Changes in the AD-AS Model - Macroeconomics

SECTION 1

Historical Context & Motivation

The Aggregate Demand–Aggregate Supply (AD-AS) model is the workhorse framework of modern macroeconomics, yet its development was anything but straightforward. Before the 1930s, classical economists assumed that markets always clear, wages and prices adjust flexibly, and the economy naturally gravitates toward full employment. The catastrophic Great Depression shattered that confidence, as output collapsed, unemployment soared beyond 25 percent in the United States, and prices fell rather than adjusting smoothly. Economists needed a new theoretical lens—one that could explain why an entire economy might get stuck well below its productive potential and how government policy could, or could not, restore prosperity.

1936 • Keynes's General Theory

John Maynard Keynes published The General Theory of Employment, Interest and Money, arguing that aggregate demand drives short-run output and that economies can remain below full employment for extended periods.

1958 • The Phillips Curve

A.W. Phillips documented an inverse relationship between unemployment and wage inflation in the UK, providing an empirical anchor for demand-side analysis and foreshadowing the short-run aggregate supply curve.

1970s • Stagflation & Supply Shocks

The OPEC oil embargoes produced simultaneous inflation and recession—stagflation—forcing economists to incorporate supply-side shifts into the model and distinguish short-run from long-run aggregate supply.

1980s–90s • New Keynesian Synthesis

Economists such as Mankiw and Romer formalized sticky wages and prices with microfoundations, producing the modern AD-AS framework widely taught in business and economics programs today.

The central question the AD-AS model addresses is deceptively simple: What causes real GDP and the price level to change in the short run, and how does the economy eventually return to long-run equilibrium? Understanding the mechanics of shifts in both aggregate demand and aggregate supply equips business professionals with the analytical toolkit needed to anticipate recessions, inflationary episodes, and the likely effects of fiscal and monetary policy.

SECTION 2

Core Principles & Definitions

Before analyzing shifts, it is essential to understand the three curves that compose the AD-AS model and the forces behind each. Aggregate demand (AD) represents the total quantity of goods and services demanded across all sectors of the economy—consumption, investment, government spending, and net exports—at each price level. The AD curve slopes downward for three interrelated reasons: the wealth effect (higher prices erode real purchasing power), the interest-rate effect (higher prices increase money demand and raise interest rates, which reduces investment), and the exchange-rate effect (higher domestic prices make exports more expensive abroad, reducing net exports). Short-run aggregate supply (SRAS) captures the total output firms are willing to produce at each price level when at least some input prices—most notably wages—are sticky or slow to adjust. The SRAS curve slopes upward because rising output prices, with wages temporarily fixed, increase profit margins and encourage firms to expand production. Long-run aggregate supply (LRAS) is a vertical line at the economy's potential output (Y*), reflecting the classical insight that in the long run, when all prices and wages have fully adjusted, real GDP depends only on real factors—technology, labor, and capital—not on the price level.

Demand Shock

Any event that shifts the AD curve. A positive demand shock (e.g., fiscal stimulus, consumer confidence surge) shifts AD right, raising both real GDP and the price level in the short run. A negative demand shock shifts AD left.

Supply Shock

Any event that shifts the SRAS curve. A negative supply shock (e.g., oil price spike) shifts SRAS left, raising the price level while reducing output—the hallmark of stagflation. A positive supply shock shifts SRAS right.

Short-Run Equilibrium

The intersection of AD and SRAS determines the economy's short-run price level and real GDP. This equilibrium can occur above, below, or exactly at potential output.

Long-Run Self-Correction

When the economy deviates from potential output, wages and input prices eventually adjust, shifting SRAS until the economy returns to the LRAS. This self-correcting mechanism may be slow without policy intervention.

Output Gap

The difference between actual real GDP and potential GDP. A recessionary (negative) gap implies unemployment above the natural rate; an inflationary (positive) gap implies overheating and upward pressure on wages and prices.

