogenous money, liquidity preference under fundamental uncertainty, and the centrality of credit creation, rejecting mainstream equilibrium approaches. More recently, Modern Monetary Theory has emerged as a distinct paradigm focusing on sovereign currency and fiscal policy space, though it remains outside the core canonical schools. These competing frameworks continue to define the theoretical landscape for understanding money's impact on economies.