From IS-LM to AD-AS: Deriving Aggregate Demand and Supply

Understanding how economies function requires powerful analytical frameworks. While the IS-LM model elegantly explains how goods and money markets interact, it assumes fixed prices-a limitation that becomes problematic when analyzing inflation or long-term economic adjustments. This is where the aggregate demand-aggregate supply model comes in, building upon IS-LM foundations while incorporating price level changes to give us a complete picture of macroeconomic dynamics.

How the aggregate demand curve emerges from IS-LM 🔗

Think of the aggregate demand curve as a clever transformation of the IS-LM model. When economists initially developed the IS-LM framework, it worked beautifully for short-run analysis-but it couldn’t explain what happens when prices change. The breakthrough came when researchers realized they could derive a new curve by systematically varying price levels and observing how equilibrium output responds.

Here’s the intuition: imagine you’re standing at an IS-LM equilibrium with a specific price level. Now, what happens if prices suddenly rise? The nominal money supply stays constant, but the real money supply falls because each rupee buys less. This reduction in real money balances causes the LM curve to shift leftward. Interest rates climb, investment spending drops, and equilibrium output falls.

Plot these combinations of price levels and corresponding output levels on a new graph-with prices on the vertical axis and output on the horizontal axis-and you get a downward-sloping aggregate demand curve. This negative relationship exists because higher prices reduce real money supply, shift LM leftward, raise interest rates, and ultimately lower equilibrium output.

What makes aggregate demand shift? 🔗

The aggregate demand curve doesn’t just sit still. Any factor that would shift either the IS or LM curve in the original model will also shift the entire AD curve in the same direction.

Fiscal policy shifts 🔗

Consider expansionary fiscal policy-perhaps the government announces a major infrastructure spending program or cuts income taxes. In the IS-LM framework, this shifts the IS curve rightward as higher government spending or lower taxes boost aggregate expenditure. Since each point on the AD curve represents an IS-LM equilibrium, this fiscal expansion shifts the entire AD curve to the right. At any given price level, the economy now demands more output. Conversely, contractionary fiscal policy shifts AD leftward, reducing aggregate demand at every price level.

Monetary policy shifts 🔗

Monetary policy works through the LM curve. When the Reserve Bank of India increases the money supply, the LM curve shifts rightward-interest rates fall, investment increases, and output rises at the original price level. This translates into a rightward shift of the aggregate demand curve. The mechanism is straightforward: more money in circulation means lower interest rates and stronger spending incentives across the economy. Tightening monetary policy does the opposite, shifting AD leftward as reduced money supply raises interest rates and dampens economic activity.

Beyond these policy tools, aggregate demand can shift due to changes in consumer confidence, business expectations, exchange rates affecting net exports, or autonomous changes in investment spending. Each factor that alters spending behavior at a given price level will reposition the AD curve.

Three perspectives on aggregate supply 🔗

While aggregate demand tells us what the economy wants to buy, aggregate supply reveals what firms are willing and able to produce. Economists have developed different views of how aggregate supply behaves, each illuminating different aspects of economic reality.

The classical long-run view 🔗

Classical economists envision a vertical long-run aggregate supply curve positioned at the economy’s potential output. This vertical line reflects a fundamental principle: in the long run, an economy’s productive capacity depends on real factors like technology, capital stock, and labor force-not on the price level. If all prices and wages are flexible and can adjust freely, the economy gravitates toward full employment of its resources regardless of whether the price level is high or low.

This classical perspective embodies Say’s Law, the idea that supply creates its own demand. In this framework, changes in aggregate demand merely alter the price level without affecting real output or employment. Want to increase actual production? You need to enhance the economy’s productive capacity through better technology, more capital investment, or workforce expansion.

The Keynesian horizontal view 🔗

John Maynard Keynes challenged this classical optimism with observations from the Great Depression. When Keynes looked around in the 1930s, he saw vast unemployment and idle factories-resources desperate to be used. In such conditions, firms can increase production without raising prices because they can hire unemployed workers at existing wages and activate dormant equipment.

This insight generates a horizontal aggregate supply curve in the short run when the economy operates far below potential. In this Keynesian zone, changes in aggregate demand directly affect output and employment with minimal price level changes. Demand truly creates its own supply here-if someone wants to buy more, firms simply produce more using their abundant spare capacity.

The upward-sloping short-run aggregate supply 🔗

Modern macroeconomics recognizes that reality often falls between these extremes. The short-run aggregate supply curve typically slopes upward, steeper as the economy approaches full capacity. This shape emerges because wages and input prices are sticky in the short run-they don’t instantly adjust to changing economic conditions.

When the price level rises but nominal wages remain temporarily fixed, real wages fall, making labor cheaper for firms. They respond by hiring more workers and increasing output. Additionally, as production expands, bottlenecks emerge-some specialized inputs become scarce, overtime premiums kick in, and less efficient equipment gets activated. These factors cause marginal costs to rise, requiring higher prices to justify continued production increases.