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KEY TAKEAWAY

Think of the AD-AS model like a thermostat system. Aggregate demand is the heat setting you choose (consumer and government spending decisions), SRAS is the furnace responding to that setting given current fuel costs (input prices), and LRAS is the home's maximum heating capacity (potential output). A demand shock is like someone cranking the thermostat up or down; a supply shock is like the price of natural gas spiking. In the long run, the house adjusts—but in the short run, you may be too hot or shivering.

SECTION 3

Visual Explanation — The AD-AS Diagram

The diagram below illustrates the baseline AD-AS model with all three curves—AD, SRAS, and LRAS—as well as the initial equilibrium point where aggregate demand meets short-run aggregate supply at potential output. Understanding this starting position is crucial because every shift analysis begins from this reference point. Note how the LRAS is vertical at potential GDP (Y*), while the SRAS slopes upward and the AD slopes downward.

The initial equilibrium E₀ occurs where AD intersects SRAS, which in this baseline scenario also sits on the LRAS at potential output Y*. The price level settles at P₀. Any shift in AD or SRAS will move the economy away from this point, creating an output gap.

In the diagram, the amber dot labeled E₀ marks the economy's starting equilibrium. The green dashed vertical line is the LRAS at Y*, representing the economy's capacity when all resources are fully employed at the natural rate of unemployment. The violet upward-sloping line is SRAS, and the blue downward-sloping line is AD. When a shock hits the economy, one (or both) of these curves shifts, moving the equilibrium to a new point and creating either a recessionary gap (output below Y*) or an inflationary gap (output above Y*).

SECTION 4

Mathematical Framework

While the AD-AS model is most often presented graphically, understanding its algebraic structure deepens your ability to reason about the magnitude and direction of shifts. The aggregate demand relationship can be derived from the income-expenditure identity combined with money market equilibrium, and the short-run aggregate supply curve arises from the sticky-wage or sticky-price assumption. The following equations capture the essential mechanics.

AGGREGATE EXPENDITURE IDENTITY

Y = C + I + G + (X − M)

Y = real GDP, C = consumption, I = investment, G = government purchases, X = exports, M = imports. A change in any component shifts the AD curve.

SPENDING MULTIPLIER

Multiplier = 1 / (1 − MPC)

MPC = marginal propensity to consume. A dollar increase in autonomous spending shifts the AD curve by more than one dollar because each round of spending generates additional income and further spending. For example, if MPC = 0.8, the multiplier = 1 / (1 − 0.8) = 5.

CHANGE IN EQUILIBRIUM GDP (DEMAND SIDE)

ΔY = Multiplier × ΔAutonomous Spending

This tells us how far the AD curve shifts horizontally in response to an initial change in spending. Note that the actual change in equilibrium output will be smaller once the price level adjusts along the SRAS curve—the multiplier gives the maximum horizontal shift of the AD curve, not the final equilibrium change.

SHORT-RUN AGGREGATE SUPPLY (STICKY-WAGE MODEL)

Y = Y* + α(P − Pᵉ)

Y* = potential GDP, P = actual price level, Pᵉ = expected price level, α = positive parameter reflecting supply responsiveness. When the actual price level exceeds expectations, firms earn higher margins with sticky wages, so output exceeds potential. When Pᵉ adjusts upward (workers renegotiate wages), the SRAS curve shifts left.

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Business Application

When you hear that the Federal Reserve is expected to cut interest rates, think of the multiplier framework. Lower rates reduce the cost of borrowing, boosting planned investment (ΔI > 0). The initial increase in investment then ripples through the economy via the multiplier, shifting the AD curve to the right. As a business manager, this signals rising demand, potential pricing power, and the possibility of expanding capacity—but also the risk of future inflation if the economy is already near potential output.

