AP Macroeconomics โ€“ 3.4 The AD-AS Model & Equilibrium
AP Macroeconomics ยท 3.4

The AD-AS Model & Equilibrium

Short-Run Equilibrium ยท Long-Run Equilibrium ยท Output Gaps ยท Self-Correction

Putting the Pieces Together

Sections 3.1 through 3.3 gave you three curves: Aggregate Demand (downward-sloping), Short-Run Aggregate Supply (upward-sloping), and Long-Run Aggregate Supply (vertical at potential GDP). Each curve told you a piece of the story. Now we combine all three into a single graph โ€” the AD-AS model โ€” and watch what the economy does.

This is the most important diagram you'll draw in AP Macroeconomics. Almost every free-response question in the course is built around it. The model lets you do three powerful things: find the economy's current equilibrium (where prices and output settle), diagnose whether the economy is in trouble (output gaps), and predict what happens next if no one intervenes (self-correction). Master this section, and the rest of the course becomes a series of well-defined variations on the same theme.

We'll work through three big ideas in this section. First, what does long-run equilibrium look like โ€” the picture of a healthy economy? Second, how do AD or SRAS shifts knock the economy out of long-run equilibrium and create output gaps (recessionary or inflationary)? Third, if the government and the Fed do nothing, how does the economy self-correct back to potential GDP? That last point is essential โ€” it sets up the entire debate over fiscal and monetary policy in the next two units.

๐Ÿ“ Why this section is heavily tested: The College Board's free-response section often gives you a starting scenario (recessionary gap, inflationary gap, or long-run equilibrium) and asks you to draw the AD-AS model accurately, label the equilibrium, then trace what happens after some shock. Every step requires fluent command of the three curves and how they interact. The work you put into 3.4 pays off again and again.

Long-Run Equilibrium: The Healthy Economy

The economy is in long-run equilibrium when all three curves โ€” AD, SRAS, and LRAS โ€” intersect at the same point. At this point, the economy is producing exactly at potential GDP, the unemployment rate equals the natural rate, prices are stable, and there's no built-in pressure for anything to change. This is the picture of a healthy macroeconomy.

Long-Run Equilibrium: The state where AD intersects SRAS exactly at the LRAS curve. At this single point, Real GDP equals potential GDP (Yf), the unemployment rate equals the natural rate, and there's no pressure for the price level or output to move.

The Foundational Diagram

Real GDP (Y) Price Level (PL) LRAS SRAS AD E PL* Yf
Long-run equilibrium: all three curves meet at point E. Real GDP equals potential GDP (Yf), and the equilibrium price level is PL*. This is the foundational picture the AP exam expects you to draw cleanly.

What Each Curve Contributes

The triple-intersection isn't accidental โ€” each curve contributes a piece of the economic story:

  • AD tells us how much output buyers want at each price level. At PL*, they collectively want exactly Yf.
  • SRAS tells us how much producers are willing to supply at each price level, given currently-sticky costs. At PL*, they're happy to supply Yf โ€” costs are exactly where producers expected them to be, so there's no incentive to expand or cut production.
  • LRAS tells us the economy's productive capacity. Producing at Yf uses all available labor (at the natural rate), capital, and technology fully โ€” but not unsustainably.

Because all three are simultaneously satisfied at point E, nothing in the economy is pushing for change. Wages match what workers expect to need given current prices, profit margins are normal, and resources are fully employed. The economy "wants" to stay here.

Why Long-Run Equilibrium Matters

Long-run equilibrium is the benchmark against which everything else is judged. When the economy is somewhere else โ€” too far left of LRAS (recession) or too far right (overheating) โ€” economists describe that as a deviation from long-run equilibrium. The deviation creates pressure for the economy to move back. Understanding that "back to E" dynamic is the heart of the next few subsections.

โš ๏ธ Draw long-run equilibrium correctly: On the AP exam, when a question begins "the economy starts in long-run equilibrium," all three curves must meet at one point on top of LRAS. A common error is drawing AD and SRAS intersecting off LRAS while still calling it long-run equilibrium. That intersection wouldn't be a long-run equilibrium โ€” it would be an output gap, which is what we'll cover next.

Output Gaps: When AD or SRAS Shifts

The economy rarely sits at long-run equilibrium for long. AD shifts because consumers get nervous, foreign demand changes, or the Fed adjusts monetary policy. SRAS shifts because input costs rise, technology improves, or a supply shock hits. Either kind of shift moves the short-run equilibrium away from LRAS, creating what we call an output gap.

