IS–LM model¶

A two-curve diagram fixing short-run equilibrium in a closed economy: the downward IS curve where the goods market clears and the upward LM curve where the money market clears cross at one point (r, Y) that pins down the interest rate and output jointly.

Version: v2 · 2026-09-19 · History
Domain-specific #: 575
Domain group: Social Sciences
Origin domain: Economics & Finance
Subdomain: Macroeconomics → Economics & Finance

Core Idea¶

The IS–LM model, introduced by Hicks (1937) to formalize Keynes, is a two-curve graphical apparatus for short-run equilibrium in a closed economy with a goods market and a money market. The IS curve traces all combinations of interest rate and output where the goods market clears (sloping down); the LM curve traces where the money market clears (sloping up). Their intersection yields the unique equilibrium (r, Y) at which both markets clear simultaneously — the model's central result.

How would you explain it like I'm…

Two Lines Find Balance

Economists who study a whole country's money draw two lines on a picture. One line shows when the amount of stuff made matches what people want to spend on, and the other shows when the money people want to keep matches the money there is. Where the two lines cross, both things are balanced at once, and that tells them how much stuff gets made and what borrowing costs for now.

The Money-and-Goods Crossing

The IS–LM model is a drawing economists use to figure out how much a country produces and what the interest rate (the price of borrowing money) will be in the short run. One line, IS, shows where spending matches what's made. It slopes down, because cheaper borrowing makes businesses build and buy more. The other line, LM, shows where the money people want to hold equals the money available. It slopes up, because a busier economy needs more money for buying things. Where the lines cross is the answer. If the government spends more, or the central bank adds money, a line moves and the crossing point moves too.

Goods-and-Money Market Equilibrium

The IS–LM model, created by John Hicks in 1937 to formalise Keynes's ideas, finds short-run equilibrium in a closed economy using two curves on a graph of interest rate (r) against output (Y). The IS curve shows where the goods market clears, meaning planned investment equals planned saving. It slopes downward because lower rates boost investment and so output. The LM curve shows where money demand equals money supply. It slopes upward because more output means more money needed for transactions, which pushes rates up when the money supply is fixed. Their intersection gives the one pair (r*, Y*) where both markets clear. Government spending shifts IS right, raising output and rates, and the higher rates 'crowd out' some private investment. Increasing the money supply shifts LM right, raising output and lowering rates, unless LM is flat (a liquidity trap), in which case extra money doesn't lower rates and monetary policy stops working.

Hicks's IS–LM model (1937) formalises Keynes's General Theory as simultaneous equilibrium in the goods and money markets of a closed economy in the short run. The IS curve is the locus of (r, Y) where planned investment equals planned saving, and it slopes down because investment is interest-sensitive. The LM curve is the locus where real money demand equals real money supply, and it slopes up because transactions demand rises with Y, requiring a higher r to clear the market at fixed money supply. The intersection determines (r*, Y*). Comparative statics drive the analysis. Fiscal expansion shifts IS right, raising Y and r, with crowding out of private investment whose size depends on the LM slope. Monetary expansion shifts LM right, raising Y and lowering r. A flat LM, the liquidity trap with highly interest-elastic money demand, makes monetary policy impotent and fiscal policy fully effective without crowding out, while a steep LM reverses this. This Keynesian-versus-classical regime classification organised policy debate from the 1940s to the 1980s and returned with the post-2008 zero lower bound. Mundell and Fleming added a balance-of-payments curve to produce the open-economy Mundell–Fleming model, from which the impossible trinity follows.

Scope of Application¶

The IS–LM model lives within macroeconomics — the subfields and uses sharing its goods-and-money closed-economy substrate, bounded by that monetary-economy frame.

Undergraduate and intermediate macro teaching — the canonical first formal model.
Short-run policy comparative-statics — "what happens to output if spending rises by X?"
Open-economy macroeconomics — the Mundell–Fleming extension yielding the impossible trinity.
History of macroeconomic thought — the bridge between Keynes and the neoclassical synthesis.
Central-bank public communication — still used to explain policy where DSGE runs internally.

Clarity¶

The model first makes visible that the goods and money markets clear jointly, not sequentially — output and the interest rate are determined together at one intersection. This dissolves the classical and naive-monetarist positions as special cases of one picture. Its sharper service converts the open-ended fiscal-versus-monetary debate into a determinate question about the slope of the LM curve, a structural parameter with observable content.

Manages Complexity¶

IS–LM compresses an unmanageable multi-market economy to two clearing loci on one (r, Y) plane and a single tracked object: the intersection. Two further compressions make it a working tool — any shock reduces to which curve it shifts and in which direction (four canonical movements), and the fiscal-versus-monetary debate contracts to one slope parameter, the interest elasticity of money demand, at the cost of everything the two curves suppress.

Abstract Reasoning¶

The model's primary move is shock-to-curve-shift decomposition, resting on a joint-determination move that corrects the "output or interest rate?" error. Its sharpest move is regime identification by curve slope, turning an ideological dispute into a parameter reading. A compositional-extension move builds Mundell–Fleming by adding a curve, and an explicit boundary condition disciplines its jurisdiction as a first-pass diagnostic.

Knowledge Transfer¶

Within macroeconomics IS–LM transfers as mechanism across the subfields sharing its closed-economy substrate — teaching, policy comparative-statics, the Mundell–Fleming extension, the history of thought, central-bank communication — carrying its limitations as a package. Beyond macroeconomics the model itself does not travel: its curves and parameters have no referent outside a goods-and-money economy. The substrate-independent insight beneath it — coupled markets clear jointly at the crossing of their clearing conditions — is carried by the parent prime general_equilibrium, not by IS–LM.

Relationships to Other Abstractions¶

Current abstraction IS–LM model Domain-specific

Parents (2) — outgoing typed relations

IS–LM model includes Comparative Statics Prime

IS–LM contains comparative statics as the curve-shift and old-versus-new intersection operation that produces its policy results.
IS–LM model presupposes Equilibrium Prime

IS–LM presupposes equilibrium because its defining point simultaneously clears the coupled goods and money markets.

Children (1) — abstractions linked here by typed relations

Aggregate Demand Domain-specific presupposes, typical IS–LM model

Aggregate demand typically presupposes IS–LM when the schedule is derived by tracing joint goods-money equilibrium output across price levels.

Hierarchy paths (2) — routes to 1 parentless root

IS–LM model → presupposes Equilibrium → is a kind of, typical Fixed Point

Show alternative path (1)

Neighborhood in Abstraction Space¶

IS–LM model sits in a moderately populated region (53^rd percentile for distinctiveness): it has near-neighbors but no dense thicket of look-alikes.

Family — Macroeconomic Growth & Equilibrium Models (22 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-10-25