SECTION 5

Detailed Shift Analysis — Demand & Supply Shocks

The analytical power of the AD-AS model lies in its ability to predict the direction and relative magnitude of changes in real GDP and the price level when shocks occur. There are four canonical scenarios: a positive demand shock (AD shifts right), a negative demand shock (AD shifts left), a negative supply shock (SRAS shifts left), and a positive supply shock (SRAS shifts right). Each produces a distinct combination of output and price-level effects.

Shock Type	Curve Shift	Real GDP	Price Level	Unemployment	Example
Positive Demand	AD → right	↑ Rises	↑ Rises	↓ Falls	Government stimulus, rate cut, tax cut
Negative Demand	AD → left	↓ Falls	↓ Falls	↑ Rises	Consumer pessimism, austerity, financial crisis
Negative Supply	SRAS → left	↓ Falls	↑ Rises	↑ Rises	Oil shock, pandemic disruption, drought
Positive Supply	SRAS → right	↑ Rises	↓ Falls	↓ Falls	Tech breakthrough, falling input costs

Four canonical AD-AS shocks and their short-run effects

Starting at E₀ (Y*, P₀), a positive demand shock shifts AD₀ to AD₁. The short-run equilibrium moves to E₁, where output rises to Y₁ and the price level increases to P₁—creating an inflationary gap. Over time, workers demand higher wages because prices have risen. This shifts SRAS₀ left to SRAS₁, and the economy self-corrects to E₂, back at Y* but at a permanently higher price level P₂.

The diagram above illustrates one of the most important sequences in macroeconomics: the short-run expansion followed by long-run self-correction. When AD shifts right, the economy initially moves along the existing SRAS to a new short-run equilibrium at E₁ with higher output and higher prices. The inflationary gap at E₁ means the labor market is tight—unemployment is below the natural rate—so workers have bargaining power to demand wage increases. As wages rise, firms' costs increase, and the SRAS curve shifts leftward. This process continues until the economy returns to potential output at E₂, but at a permanently higher price level. The key insight for business strategy is that demand-driven booms are inherently temporary; the price adjustment that follows erodes the initial output gains.

Factors that shift AD right: Increases in consumer confidence, expansionary fiscal policy (higher G, lower taxes), expansionary monetary policy (lower interest rates, quantitative easing), a weaker domestic currency boosting net exports, increased foreign demand.
Factors that shift AD left: Consumer pessimism, contractionary fiscal policy (austerity), higher interest rates, a stronger domestic currency, declining foreign income, financial crises that freeze credit markets.
Factors that shift SRAS left: Rising input prices (oil, raw materials), higher nominal wages, supply chain disruptions, adverse weather, new regulations that increase costs.
Factors that shift SRAS right: Falling input prices, technological improvements, favorable weather, deregulation, immigration that increases labor supply.

SECTION 6

Worked Example — Oil Price Shock & Policy Response

Consider an economy initially at long-run equilibrium with real GDP equal to potential output of $20 trillion and a price level index of 100. An oil cartel restricts supply, causing energy prices to spike. Simultaneously, the central bank decides to respond with an expansionary monetary policy. We will trace the effects step by step.

Negative Supply Shock with Monetary Policy Response

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Step 1 — Identify the Initial Equilibrium

The economy begins at E₀ where AD₀ intersects SRAS₀ on the LRAS at Y* = $20 trillion and P₀ = 100. The output gap is zero, and unemployment is at the natural rate.

Y₀ = $20 trillion, P₀ = 100, output gap = 0
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Step 2 — Apply the Supply Shock

The oil price spike raises production costs across the economy. This shifts the SRAS curve to the left from SRAS₀ to SRAS₁. At every price level, firms are willing to produce less output. The new short-run equilibrium E₁ is found at the intersection of AD₀ and SRAS₁. The result is stagflation: real GDP falls to, say, $19.2 trillion and the price level rises to 106.

