Changes in the AD-AS Model in the Short Run

SECTION 1

Historical Context & Motivation

The aggregate demand–aggregate supply (AD-AS) model is the workhorse framework macroeconomists use to explain short-run fluctuations in real GDP and the overall price level. Before this model crystallized in the mid-twentieth century, economists lacked a unified way to explain why economies oscillate between booms and recessions, or why prices sometimes rise even as output falls. The AD-AS framework grew out of intense debates between classical and Keynesian thinkers, each grappling with the devastating reality of the Great Depression and subsequent wartime inflation.

1936

Keynes's General Theory

John Maynard Keynes argued that insufficient aggregate demand—not wage rigidity alone—could produce prolonged recessions. His work laid the conceptual foundation for the demand side of the AD-AS model.

1958

The Phillips Curve

A.W. Phillips documented an inverse relationship between unemployment and wage inflation in the UK, reinforcing the idea that short-run output and price-level changes are linked through aggregate demand shifts.

1970s

Stagflation and Supply Shocks

Oil price shocks produced simultaneous inflation and recession—stagflation—revealing that supply-side disturbances shift the short-run aggregate supply curve leftward, a phenomenon pure demand-side models could not explain.

1980s

New Keynesian Synthesis

Economists such as N. Gregory Mankiw and David Romer formalized sticky wages and prices within a microfounded framework, giving the upward-sloping SRAS curve its modern theoretical justification.

2008–2020

Great Recession & COVID-19

Severe demand contractions (2008) and simultaneous supply-and-demand shocks (2020) demonstrated that the AD-AS model remains essential for diagnosing real-world macroeconomic crises and guiding fiscal and monetary responses.

The central question the AD-AS model addresses in the short run is: What happens to real GDP and the aggregate price level when either aggregate demand or short-run aggregate supply shifts? Answering this question allows economists—and AP Macroeconomics students—to predict whether an economy will experience an inflationary gap, a recessionary gap, or stagflation, and to evaluate the appropriate policy response.

SECTION 2

Core Principles & Definitions

Before analyzing shifts, you need a firm grasp of the three curves in the AD-AS diagram and the equilibrium they determine. Aggregate demand (AD) slopes downward because a lower price level raises the real value of money (wealth effect), reduces interest rates (interest-rate effect), and makes domestic goods cheaper relative to foreign goods (exchange-rate effect). The short-run aggregate supply (SRAS) curve slopes upward because many input costs—especially nominal wages—are sticky in the short run, so rising output prices widen profit margins and encourage greater production. The long-run aggregate supply (LRAS) curve is vertical at the economy's potential output (full-employment GDP), reflecting the classical notion that, once all prices and wages adjust, output depends only on real factors such as technology, capital, and labor.

Short-Run Equilibrium

The economy's short-run equilibrium is the intersection of AD and SRAS. This determines the actual real GDP (Y) and the aggregate price level (PL). Equilibrium may be above, below, or exactly at potential output.

Demand Shock

A demand shock is any event that shifts the AD curve. Positive demand shocks (increased consumer confidence, expansionary policy) shift AD rightward; negative demand shocks (reduced investment, contractionary policy) shift AD leftward.

Supply Shock

A supply shock shifts SRAS. Negative supply shocks (rising input prices, resource scarcity) shift SRAS leftward, raising the price level and reducing output. Positive supply shocks (technological gains, falling resource costs) shift SRAS rightward.

Output Gaps

An inflationary gap exists when actual GDP exceeds potential (Y > Y_f). A recessionary gap exists when actual GDP falls short of potential (Y < Y_f). Both gaps create pressure for eventual self-correction toward long-run equilibrium.

Sticky Wages & Prices

The upward slope of SRAS relies on the assumption that nominal wages and some input prices adjust slowly. Contracts, menu costs, and money illusion prevent instantaneous wage adjustments, allowing short-run deviations from potential output.

✦ KEY TAKEAWAY

Think of the AD-AS model like a thermostat system. Aggregate demand is the thermostat setting (how much economic 'heat' households, firms, governments, and foreigners want), SRAS represents the furnace's capacity at current fuel prices, and LRAS is the home's insulation—the structural limit on how much warmth the house can hold. In the short run, turning the thermostat up (shifting AD right) raises both the temperature (price level) and the furnace output (real GDP). A supply shock is like a sudden spike in fuel prices: the furnace delivers less heat at a higher cost.

