The IS-LM Model Explained: A Step-by-Step Guide
The IS-LM model is one of the central analytical frameworks in macroeconomics. It combines two markets — the goods market and the money market — into a single diagram to show how interest rates and national output are determined simultaneously. Developed by John Hicks and Alvin Hansen as a formalization of Keynes's General Theory, the IS-LM model remains a standard tool for analyzing the effects of fiscal and monetary policy.
The Two Markets
The IS-LM framework rests on two equilibrium conditions, one for each market. The goods market is in equilibrium when total output (Y) equals total desired spending — consumption, investment, government expenditure, and net exports. The money market is in equilibrium when the demand for money equals the supply of money, which determines the interest rate (r).
The key insight is that these two markets are linked. Investment in the goods market depends on the interest rate determined in the money market. Changes in output affect the demand for money. The model finds the interest rate and output level that satisfies both equilibrium conditions at once.
Key Concept: The IS-LM Diagram. The IS-LM model is typically drawn on a graph with the interest rate (r) on the vertical axis and real output or income (Y) on the horizontal axis. The IS curve and the LM curve both appear on this graph. Their intersection is the economy's short-run equilibrium: the unique combination of r and Y at which both markets clear simultaneously.
The IS Curve: Goods Market Equilibrium
The IS curve shows all combinations of the interest rate and output at which the goods market is in equilibrium — that is, where investment equals saving (hence "IS"). It slopes downward.
The reasoning is straightforward. At a lower interest rate, borrowing is cheaper, so firms invest more. Higher investment raises aggregate demand, which through the multiplier process increases equilibrium output. Therefore, lower interest rates correspond to higher output on the IS curve, giving it a negative slope.
What Shifts the IS Curve?
The IS curve shifts when anything other than the interest rate changes aggregate demand. An increase in government spending shifts IS to the right: at every interest rate, output is now higher. A tax cut also shifts IS rightward by boosting consumption. A fall in business confidence shifts IS leftward by reducing investment at every interest rate. Net export changes driven by foreign income or exchange rates also shift the IS curve.
The LM Curve: Money Market Equilibrium
The LM curve shows all combinations of the interest rate and output at which the money market is in equilibrium — where money demand equals money supply (hence "LM" for Liquidity preference-Money supply). It slopes upward.
The logic runs as follows. Higher output means more transactions, which increases the demand for money as a medium of exchange (the transactions motive). For a fixed money supply, the interest rate must rise to reduce speculative money demand and bring total money demand back into balance with supply. Therefore, higher output corresponds to higher interest rates on the LM curve.
Key Concept: Liquidity Preference. Keynes argued that people hold money not only to make transactions but also because money is liquid — easily converted into other assets. At higher interest rates, the opportunity cost of holding money rises, so people hold less money and more interest-bearing bonds. This inverse relationship between the interest rate and money demand (liquidity preference) is the foundation of the LM curve.
What Shifts the LM Curve?
The LM curve shifts when the money supply changes. An increase in the money supply — through the central bank buying bonds — shifts LM to the right: at every output level, the interest rate is now lower because there is more money available. A decrease in the money supply shifts LM leftward, raising interest rates at every output level. Changes in the price level also shift LM: a higher price level reduces the real money supply and shifts LM leftward.
IS-LM Equilibrium
Equilibrium in the IS-LM model occurs at the intersection of the two curves. This point gives the unique interest rate and output level at which both the goods market and the money market are simultaneously in balance. If the economy is off this point, adjustment forces push it back toward equilibrium.
Suppose the economy is initially in equilibrium at output Y* and interest rate r*. Now the government increases spending. This shifts IS to the right. At the old interest rate r*, the new IS curve implies higher output, but this higher output raises money demand. With the money supply unchanged, the interest rate rises. The economy settles at a new equilibrium with both higher output and a higher interest rate.
The rise in the interest rate partially crowds out private investment, so the final increase in output is less than the simple Keynesian multiplier would suggest. This crowding-out effect is built directly into the IS-LM framework.
Fiscal Policy in the IS-LM Model
Expansionary fiscal policy — higher government spending or lower taxes — shifts the IS curve to the right. In the IS-LM diagram, this raises equilibrium output and the equilibrium interest rate. The output increase is smaller than the simple multiplier predicts because the higher interest rate reduces investment.
How much output rises relative to how much the interest rate rises depends on the slopes of the two curves. If the LM curve is relatively flat (which happens when money demand is very interest-sensitive), the interest rate rise is small and the output increase is large. If the LM curve is steep, the interest rate rise is large and more crowding out occurs.
Monetary Policy in the IS-LM Model
Expansionary monetary policy — an increase in the money supply — shifts the LM curve to the right. This lowers the equilibrium interest rate and raises equilibrium output. Lower interest rates stimulate investment, which increases aggregate demand and output.
A special case is the liquidity trap: when interest rates are already very low, the LM curve becomes nearly horizontal and monetary policy has little effect on output. Additional money is simply held as cash rather than driving rates lower. This was a central concern during the Great Depression and again after the 2008 financial crisis.
Key Concept: The Liquidity Trap. In a liquidity trap, interest rates are so low that people expect them to rise in the future, making bonds unattractive. Any increase in the money supply is absorbed entirely into idle money holdings. Monetary policy becomes ineffective, and fiscal policy — shifting the IS curve — becomes the dominant stabilization tool.
Limitations and Extensions
The IS-LM model is a short-run framework that holds prices fixed. It does not explain inflation or long-run growth. Modern extensions add an aggregate supply curve (creating the AS-AD model) or incorporate expectations explicitly (as in the New Keynesian framework). Despite its limitations, IS-LM remains valuable for building intuition about how goods markets and money markets interact and for tracing the first-order effects of policy changes.
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Frequently Asked Questions
What does IS-LM stand for?
IS stands for Investment-Saving and represents the goods market. LM stands for Liquidity preference-Money supply and represents the money market. Together, the IS-LM model finds the combinations of interest rates and output (GDP) at which both the goods market and the money market are simultaneously in equilibrium.
Why does the IS curve slope downward?
The IS curve slopes downward because lower interest rates encourage more investment spending, which increases aggregate demand and raises equilibrium output. Conversely, higher interest rates reduce investment, lowering output. This inverse relationship between the interest rate and output is what gives the IS curve its negative slope.
How does fiscal policy affect the IS-LM model?
An increase in government spending or a tax cut shifts the IS curve to the right, because it raises aggregate demand at any given interest rate. In the IS-LM framework, this raises both output and the interest rate — the higher interest rate partially crowds out private investment, limiting the full multiplier effect of the fiscal expansion.