Y₁ = $19.2 trillion, P₁ = 106 — a recessionary gap of $0.8 trillion with rising prices (stagflation)
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Step 3 — Evaluate the Policy Dilemma

The central bank faces a dilemma. If it does nothing, the recessionary gap will eventually cause wages to fall (because unemployment is above the natural rate), shifting SRAS back to the right and restoring Y* at P₀—but this self-correction may take years. Alternatively, the central bank can cut interest rates to stimulate aggregate demand, closing the output gap faster but at the cost of a permanently higher price level.
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Step 4 — Apply Expansionary Monetary Policy

The central bank lowers interest rates, which stimulates investment and consumption. The AD curve shifts right from AD₀ to AD₁. The new short-run equilibrium E₂ is the intersection of AD₁ and SRAS₁. Output recovers to $20 trillion (back to Y*), but the price level rises further to, say, 110.

Y₂ = $20 trillion (output gap closed), P₂ = 110 — output restored but at a higher price level
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Step 5 — Assess the Trade-Off

The monetary policy successfully closed the recessionary gap, preventing extended unemployment. However, it accommodated the inflation triggered by the supply shock, resulting in a price level of 110 rather than the original 100. This is the classic policy trade-off following a supply shock: policymakers can stabilize output or stabilize prices, but not both simultaneously. For business strategists, the key implication is that the post-shock price level will be higher regardless, but the speed at which the output recovery occurs depends heavily on policy choices.

Policy trade-off: faster recovery at the cost of 10% higher price level

SECTION 7

Policy Responses — Strengths & Limitations

The AD-AS model not only helps us diagnose economic conditions but also serves as a framework for evaluating policy responses. Both fiscal policy (government spending and taxation changes) and monetary policy (interest rate and money supply adjustments) work primarily through the demand side, shifting the AD curve. Supply-side policies—deregulation, investment in infrastructure, education, and technology—shift the LRAS and SRAS curves over longer horizons. Each approach has distinct advantages and constraints.

Policy Tool	Strengths	Limitations
Expansionary Fiscal Policy (↑G or ↓T)	Direct impact on demand; can target specific sectors or populations; effective at the zero lower bound when monetary policy is exhausted	Implementation lags (legislative process); may crowd out private investment via higher interest rates; increases government debt; political constraints
Expansionary Monetary Policy (↓interest rates)	Faster implementation (central bank independence); operates through multiple channels (investment, housing, exchange rates); can be reversed quickly	Ineffective at the zero lower bound (liquidity trap); transmission depends on bank willingness to lend; may inflate asset prices without boosting real output
Supply-Side Policies (deregulation, infrastructure)	Expand potential output (shift LRAS right); can raise output and lower prices simultaneously; address structural problems	Very long time horizons (years to decades); benefits difficult to quantify in advance; politically contentious distributional effects

Comparison of demand-side and supply-side stabilization tools

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KEY TAKEAWAY

Demand-side policies (fiscal and monetary) are like applying the accelerator or brake in a car—they can speed up or slow down the economy relatively quickly, but they only change the price level and output along the existing supply curve. Supply-side policies are more like upgrading the engine itself—they expand the economy's capacity, but the redesign takes time. The best macroeconomic strategies, much like the best business strategies, combine short-run responsiveness with long-run capacity building.

SECTION 8

Connection to Advanced Macroeconomic Theory

The AD-AS model presented in this lesson is a simplified but powerful framework. In more advanced macroeconomics courses and in professional economic analysis, the model is extended and refined in several important ways. Understanding these connections gives you a sense of where the field is heading and why certain policy debates persist.