SECTION 3

Visual Explanation — The AD-AS Diagram

The diagram below illustrates the baseline AD-AS model with all three curves and the initial short-run equilibrium. The vertical LRAS curve anchors at full-employment output (Y_f), while the intersection of AD and SRAS determines the economy's actual short-run real GDP and price level. Pay close attention to how the three curves relate to one another—shifts in AD and SRAS will reposition this equilibrium.

Baseline AD-AS diagram showing LRAS (vertical green dashed line at full-employment output Y_f), SRAS (upward-sloping amber curve), and AD (downward-sloping cyan curve). The initial equilibrium E₁ occurs near potential output at price level PL₁ and real GDP Y₁.

In this baseline scenario, the short-run equilibrium (E₁) sits very close to the LRAS line, meaning actual output (Y₁) is approximately equal to potential output (Y_f). When either AD or SRAS shifts, the equilibrium moves to a new intersection, creating an output gap. The direction and magnitude of this shift determine whether the economy experiences inflation, recession, or the uncomfortable combination of both.

SECTION 4

How Shifts Work — Determinants & Transmission

Determinants of Aggregate Demand

Recall that GDP measured from the expenditure side is Y = C + I + G + NX. Any non-price-level factor that changes consumption (C), investment (I), government spending (G), or net exports (NX) will shift the AD curve. For instance, an increase in consumer confidence raises autonomous consumption, shifting AD to the right. Similarly, a decrease in the money supply raises interest rates, reduces investment, and shifts AD to the left. It is crucial to distinguish between a movement along AD (caused by a change in the price level) and a shift of AD (caused by a change in a determinant of aggregate spending at every price level).

AGGREGATE EXPENDITURE IDENTITY

Y = C + I + G + (X − M)

C = consumption, I = gross private investment, G = government purchases, X = exports, M = imports. A change in any component at every price level shifts the AD curve.

Determinants of Short-Run Aggregate Supply

The SRAS curve shifts when there is a change in per-unit production costs that is independent of the current price level. The most frequently tested determinants include changes in nominal wages, input resource prices (especially energy), productivity, business taxes and subsidies, and supply-chain disruptions. An increase in nominal wages, for example, raises the cost of producing every unit of output, shifting SRAS leftward. Conversely, a technological improvement that raises labor productivity lowers per-unit costs, shifting SRAS rightward.

PER-UNIT PRODUCTION COST

Per-Unit Cost = Total Input Costs ÷ Total Output

When per-unit cost rises (e.g., higher wages or oil prices), SRAS shifts left. When per-unit cost falls (e.g., technological improvement), SRAS shifts right.

The Spending Multiplier Effect

When the government increases spending by ΔG, the total change in aggregate demand is amplified by the spending multiplier. This multiplier effect means that the horizontal distance of the AD shift exceeds the initial change in spending. However, the actual change in equilibrium real GDP is smaller than the full multiplier suggests because the upward-sloping SRAS dampens the expansion through a rising price level.

SPENDING MULTIPLIER

Multiplier = 1 ÷ (1 − MPC) = 1 ÷ MPS

MPC = marginal propensity to consume, MPS = marginal propensity to save. If MPC = 0.8, the multiplier = 1 ÷ 0.2 = 5. An initial $10 billion increase in G shifts AD rightward by $50 billion at every price level.

TAX MULTIPLIER

Tax Multiplier = −MPC ÷ (1 − MPC)

The tax multiplier is smaller in absolute value than the spending multiplier because a portion of a tax cut is saved rather than spent. A $10 billion tax cut with MPC = 0.8 shifts AD rightward by $40 billion.

SECTION 5

Four Key Short-Run Scenarios

The AP exam consistently tests four fundamental shift scenarios. Each produces a distinct combination of changes in real GDP, the price level, and unemployment. The diagram below overlays all four shifts on a single set of axes to facilitate comparison, and the table that follows summarizes the outcomes systematically.

Composite diagram showing four shift scenarios from the original equilibrium E₁. E₂ results from a rightward AD shift (inflationary gap). E₃ results from a leftward AD shift (recessionary gap). E₄ results from a leftward SRAS shift (stagflation). E₅ results from a rightward SRAS shift (growth with lower prices).

