IS-LM Model: What It Is, IS and LM Curves, Characteristics, Limitations

What Is the IS-LM Model?

The IS-LM model, which stands for “investment-saving” (IS) and “liquidity preference-money supply” (LM), is a Keynesian macroeconomic model that shows how the market for economic goods interacts with the loanable funds market, or money market. It is represented as a graph in which the IS and LM curves intersect to show the short-run equilibrium between interest rates and output.

Understanding the IS-LM Model

British economist John Hicks first introduced the IS-LM model in 1937, not long after fellow British economist John Maynard Keynes published "The General Theory of Employment, Interest, and Money" in 1936. Hicks’ model served as a formalized graphical representation of Keynes’ theories, though it is used mainly as a heuristic device today.

The three critical exogenous (or external) variables in the IS-LM model are liquidity, investment, and consumption. According to the theory, liquidity is determined by the size and velocity of the money supply. The levels of investment and consumption are determined by the marginal decisions of individual actors.

The IS-LM graph examines the relationship between output, or gross domestic product (GDP), and interest rates. The entire economy is boiled down to just two markets—output and money—and their respective supply and demand characteristics push the economy toward an equilibrium point.

Characteristics of the IS-LM Graph

The IS-LM graph consists of two curves: IS and LM. GDP is placed on the horizontal axis, increasing to the right. The interest rate makes up the vertical axis.

The IS Curve

The IS curve depicts the set of all levels of interest rates and output (GDP) at which total investment (I) equals total saving (S). At lower interest rates, investment is higher, which translates into more total output (GDP), so the IS curve slopes downward and to the right.

The LM Curve

The LM curve depicts the set of all levels of income (GDP) and interest rates at which money supply equals money (liquidity) demand. The LM curve slopes upward because higher levels of income (GDP) induce increased demand to hold money balances for transactions, which requires a higher interest rate to keep money supply and liquidity demand in equilibrium.

The Intersection of the IS and LM Curves

The intersection of the IS and LM curves shows the equilibrium point of interest rates and output when money markets and the real economy are in balance. Multiple scenarios or points in time may be represented by adding additional IS and LM curves.

In some versions of the graph, curves display limited convexity or concavity. Shifts in the position and shape of the IS and LM curves, representing changing preferences for liquidity, investment, and consumption, alter the equilibrium levels of income and interest rates.

Limitations of the IS-LM Model

Many economists, including many Keynesians, object to the IS-LM model for its simplistic and unrealistic assumptions about the macroeconomy. It cannot account for simultaneous high unemployment and inflation in the economy. It is also undercut by the change by central banks to using an interest-rate rule rather than targeting the money supply.

Even Hicks later admitted that the model’s flaws were fatal, and it was probably best used as “a classroom gadget, to be superseded, later on, by something better.” Subsequent revisions have taken place for so-called “new” or “optimized” IS-LM frameworks.

The model is a limited policy tool, as it cannot explain how tax or spending policies should be formulated with any specificity. This significantly limits its functional appeal. It has very little to say about inflation, rational expectations, or international markets, although later models do attempt to incorporate these ideas. The model also ignores the formation of capital and labor productivity.

The Bottom Line

The IS-LM model is a tool for looking at how the market for economic goods intersects with the loanable funds market. It depicts the short-term equilibrium point between interest rates and output, with its three variables being liquidity, investment, and consumption.

Because it is a highly simplistic device, it is only useful when snap decisions must be made, as it lacks the sophistication necessary for setting tax and spending policies.