ECON30005 · Money and Banking
Money Demand and the Quantity Theory of Money
Week 2 derives the demand for money three ways: the classical quantity theory via the equation of exchange MV=PY, Keynes's liquidity-preference theory with its transactions, precautionary and speculative motives, and portfolio theory. It shows why the nominal interest rate is the opportunity cost of holding money and why velocity is not in fact constant. Expect true/false on the quantity-theory assumptions, short essays on the motives for holding money, and MV=PY arithmetic (levels or growth rates) in the quiz and exam.
What this chapter covers
- 01The equation of exchange MV = PY: an identity that becomes a theory of inflation when V is stable and Y is at potential
- 02Implied classical money demand M = (1/V)PY — proportional to nominal income, independent of the interest rate
- 03Growth-rate form: gM + gV ≈ π + gY, so with V constant π ≈ gM − gY
- 04Keynesian liquidity preference: transactions, precautionary and speculative motives → M/P = L(i, Y), increasing in Y, decreasing in i
- 05The nominal interest rate as the opportunity cost of holding money
- 06Portfolio theory: asset demand rises with wealth, expected relative return and liquidity, falls with relative risk
- 07Why velocity is not constant in the data — payment technology, interest rates and financial innovation move V
Velocity from the equation of exchange, then inflation from growth rates
- +1Write the equation of exchange MV = PY, so velocity V = PY/M. Substitute: V = 6000/800 = 7.5. Each dollar of money supports 7.5 dollars of nominal spending per year.
- +1Take the growth-rate form. Log-differentiating MV = PY gives gM + gV ≈ π + gY, where π is the inflation rate. Rearrange for inflation: π ≈ gM + gV − gY.
- +1Substitute the growth rates. With velocity stable, gV = 0, so π ≈ 9% + 0% − 3% = 6%.
- +1State the assumption. The result relies on velocity being stable (gV = 0) — the quantity-theory assumption. If financial innovation or interest-rate changes move velocity, the money-growth/inflation link loosens, which is why the theory fits the long run better than the short run.
Key terms
- Equation of exchange
- The identity MV = PY linking the money stock M and its velocity V to the price level P and real output Y. It holds by definition of velocity; the quantity theory adds behavioural assumptions to turn it into a theory of inflation.
- Velocity of money
- The average number of times a unit of money is spent on final goods per period, V = PY/M. The quantity theory assumes it is stable, but in the data it moves with payment technology, interest rates and financial innovation.
- Liquidity preference
- Keynes's theory that money is demanded for three motives — transactions, precautionary and speculative — giving M/P = L(i, Y), increasing in income and decreasing in the nominal interest rate.
- Speculative motive
- The demand to hold money as a store of wealth; a higher nominal interest rate on bonds raises the opportunity cost of holding money and lowers speculative money demand.
- Opportunity cost of holding money
- The nominal interest rate — the return forgone by holding non-interest-bearing money rather than bonds. It is why money demand falls as the nominal rate rises.
- Portfolio theory of money demand
- The view that money demand, like any asset demand, rises with wealth, expected relative return and liquidity and falls with relative risk; it provides a micro-foundation for the Keynesian money-demand function.
Money Demand and the Quantity Theory of Money FAQ
Why is MV = PY an identity but the quantity theory a theory?
MV = PY is true by construction: velocity is defined as PY/M, so the equation always holds. It becomes the quantity theory of money — a theory of inflation — only once you add assumptions: that velocity is stable (set by transaction institutions) and that real output is at its full-employment level. Under those assumptions changes in the money supply map into proportional changes in the price level.
Does the interest rate affect money demand or not?
It depends on the theory. In the classical quantity theory money demand M = (1/V)PY is proportional to nominal income and does not depend on the interest rate. In Keynesian liquidity preference and portfolio theory it does: a higher nominal interest rate is the opportunity cost of holding money, so money demand falls. Empirically the interest-rate effect is real, which is one reason velocity is not constant.
Why is velocity not actually constant?
Velocity moves with the payment technology (faster payments let each dollar turn over more), with interest rates (higher rates push people to economise on money balances), and with financial innovation (new deposit-like products). A payments outage, for example, forces settlement through cash and deposits and lowers velocity. Because velocity drifts, the quantity theory predicts long-run inflation well but short-run inflation poorly.
Can AI help me with the quantity theory in ECON30005?
Yes. Sia can walk you through MV = PY in both level and growth-rate form, show why the stable-velocity assumption matters, and check your inflation arithmetic on fresh numbers. Use it to rehearse the method and to test the three money-demand theories against each other; it does not complete graded quizzes for you, and you should confirm assessment rules on Canvas.
Exam move
Master MV = PY in both forms and know when each applies: the level form V = PY/M for a snapshot of velocity, the growth-rate form gM + gV = π + gY for inflation. Drill the π ≈ gM − gY shortcut and always attach the 'stable velocity' caveat — examiners reward the assumption clause. For the theory side, be able to contrast the three money-demand stories on one page: quantity theory (interest-rate-independent), Keynesian liquidity preference (three motives, M/P = L(i, Y)), and portfolio theory (wealth, return, liquidity, risk). Keep two or three real reasons velocity moves, because a favourite true/false is 'velocity is constant' (false in the data). This week feeds directly into the OLG model, where the same money-demand comparative statics are derived analytically. Confirm the assessment structure on Canvas.
Working through Money Demand and the Quantity Theory of Money in ECON30005? Sia is AskSia’s AI Economics tutor — ask any ECON30005 Money Demand and the Quantity Theory of Money question and get a clear, step-by-step explanation grounded in how ECON30005 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