Introductory AD-AS	Advanced Extension
Static model — compares one equilibrium to another	Dynamic AD-AS (DAD-DAS) — tracks inflation rates over time, incorporating adaptive or rational expectations
Price level on the vertical axis	Inflation rate on the vertical axis — aligns with how central banks actually conduct policy (inflation targeting)
Self-correction via wage adjustment (mechanism left vague)	Phillips Curve micro-foundations — explicit modeling of inflation expectations, menu costs, and staggered wage contracts
Multiplier is constant	Multiplier varies with the state of the economy — larger in recessions, smaller near full employment; depends on monetary policy reaction
LRAS is fixed at Y*	LRAS can shift over time due to capital accumulation, population growth, institutional change, and technology — linked to economic growth theory

From introductory AD-AS to advanced macroeconomic modeling

For business students, the most immediately relevant advanced extension is the expectations-augmented framework. When firms and consumers form expectations about future inflation, those expectations become self-fulfilling: if businesses expect higher input costs, they raise prices preemptively; if workers expect higher inflation, they demand larger wage increases. Central banks therefore invest heavily in managing inflation expectations through clear communication and credible commitments—a practice known as forward guidance. As you move into courses on business strategy, corporate finance, and risk management, understanding how macroeconomic shifts alter the cost of capital, consumer demand, and competitive landscapes will be an invaluable part of your analytical toolkit.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

Explain why a negative supply shock (SRAS shifts left) poses a more difficult policy challenge for central bankers than a negative demand shock (AD shifts left). In your answer, describe what happens to both real GDP and the price level in each scenario and why the appropriate policy response differs.

Show answer

A negative demand shock reduces both real GDP and the price level, so the policy prescription is relatively straightforward: expansionary monetary or fiscal policy shifts AD back to the right, restoring output and raising the price level back toward its original level—both goals move in the same direction. A negative supply shock, by contrast, reduces real GDP while simultaneously raising the price level (stagflation). If the central bank expands demand to restore output, it worsens inflation; if it tightens policy to control inflation, it deepens the recession. This inherent conflict between stabilizing output and stabilizing prices is what makes supply shocks particularly vexing. The central bank must choose which objective to prioritize, and any choice involves an explicit trade-off.

Question 2: BASIC CALCULATION

Suppose the marginal propensity to consume (MPC) in an economy is 0.75. The government increases spending by $50 billion. Calculate the spending multiplier and determine how far the AD curve shifts horizontally (ignoring price-level effects). Then explain why the actual change in equilibrium real GDP will be smaller than this horizontal shift.

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The spending multiplier is 1 / (1 − MPC) = 1 / (1 − 0.75) = 1 / 0.25 = 4. The horizontal shift of the AD curve is ΔY = Multiplier × ΔG = 4 × $50 billion = $200 billion. However, $200 billion represents the shift in the AD curve at the original price level—it is the maximum possible increase in real GDP. In the AD-AS model, as AD shifts right, the economy moves along the upward-sloping SRAS curve, meaning part of the increased demand is absorbed by a rising price level rather than by increased real output. The steeper the SRAS curve, the more the price level absorbs and the smaller the actual change in real GDP. Only if the SRAS were perfectly horizontal (Keynesian extreme) would the full $200 billion translate into real output gains.

Question 3: INTERMEDIATE

An economy is at long-run equilibrium when two simultaneous events occur: (1) the central bank raises interest rates significantly, and (2) a new technology dramatically reduces manufacturing costs. Using the AD-AS model, determine the direction of change (increase, decrease, or ambiguous) for real GDP and the price level in the short run. Justify your reasoning.

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The interest rate hike is contractionary monetary policy, which reduces investment and consumption, shifting the AD curve to the left. The technological improvement reduces production costs, shifting the SRAS curve to the right. Both shifts push the price level downward, so the direction of change in the price level is unambiguously negative—it will fall. However, the two shocks have opposite effects on real GDP: the AD leftward shift reduces output, while the SRAS rightward shift increases output. The net effect on real GDP is therefore ambiguous and depends on the relative magnitude of the two shifts. If the technology shock is large relative to the monetary tightening, GDP could still rise; if monetary tightening dominates, GDP falls. Without information about the magnitudes, we cannot determine the direction. This type of mixed-shock analysis is a common exam scenario and illustrates why economists must consider both curves simultaneously.