Summary of the four short-run shift scenarios and their macroeconomic effects

Scenario	Curve Shift	Real GDP	Price Level	Unemployment	Gap Created
Positive AD Shock	AD shifts right	Increases ↑	Increases ↑	Decreases ↓	Inflationary
Negative AD Shock	AD shifts left	Decreases ↓	Decreases ↓	Increases ↑	Recessionary
Negative SRAS Shock	SRAS shifts left	Decreases ↓	Increases ↑	Increases ↑	Stagflation
Positive SRAS Shock	SRAS shifts right	Increases ↑	Decreases ↓	Decreases ↓	Beneficial growth

📝 AP Exam Tip

On the free-response section, always label your axes ("Price Level" on the vertical, "Real GDP" on the horizontal), label all curves, show the direction of the shift with an arrow, and clearly mark both the original and new equilibrium points. Incomplete graphs lose points even if the written explanation is correct.

SECTION 6

Worked Example — Fiscal Stimulus and AD Shift

Suppose the economy is in a recessionary gap. The government enacts a $20 billion increase in government spending. The marginal propensity to consume (MPC) is 0.75. Determine the full shift of the AD curve, the type of output gap before and after the policy, and the expected effects on the price level, real GDP, and unemployment.

Fiscal Stimulus in a Recessionary Gap

Step 1 — Identify the Initial SituationThe economy is operating below full-employment output (Y < Y_f), so there is a recessionary gap. Unemployment is above the natural rate, and the price level is relatively low.

Initial condition: Recessionary gap (Y < Y_f)

Step 2 — Calculate the Spending MultiplierThe spending multiplier equals 1 ÷ (1 − MPC). With MPC = 0.75, the multiplier = 1 ÷ (1 − 0.75) = 1 ÷ 0.25 = 4.

Spending multiplier = 4

Step 3 — Determine the Maximum Potential AD ShiftThe AD curve shifts rightward by ΔG × multiplier = $20 billion × 4 = $80 billion. This represents the horizontal distance the AD curve moves at every price level. Note that this is the maximum potential change in GDP if the price level did not change (i.e., along a perfectly horizontal SRAS).

AD shifts right by $80 billion

Step 4 — Analyze the New Short-Run EquilibriumBecause SRAS slopes upward, the new equilibrium involves both a higher price level and higher real GDP. The actual increase in real GDP will be less than $80 billion because the rising price level partially offsets the demand stimulus. Unemployment falls as firms hire more workers to meet increased demand.

Real GDP ↑ (but < $80B), Price Level ↑, Unemployment ↓

Step 5 — Evaluate the Output GapIf the rightward shift of AD is large enough, the economy may move from a recessionary gap to an inflationary gap (Y > Y_f). If the shift is precisely calibrated, the economy returns to full employment. The AP exam frequently asks students to identify whether the policy overshoots, undershoots, or perfectly closes the gap.

Gap outcome depends on the size of the shift relative to the recessionary gap

SECTION 7

Policy Responses vs. Self-Correction

Once a short-run disequilibrium emerges, policymakers face a choice: intervene with fiscal or monetary policy, or allow the economy to self-correct through eventual wage and price adjustments. Understanding the tradeoffs between these approaches is essential for both AP free-response questions and real-world macroeconomic debate.

Comparison of policy responses and self-correction for short-run disequilibria

Approach	Strengths	Limitations
Expansionary Fiscal Policy (↑G or ↓T to shift AD right)	Can target specific sectors; directly raises aggregate demand; works through the spending or tax multiplier.	Time lags (recognition, legislative, implementation); crowding-out effect may reduce private investment; increases government debt.
Expansionary Monetary Policy (↑MS to lower interest rates, shift AD right)	Faster implementation than fiscal policy; Fed can act independently of Congress; affects investment and interest-sensitive consumption.	May be ineffective in a liquidity trap (interest rates near zero); transmission depends on bank lending willingness; can create asset bubbles.
Self-Correction (wages/prices adjust, SRAS shifts)	No fiscal cost; avoids government failure and political distortion; maintains long-run price flexibility.	Potentially very slow, especially for recessionary gaps where nominal wages are sticky downward; prolonged unemployment causes human suffering and hysteresis effects.
Contractionary Policy (↓G, ↑T, or ↓MS to shift AD left)	Effective against demand-pull inflation; can reduce inflationary gaps and restore price stability; reduces budget deficits.	Politically unpopular; risks pushing economy into recession if overdone; time lags may cause policy to take effect after inflation has already subsided.

✦ KEY TAKEAWAY

The policy-vs.-self-correction debate in macroeconomics mirrors the choice an engineer faces when a feedback control system oscillates: should you actively dampen the oscillation with a counter-signal (policy intervention), or trust that the system's built-in restoring forces will bring it back to steady state (self-correction)? Active intervention is faster but risks overcorrection; passive adjustment is reliable but may tolerate prolonged deviation from the target.