Output Gap: The difference between actual Real GDP and potential GDP (Yf). When actual GDP is below potential, there is a recessionary gap. When actual GDP is above potential, there is an inflationary gap.

The two kinds of output gaps look very different on the graph and call for very different policy responses. The AP exam tests both, and you need to recognize each one at a glance.

GAP TYPE 1
Recessionary Gap
What it is: Actual Real GDP is below potential GDP. The short-run equilibrium sits to the left of LRAS.

What you observe: Unemployment is above the natural rate (extra cyclical unemployment). The price level is lower than it would be at potential. Resources are underutilized โ€” factories run below capacity, workers are looking for jobs they can't find.

Typical cause: A leftward shift of AD (e.g., a fall in consumer confidence, contractionary monetary or fiscal policy, a global recession reducing exports).

Policy response: Expansionary fiscal policy (โ†‘ G or โ†“ T) or expansionary monetary policy (โ†“ interest rates). The goal is to shift AD right, back to potential.
GAP TYPE 2
Inflationary Gap
What it is: Actual Real GDP is above potential GDP. The short-run equilibrium sits to the right of LRAS.

What you observe: Unemployment is below the natural rate (workers are stretched, labor markets are tight). The price level is elevated. The economy is "overheating" โ€” producing beyond its sustainable capacity.

Typical cause: A rightward shift of AD (e.g., a consumer spending boom, expansionary monetary or fiscal policy, a surge in exports).

Policy response: Contractionary fiscal policy (โ†“ G or โ†‘ T) or contractionary monetary policy (โ†‘ interest rates). The goal is to shift AD left, back to potential.

The Two Gaps Visualized

Recessionary Gap AD shifted left; Y < Yf Real GDP PL LRAS Yf SRAS AD0 AD1 E1 Rec. Gap Inflationary Gap AD shifted right; Y > Yf Real GDP PL LRAS Yf SRAS AD0 AD1 E1 Inf. Gap
Two output gap scenarios. Left: AD shifts from AD0 to AD1, leaving short-run equilibrium E1 to the left of LRAS โ€” a recessionary gap. Right: AD shifts from AD0 to AD1, leaving E1 to the right of LRAS โ€” an inflationary gap.

A Side-by-Side Diagnostic Table

Variable Recessionary Gap Inflationary Gap
Real GDP relative to Yf Y < Yf (below potential) Y > Yf (above potential)
Unemployment vs. natural rate Higher than natural rate (positive cyclical unemployment) Lower than natural rate (labor markets tight)
Price level vs. long-run Below long-run equilibrium PL* Above long-run equilibrium PL*
Position on graph Short-run equilibrium left of LRAS Short-run equilibrium right of LRAS
What's "wrong" Resources are idle; the economy could produce more The economy is producing unsustainably; inflation pressure builds

๐ŸŽฏ The diagnostic shortcut: When you see an AD-AS graph, look at where the AD/SRAS intersection sits relative to LRAS. To the left = recessionary gap (too little output). To the right = inflationary gap (too much output, overheating). Right on LRAS = long-run equilibrium. This three-way diagnosis takes two seconds once you've internalized it.

Supply-Shock Output Gaps

Output gaps can also arise from SRAS shifts, not just AD shifts. A leftward SRAS shift (negative supply shock) creates a recessionary gap with rising prices โ€” the stagflation scenario we covered in Section 3.2. A rightward SRAS shift creates an inflationary gap with falling prices. The diagnostic logic is the same: look at where the new short-run equilibrium sits relative to LRAS.

โš ๏ธ Two ways to create a recessionary gap: Most recessionary gaps come from AD shifting left (prices and output both fall). But a leftward SRAS shift creates a recessionary gap with prices rising โ€” that's stagflation. The position relative to LRAS is the same; the cause and price-level direction differ. Always check both AD and SRAS movements when analyzing a scenario.

Self-Correction: How the Economy Returns to Yf

Here's the most important policy idea in all of macroeconomics: output gaps don't last forever โ€” even without any government action. The economy has a built-in mechanism that pulls it back to long-run equilibrium over time. The mechanism is slow, sometimes painful, but inevitable. Understanding it is essential because it tells you what the economy would do without intervention โ€” and that's the baseline against which all fiscal and monetary policy gets evaluated.