Question 4: APPLIED

In early 2020, the COVID-19 pandemic simultaneously reduced consumer spending (as lockdowns curtailed economic activity) and disrupted global supply chains (reducing firms' ability to produce). Using the AD-AS model, describe the predicted short-run effects on real GDP and the price level. Then explain why the massive fiscal stimulus packages enacted in many countries could have produced inflation once supply constraints became the binding problem.

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COVID-19 acted as a simultaneous negative demand shock (AD shifted left due to lockdowns, uncertainty, and reduced consumption) and negative supply shock (SRAS shifted left due to factory closures, supply chain disruptions, and labor shortages). Both shifts reduce real GDP, so the decline in output was unambiguously large. The effect on the price level was initially ambiguous—the demand decline pushed prices down while the supply disruption pushed them up—but in practice, deflationary pressure initially dominated in early 2020. Governments then enacted massive fiscal stimulus (checks to households, enhanced unemployment benefits, PPP loans), which shifted AD sharply to the right. While aggregate supply remained constrained by ongoing supply chain problems, this large rightward AD shift against a leftward-shifted SRAS created a classic inflationary gap: too much demand chasing too few goods. The result was the significant inflation experienced in 2021–2022 across many economies. This episode vividly demonstrates that demand-side stimulus is most effective when supply is elastic; when supply is constrained, stimulating demand primarily raises prices rather than output.

Question 5: CRITICAL THINKING

Some economists argue that the AD-AS model's assumption of a stable, vertical LRAS is unrealistic because potential output itself can be permanently altered by severe recessions (a concept known as hysteresis). If hysteresis exists—meaning a deep recession permanently reduces potential GDP—how would this change the self-correction mechanism depicted in the model? Discuss the implications for the debate over whether governments should aggressively use fiscal policy during recessions.

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In the standard AD-AS model, the LRAS is fixed at Y*, and after a negative demand shock the economy eventually self-corrects as wages and prices fall, shifting SRAS right and restoring output to Y*. If hysteresis exists, however, a prolonged recession can cause permanent damage—workers lose skills during extended unemployment (human capital depreciation), discouraged workers exit the labor force, firms defer investment, and research and development spending is cut. These effects would shift the LRAS leftward to a new, lower Y*'. The self-correction mechanism would still operate, but the economy would converge to a lower potential output than it had before the recession. This has profound policy implications: if recessions can permanently scar the economy's productive capacity, then the cost of allowing a deep recession to persist (relying on slow self-correction) is much higher than the standard model suggests. The case for aggressive, early fiscal intervention becomes stronger because preventing the recession from deepening may preserve human and physical capital that would otherwise be lost. This strengthens the Keynesian argument for countercyclical fiscal policy, but it also raises the stakes of policy errors—over-stimulating the economy could create inflation, while under-stimulating could lock in permanently lower output.

SUMMARY

Lesson Summary

Summary

The AD-AS model is the foundational framework for analyzing short-run fluctuations in real GDP and the price level. Demand shocks shift the AD curve and move output and the price level in the same direction (both rise or both fall), while supply shocks shift the SRAS curve and move output and the price level in opposite directions—the defining feature of stagflation. In the short run, the economy's equilibrium is determined by the intersection of AD and SRAS, which may diverge from potential output (Y*), creating recessionary or inflationary gaps.

The self-correction mechanism works through wage and input price adjustments: in a recessionary gap, falling wages shift SRAS right; in an inflationary gap, rising wages shift SRAS left—both processes return the economy to the LRAS at Y*. Fiscal and monetary policy can accelerate this adjustment by shifting AD, but supply shocks present a fundamental trade-off between stabilizing output and stabilizing prices. The spending multiplier (1 / (1 − MPC)) determines the horizontal shift of AD for a given change in autonomous spending, though the actual change in equilibrium GDP is moderated by the slope of the SRAS curve. For business professionals, mastering these dynamics is essential for anticipating macroeconomic turning points and formulating strategies that are resilient to both demand-driven recessions and supply-driven disruptions.