Quantity theory of money

The quantity theory of money (often abbreviated QTM) is a theory from monetary economics which states that the general price level of goods and services is directly proportional to the amount of money in circulation (i.e., the money supply), and that the causality runs from money to prices. This implies that the theory potentially explains inflation. It originated in the 16th century and has been proclaimed the oldest surviving theory in economics.

According to some, the theory was originally formulated by Renaissance mathematician Nicolaus Copernicus in 1517, whereas others mention Martín de Azpilcueta and Jean Bodin as independent originators of the theory. It has later been discussed and developed by several prominent thinkers and economists including John Locke, David Hume, Irving Fisher and Alfred Marshall. Milton Friedman made a restatement of the theory in 1956 and made it into a cornerstone of monetarist thinking.

The theory is often stated in terms of the equation MV = PY, where M is the money supply, V is the velocity of money, and PY is the nominal value of output or nominal GDP (P itself being a price index and Y the amount of real output). This equation is known as the quantity equation or the equation of exchange and is itself uncontroversial, as it can be seen as an accounting identity, residually defining velocity as the ratio of nominal output to the supply of money. Assuming additionally that Y is exogenous, being independently determined by other factors, that V is constant, and that M is exogenous and under the control of the central bank, the equation is turned into a theory which says that inflation (the change in P over time) can be controlled by setting the growth rate of M. However, all three assumptions are arguable and have been challenged over time. Output is generally believed to be affected by monetary policy at least temporarily, velocity has historically changed in unanticipated ways because of shifts in the money demand function, and some economists believe the money supply to be endogenously determined and hence not controlled by the monetary authorities.

The QTM played an important role in the monetary policy of the 1970s and 1980s when several leading central banks (including the Federal Reserve, the Bank of England and Bundesbank) based their policies on a money supply target in accordance with the theory. However, the results were not satisfactory, and strategies focusing specifically on monetary aggregates were generally abandoned during the 1980s and 1990s. Today, most major central banks in practice follow inflation targeting by suitably changing interest rates, and monetary aggregates play little role in monetary policy considerations in most countries.