The mechanism works through the same sticky-wage logic we built up in Section 3.2. The short run is the period when wages can't adjust. Self-correction is what happens when enough time passes for wages to finally adjust. Once they do, SRAS shifts, and the economy lands back at potential GDP.

Self-Correction From a Recessionary Gap

Suppose the economy starts in long-run equilibrium, then AD shifts left (maybe consumer confidence collapsed). We're now sitting at a short-run equilibrium with output below Yf โ€” a recessionary gap. What happens next, if no policymaker intervenes?

1

Starting point: recessionary gap

AD has shifted left. Short-run equilibrium sits to the left of LRAS. Output is below Yf, unemployment is above the natural rate, and the price level has fallen below its long-run level.

2

Wages and other input costs begin to fall

With unemployment elevated, workers face weaker bargaining power. Over time, wages either stagnate or fall outright; suppliers cut prices to compete for shrinking demand. Input costs across the economy start to drop.

3

Lower costs shift SRAS to the right

Cheaper inputs mean firms can profitably produce more at every price level. The SRAS curve shifts gradually rightward, expanding the economy's short-run output capacity.

4

New long-run equilibrium at lower prices

SRAS continues shifting right until the short-run equilibrium reaches the LRAS line again. Output returns to Yf, unemployment returns to the natural rate, but the price level is now lower than it was originally. The economy is healed โ€” just at a lower PL.

Real GDP (Y) Price Level (PL) LRAS Yf SRAS0 SRAS1 AD0 AD1 E0 E1 E2 SRAS shifts right
Self-correction from a recessionary gap. The economy starts at E0 (long-run equilibrium). AD shifts left to AD1, creating a recessionary gap at E1. Over time, wages fall and SRAS shifts right from SRAS0 to SRAS1, returning the economy to potential GDP at E2 โ€” but at a permanently lower price level.

Self-Correction From an Inflationary Gap

The mirror-image scenario works in reverse. Start in long-run equilibrium, then AD shifts right (maybe a consumer boom or expansionary policy). We end up at a short-run equilibrium to the right of LRAS โ€” an inflationary gap. What happens next?

1

Starting point: inflationary gap

AD has shifted right. Short-run equilibrium sits to the right of LRAS. Output is above Yf, unemployment is below the natural rate, and the price level is elevated.

2

Wages and input costs rise

With unemployment unusually low, workers have strong bargaining power. They notice prices have risen and negotiate higher wages. Suppliers raise their prices too. Input costs across the economy begin to climb.

3

Higher costs shift SRAS to the left

More expensive inputs mean firms can profitably produce less at every price level. The SRAS curve shifts gradually leftward, contracting the economy's short-run output capacity.

4

New long-run equilibrium at higher prices

SRAS continues shifting left until the short-run equilibrium meets LRAS again. Output returns to Yf, unemployment returns to the natural rate, but the price level is now permanently higher than before the boom.

The Pattern in One Sentence

Self-correction always works the same way: SRAS shifts in the direction that closes the output gap. From a recessionary gap, SRAS shifts right (because falling wages reduce costs). From an inflationary gap, SRAS shifts left (because rising wages increase costs). Either way, output returns to Yf, but the price level ends up different from where it started.

The Self-Correction Rule: Output gaps eventually close on their own as wages and other input costs adjust. SRAS shifts in whichever direction returns output to Yf โ€” right after a recessionary gap, left after an inflationary gap. AD does not shift back during self-correction; it's the supply side that does the work.

Why Policymakers Don't Always Wait

If the economy heals itself, why does anyone bother with fiscal or monetary policy? Three reasons make self-correction a less appealing option than it sounds:

  • It's slow. Sticky wages are sticky for a reason โ€” long-term labor contracts, social norms against wage cuts, and worker resistance can delay adjustment for years. A recessionary gap might persist for several years before self-correction finishes.
  • It's painful. During a recessionary gap, real people are unemployed and suffering. Waiting for wages to fall enough to restore equilibrium can mean years of joblessness for millions. The political pressure to act is enormous.
  • It can be incomplete. In severe downturns, wages often fall stickier than the model suggests, and the economy can sit in a prolonged recessionary gap that policy could shorten dramatically.

These three reasons set up the entire debate over fiscal policy (Section 3.5) and monetary policy (Unit 4). Active policy can shift AD directly, bypassing the slow SRAS adjustment. Whether to wait or to intervene is one of the central questions in macroeconomics โ€” and you'll be asked it from many angles across the rest of the course.

