The Quantity Theory of Money
The equation of exchange, MV = PY, is one of the oldest identities in economics. The money supply (M) times its velocity (V) — how fast money circulates — equals the price level (P) times real output (Y). In growth rate form, this becomes: inflation = money growth + velocity growth - real GDP growth. Milton Friedman's famous dictum, 'Inflation is always and everywhere a monetary phenomenon,' captures the essence: persistent inflation requires persistent money supply growth beyond what real economic growth absorbs.
Money Growth and Inflation
When central banks increase the money supply faster than the economy grows, the excess money chases the same amount of goods, driving prices up. The relationship is not instantaneous — Friedman estimated 'long and variable lags' of 12-24 months. But over decades, the correlation between money growth and inflation is remarkably tight across countries and eras. Hyperinflation episodes — Weimar Germany (1923), Zimbabwe (2008), Venezuela (2018) — all involved explosive money supply growth.
The Fisher Equation
Irving Fisher's 1930 insight connects inflation to interest rates: nominal rate = real rate + expected inflation. This is why savings accounts paying 2% actually lose money when inflation is 3% — the real return is -1%. It also explains central bank behavior: raising interest rates fights inflation by making borrowing more expensive, slowing money creation through the banking system.
Purchasing Power Erosion
The top chart shows the CPI rising over time while purchasing power declines — the two lines moving in opposite directions tell the same story from different perspectives. Even 'moderate' 3% inflation halves your purchasing power in 24 years. The shrinking $100 bill illustrates this viscerally: your cash is losing value every day. This is why financial literacy emphasizes investing over saving — your returns must exceed inflation to preserve real wealth.
Decomposing Inflation
The bottom section decomposes inflation into its components. Money growth pushes prices up. Velocity changes can amplify or dampen this effect. Real GDP growth absorbs some monetary expansion without inflation — a growing economy needs more money to facilitate more transactions. When money growth is 5% and real GDP grows at 2%, roughly 3% shows up as inflation. The policy implication: if you want price stability, money supply growth should approximately equal real GDP growth.