SECTION 8

From Short Run to Long Run — Self-Correction Mechanism

The short-run AD-AS analysis is only part of the story. In the long run, the economy gravitates back to potential output as wages and resource prices fully adjust. Understanding this self-correction mechanism deepens your grasp of why short-run policy has only temporary effects on real GDP and why the long-run Phillips curve is vertical at the natural rate of unemployment.

Short-run vs. long-run behavior in the AD-AS model

Feature	Short Run	Long Run
Wages and input prices	Sticky (fixed by contracts, menu costs, money illusion)	Fully flexible; adjust to reflect actual price level
Aggregate supply curve	Upward-sloping SRAS	Vertical LRAS at Y_f
Output	Can differ from potential (output gaps possible)	Returns to potential output (Y_f)
Effect of AD shift	Changes both real GDP and price level	Changes only the price level; real GDP unaffected
Self-correction mechanism	Not yet operative; sticky inputs prevent full adjustment	Complete: SRAS shifts until equilibrium returns to LRAS

Consider an inflationary gap: actual GDP exceeds Y_f. In the short run, workers eventually recognize that the price level has risen and negotiate higher wages. Higher nominal wages raise per-unit production costs, shifting SRAS leftward. This process continues until the economy returns to Y_f at a permanently higher price level. For a recessionary gap, the reverse occurs—but much more slowly, because nominal wages are notoriously sticky downward. This asymmetry is a key reason Keynesian economists argue for active stabilization policy during recessions rather than waiting for self-correction.

🔗 Looking Ahead

The self-correction mechanism connects directly to the Phillips curve framework you will encounter in the Inflation, Unemployment, and Stabilization Policies unit. A leftward SRAS shift during self-correction from an inflationary gap maps to movement along the short-run Phillips curve as the economy trades lower unemployment for higher inflation.

SECTION 9

Practice Problems

If the economy is currently operating at full-employment output and consumer confidence suddenly drops, what is the most likely short-run effect on real GDP and the aggregate price level?

The government increases its spending by $15 billion. The marginal propensity to consume is 0.6. By how much does the aggregate demand curve shift to the right?

An economy experiences a simultaneous increase in government spending and a sharp rise in oil prices. Which of the following outcomes is most likely in the short run?

PROBLEM 4 — APPLIED

Country X is experiencing a recessionary gap. The full-employment output (Y_f) is $800 billion, and the current equilibrium real GDP is $720 billion. The marginal propensity to consume is 0.80. (a) Calculate the spending multiplier. (b) Calculate the minimum increase in government spending needed to shift AD enough to close the recessionary gap. (c) Explain why the actual increase in real GDP from this policy will likely be less than $80 billion, and identify the phenomenon responsible. (d) Draw a correctly labeled AD-AS graph showing the initial recessionary gap, the shift in AD, and the new short-run equilibrium. Indicate the change in the price level.

PROBLEM 5 — CRITICAL THINKING

During the COVID-19 pandemic of 2020, economies experienced simultaneous leftward shifts in both aggregate demand and short-run aggregate supply. (a) Explain one factor that shifted AD to the left during the pandemic. (b) Explain one factor that shifted SRAS to the left during the pandemic. (c) When both AD and SRAS shift leftward simultaneously, what is the definite effect on real GDP and what is the indeterminate effect? Explain your reasoning.

SUMMARY

Summary — Changes in the AD-AS Model in the Short Run

The AD-AS model is the central framework for analyzing short-run macroeconomic fluctuations. A rightward AD shift (caused by increases in C, I, G, or NX) raises both real GDP and the price level, potentially creating an inflationary gap. A leftward AD shift lowers both real GDP and the price level, creating a recessionary gap. A leftward SRAS shift (a negative supply shock) simultaneously raises the price level and lowers real GDP—a condition known as stagflation. A rightward SRAS shift (a positive supply shock) raises real GDP while lowering the price level—the most beneficial outcome.

The magnitude of AD shifts is amplified by the spending multiplier (1 ÷ MPS) and the tax multiplier (−MPC ÷ MPS). Policymakers can respond with fiscal policy (changing G or T) or monetary policy (changing the money supply) to shift AD, or they can allow self-correction to occur as wages and resource prices adjust, shifting SRAS over time until the economy returns to full-employment output along the vertical LRAS curve. On the AP exam, always label your axes, show the direction of shifts with arrows, and identify both original and new equilibrium points.