๐Ÿ“ FRQ-killer point: When the AP free-response asks you to "show the long-run effect" of an AD shock, the right answer is not just the short-run gap. You need to show SRAS shifting in the appropriate direction so output ends back at Yf. Drawing only the short-run effect loses long-run points every time.

Three Golden Rules of AD-AS

To wrap up the model, three rules summarize how AD-AS thinking works in every AP scenario. Internalize these, and most exam questions reduce to mechanical application.

Rule 1: In the long run, output always returns to Yf

No matter what shock hits the economy โ€” AD shift, SRAS shift, supply shock, policy change โ€” the long-run resting place is always at potential GDP. The economy can be temporarily above or below Yf, but it can't stay there. Self-correction (or active policy) always brings it back. The only way to permanently change output is to shift LRAS itself.

Rule 2: AD shifts change the price level in the long run, not output

A rightward AD shift raises output in the short run, but the long-run effect is purely on prices โ€” output returns to Yf, but at a higher price level. A leftward AD shift lowers output in the short run, but long-run output returns to Yf at a lower price level. This is sometimes called monetary neutrality when the AD shift comes from the money supply: nominal changes don't affect real output in the long run.

Rule 3: Only LRAS shifts permanently change output

To grow the economy's long-run output, you must shift LRAS to the right โ€” by adding workers, accumulating capital, advancing technology, or finding more natural resources. No demand-side action can do this. Tax cuts, stimulus packages, money supply changes โ€” these all shift AD, not LRAS. Long-run economic growth (Unit 5) is fundamentally a supply-side story.

AD shifts โ†’ Short-run gap โ†’ Self-corrects to Yf with different PL

LRAS shifts โ†’ New permanent Yf

โš ๏ธ Don't confuse short-run and long-run effects: AP free-response questions often ask for both. Always answer in two parts: what happens immediately (short run, before SRAS adjusts) and what happens eventually (long run, after self-correction). Skipping either part loses points.

Common Misconceptions

The AD-AS model is the most heavily tested topic in Unit 3, and several aspects consistently confuse students. Clear these up before test day.

  • "AD intersects SRAS at long-run equilibrium." Half right. Long-run equilibrium requires AD and SRAS to intersect on the LRAS curve. An AD-SRAS intersection off LRAS is just a short-run equilibrium, not a long-run one. The three-way intersection is what makes it "long-run."
  • "A recessionary gap means AD is below LRAS." The shift might be a leftward AD shift, but the gap itself is measured by the position of the short-run equilibrium (where AD meets SRAS), not by AD alone. Always look at the equilibrium point relative to LRAS.
  • "Self-correction means AD shifts back." No. Self-correction is a supply-side process. SRAS shifts (right after a recessionary gap, left after an inflationary gap) as wages adjust. AD stays where the shock left it. This is the most common AP error on FRQs that ask about long-run effects.
  • "Self-correction restores the original price level." No. Self-correction restores output to Yf, but the price level ends up different from where it started. After a recessionary gap, PL ends lower; after an inflationary gap, PL ends higher. Output is real, the price level is nominal โ€” they don't move together in the long run.
  • "An inflationary gap means inflation is rising forever." No. An inflationary gap means the price level is currently elevated above its long-run equilibrium, but the gap is temporary. As SRAS shifts left (wages catching up), the gap closes โ€” but the higher price level persists at the new long-run equilibrium. The "gap" is in output, not in the rate of inflation continuing forever.
  • "In long-run equilibrium, the economy is producing the maximum possible output." Not quite. Yf is the sustainable maximum, not the absolute maximum. The economy can temporarily produce above Yf during an inflationary gap (workers stretched, factories overworked), but it can't sustain that level. Long-run equilibrium is the level the economy can produce indefinitely without overheating.
  • "AD and SRAS shifts have permanent effects on output." Only in the short run. In the long run, AD shifts only affect prices, and SRAS shifts trigger self-correction that returns output to Yf. The only permanent output changes come from LRAS shifts.
  • "Drawing the gap on the AD curve." A recessionary or inflationary gap is the horizontal distance between the current short-run equilibrium Y and Yf, drawn as a bracket on the x-axis. Some students confusingly try to mark it on AD itself โ€” but the gap is in output (the x-axis), not in demand.