The SRAS curve essentially combines Keynesian and classical insights: at low output levels where unemployment is high, it’s relatively flat like the Keynesian view; near potential GDP where resources are fully employed, it becomes nearly vertical like the classical perspective; in between lies an intermediate zone where both output and prices respond to demand changes.

Finding equilibrium where demand meets supply 🔗

Economic equilibrium occurs at the intersection of the aggregate demand and aggregate supply curves, determining both the economy’s output level and its price level simultaneously. Understanding this equilibrium helps explain business cycles, inflation dynamics, and policy effectiveness.

Short-run versus long-run equilibrium 🔗

In the short run, equilibrium happens where AD crosses the short-run aggregate supply curve. This intersection reveals the current state of the economy-the actual output being produced and the prevailing price level. But this short-run equilibrium might not coincide with the economy’s potential output marked by the vertical long-run aggregate supply curve.

Imagine aggregate demand intersects SRAS to the left of LRAS. The economy operates below potential with high unemployment-a recessionary gap. Workers are willing to work but can’t find jobs; factories could produce more but lack customers. Over time, unemployment puts downward pressure on wages, shifting SRAS rightward until the economy reaches its potential output at a lower price level.

Conversely, if short-run equilibrium occurs to the right of LRAS, the economy overheats with an inflationary gap. Firms strain to produce beyond sustainable levels, overtime becomes standard, and tight labor markets push wages upward. Eventually, rising input costs shift SRAS leftward, returning output to potential but at a higher price level.

Analyzing demand shocks 🔗

Consider what happens when aggregate demand suddenly increases-perhaps due to a government stimulus package or a surge in export demand. The AD curve shifts rightward, creating a new intersection with SRAS at higher output and higher prices. In the short run, the economy enjoys increased production and employment, but experiences inflationary pressure.

What happens next depends on where the economy started. If it began in the Keynesian zone with high unemployment, the output expansion causes little inflation-exactly what policymakers want. But if the economy was already near full capacity in the neoclassical zone, the demand shock primarily raises prices with minimal output gains. In the intermediate zone, we get both more output and higher inflation, creating policy tradeoffs.

Over time, if the new short-run equilibrium exceeds potential output, rising wages and input costs shift SRAS leftward. The economy gradually returns to potential GDP but at a permanently higher price level. This adjustment process reveals an important truth: demand management policies can influence output in the short run, but long-run output depends on the economy’s productive capacity.

Supply shocks and stagflation 🔗

The AD-AS framework also helps explain supply shocks-sudden changes in production costs or capacity. A negative supply shock like an oil price spike shifts SRAS leftward, causing output to fall while prices rise simultaneously. This unpleasant combination-stagnant output with inflation, or stagflation-poses difficult challenges for policymakers. Stimulating aggregate demand might restore output but worsen inflation; fighting inflation by reducing demand further depresses output.

Positive supply shocks work more favorably, shifting SRAS rightward and delivering the pleasant combination of higher output with lower prices. Technological breakthroughs, improved productivity, or reductions in input costs can generate these beneficial supply-side improvements.

Bridging theory and policy 🔗

The AD-AS model’s real power lies in connecting abstract economic theory to practical policy questions. Should the government increase spending during a recession? The answer depends on which zone of the AS curve the economy occupies. In the Keynesian zone with high unemployment, fiscal stimulus shifts AD rightward, significantly boosting output with minimal inflation-the ideal outcome. But in the neoclassical zone near full employment, the same policy primarily generates inflation with little output gain.

This framework also clarifies monetary policy transmission. When the central bank adjusts interest rates, it shifts the LM curve and therefore the AD curve, but the ultimate effects on output and prices depend on aggregate supply conditions. Understanding whether the economy faces demand deficiency or supply constraints fundamentally shapes appropriate policy responses.

The journey from IS-LM to AD-AS represents more than technical extension-it marks the evolution from a fixed-price world to a framework encompassing both output and inflation dynamics. By integrating classical insights about long-run supply constraints with Keynesian understanding of short-run demand management, the AD-AS model provides a balanced, practical tool for analyzing how economies actually behave. Whether examining historical episodes like the 1970s stagflation or contemporary challenges like pandemic recovery, this framework helps organize our thinking about the complex interplay between aggregate demand, aggregate supply, and macroeconomic outcomes.

What do you think? Looking at current economic conditions in your country, which zone of the aggregate supply curve do you think the economy is operating in right now? How would this assessment change your view on whether expansionary fiscal or monetary policy would be more effective?

References

1. https://en.wikipedia.org/wiki/AD–AS_model
2. https://banotes.org/macroeconomics-ii/aggregate-demand-curve-insights-is-lm-analysis/
3. https://www.economicsdiscussion.net/demand/aggregate-demand-curve/derivation-of-aggregate-demand-curve-with-diagram-is-lm-model/15826
4. https://analystprep.com/cfa-level-1-exam/economics/explain-the-is-lm-curves-and-how-they-combine-to-generate-the-aggregate-demand-curve/
5. https://courses.lumenlearning.com/wm-macroeconomics/chapter/keynes-law-and-says-law-in-the-adas-model-2/
6. https://inomics.com/terms/ad-as-model-1546932