AP Syllabus focus: ‘When the money market is in disequilibrium, surpluses or shortages push nominal interest rates toward equilibrium.’
Money markets do not always start in equilibrium. When people try to hold more or less money than is available at the current nominal interest rate, financial trades change bond prices and interest rates until the money market clears again.
Core idea: disequilibrium creates pressure on the nominal interest rate
The money market compares the quantity of money people want to hold with the quantity of money available.
A standard money market diagram with a vertical money supply curve and a downward-sloping money demand curve. The intersection identifies the equilibrium nominal interest rate, where money holdings demanded equal the quantity of money supplied. This visual reinforces why movements in (rather than movements in ) are the key adjustment back to equilibrium. Source
If the market is not at its clearing interest rate, people adjust their portfolios (money versus interest-bearing assets), and that behavior moves the nominal interest rate back toward equilibrium.
Disequilibrium outcomes
Money market surplus (too much money supplied)
A surplus occurs when the quantity of money supplied is greater than the quantity demanded at the current nominal interest rate.
Money market surplus: a situation where at the current nominal interest rate, meaning people are holding more money than they want.
When households or firms have excess money balances, they try to get rid of them by buying interest-bearing assets (commonly modelled as bonds). This portfolio shift is the key mechanism that changes interest rates.
Money market shortage (too little money supplied)
A shortage occurs when the quantity of money demanded is greater than the quantity supplied at the current nominal interest rate.
Money market shortage: a situation where at the current nominal interest rate, meaning people want to hold more money than is available.
In a shortage, people try to increase money holdings by selling interest-bearing assets to obtain cash or deposits, which puts upward pressure on interest rates.
The interest rate adjustment mechanism (how the market moves back to equilibrium)
The nominal interest rate adjusts because trades in the bond market change bond prices, and bond prices and interest rates move in opposite directions. The central logic is: when people prefer bonds over money, bond prices rise and interest rates fall; when people prefer money over bonds, bond prices fall and interest rates rise.
= Quantity of money demanded (dollars)
= Quantity of money supplied (dollars)
= Nominal interest rate (percent per year)
This adjustment occurs because money demand is inversely related to the nominal interest rate: when falls, holding money becomes less costly (lower foregone interest), so the quantity of money demanded rises; when rises, holding money becomes more costly, so the quantity of money demanded falls.
Step-by-step: surplus drives the interest rate down
Initial condition: at the current nominal interest rate.
People attempt to reduce excess money holdings by buying bonds (or other interest-bearing assets).
Increased demand for bonds raises bond prices.
Higher bond prices imply a lower nominal interest rate.
As falls, the quantity of money demanded increases, reducing the surplus.
The process continues until , eliminating the surplus.
Step-by-step: shortage drives the interest rate up
Initial condition: at the current nominal interest rate.
People attempt to build money balances by selling bonds to obtain money.
Increased supply of bonds lowers bond prices.
Lower bond prices imply a higher nominal interest rate.
As rises, the quantity of money demanded decreases, reducing the shortage.
The process continues until , eliminating the shortage.
What to emphasise in an AP-style explanation
Direction first: identify whether the market has a surplus or shortage of money.
Portfolio behaviour: explain why people buy or sell bonds to adjust money holdings.
Bond prices to interest rates: connect the trades to falling or rising.
Return to equilibrium: show that changing changes until it equals .
FAQ
Bonds are a convenient stand-in for interest-bearing assets because their prices adjust quickly in organised markets.
Using bonds highlights the key link: trading assets changes asset prices, which changes the market interest rate.
Yes. Disequilibrium can occur if people’s desired money holdings shift abruptly due to payment timing, uncertainty, or changes in spending plans.
With a fixed $Q^s_M$, the adjustment happens through changes in $i$ rather than changes in money available.
The model assumes rates can adjust via asset trading, but in reality rates may adjust with delays.
Even with frictions, the predicted direction of pressure remains: surplus pushes $i$ down; shortage pushes $i$ up.
Money demand is tied to the opportunity cost of holding money measured in money terms, which is the nominal return on interest-bearing alternatives.
Inflation expectations matter in practice, but the clearing mechanism here is framed through the nominal rate.
Not necessarily. Many people adjust via bank accounts, money market funds, or other intermediaries.
Those intermediaries still rebalance portfolios in bond and short-term debt markets, transmitting the same price-and-interest-rate adjustment.
Practice Questions
(2 marks) If the nominal interest rate is above the money market equilibrium rate, is there a money surplus or shortage, and what happens to the nominal interest rate over time?
Identifies surplus of money (1)
States nominal interest rate falls toward equilibrium (1)
(6 marks) Explain how a money market shortage leads to a change in bond prices and an adjustment of the nominal interest rate back to equilibrium, assuming the money supply is fixed.
States that at current , (1)
People sell bonds to increase money holdings (1)
Bond prices fall due to increased bond supply (1)
Falling bond prices imply rises (inverse relationship) (1)
Higher reduces quantity of money demanded (1)
Equilibrium restored when (1)