โšก 3.4 Quiz: 5 Questions

Click an answer to lock it in. Every option gets a full breakdown โ€” what's right, what's wrong, and the AP-favorite trap each distractor is designed to catch.

1. An economy is in long-run equilibrium when:

  • (A) Aggregate Demand intersects Short-Run Aggregate Supply at any point.
  • (B) The unemployment rate equals zero.
  • (C) The price level equals 100.
  • (D) AD, SRAS, and LRAS all intersect at the same point.
  • (E) The economy is producing the maximum possible output regardless of sustainability.

โœ“ Correct answer: (D)

Long-run equilibrium is the unique state where all three curves meet at a single point. At this triple intersection, Real GDP equals potential GDP (Yf), the unemployment rate equals the natural rate, and there's no pressure for prices or output to change. The three-way intersection is what makes it "long-run" rather than just "short-run."

โš ๏ธ The "any intersection counts" trap: Option (A) is the most tempting wrong answer. AD and SRAS always intersect somewhere โ€” that's the short-run equilibrium. But that intersection is only a long-run equilibrium if it also sits on LRAS. Off LRAS, you have an output gap, not long-run equilibrium.
Why the other options miss the mark
  • (A) Any AD-SRAS intersection is a short-run equilibrium. It's only a long-run equilibrium if it also lies on LRAS. This is the most common misconception about the AD-AS model.
  • (B) Zero unemployment is impossible. At long-run equilibrium, unemployment equals the natural rate (frictional + structural), typically 4โ€“5%. This is the same trap from Section 2.2 carrying forward.
  • (C) The price level can be any value at long-run equilibrium โ€” it's whatever LRAS, SRAS, and AD happen to meet at. PL = 100 only matters as a base-year reference for indices, not as a definition of equilibrium.
  • (E) Yf is the sustainable maximum, not the absolute maximum. The economy can produce above Yf temporarily in an inflationary gap โ€” but not sustainably. Long-run equilibrium is what the economy can produce indefinitely.

๐Ÿ”— Review: Re-read the "Long-Run Equilibrium: The Healthy Economy" section, especially the foundational diagram. The triple intersection is the defining feature โ€” AP graders look for it specifically on FRQs.

2. An economy starts in long-run equilibrium. Consumer confidence then falls sharply, causing AD to shift left. In the short run, which of the following best describes the new situation?

  • (A) An inflationary gap with rising prices and unemployment below the natural rate.
  • (B) A recessionary gap with falling prices and unemployment above the natural rate.
  • (C) The economy stays at potential GDP because LRAS is vertical.
  • (D) Real GDP rises while the price level falls.
  • (E) A recessionary gap with rising prices and rising unemployment.

โœ“ Correct answer: (B)

A leftward AD shift moves the short-run equilibrium down and to the left along the SRAS curve. The new short-run equilibrium sits to the left of LRAS โ€” a recessionary gap. Output falls below Yf, unemployment rises above the natural rate (cyclical unemployment appears), and the price level falls. This is the textbook AD-driven recession scenario.

โš ๏ธ The "LRAS is vertical so nothing changes" trap: Option (C) confuses the long-run and short-run pictures. LRAS being vertical means output is unaffected by price-level changes in the long run, but in the short run, AD shifts absolutely change output via SRAS. Always answer short-run questions using AD and SRAS, not LRAS alone.
Why the other options miss the mark
  • (A) Direction reversed. AD shifted left (consumer confidence fell), which creates a recessionary gap, not inflationary. Inflationary gaps come from rightward AD shifts.
  • (C) Misapplies the vertical LRAS rule. The short-run effect of an AD shift definitely changes output. LRAS being vertical only matters for the long-run effect, after SRAS has time to adjust.
  • (D) When AD shifts left along an upward-sloping SRAS, both output AND the price level fall โ€” not output rising and prices falling. This option mixes up the directions.
  • (E) A leftward AD shift makes prices fall, not rise. Rising prices with a recessionary gap would be a stagflation scenario (leftward SRAS shift), which isn't what the question describes.

๐Ÿ”— Review: Walk through the "Recessionary Gap" card and the dual-graph SVG showing both gap types. Trace what happens to Y and PL when AD shifts left along SRAS โ€” both fall.

3. An economy is experiencing a recessionary gap. If no policymakers intervene, what is the most likely long-run self-correction mechanism?

  • (A) AD will shift right as consumer confidence automatically rebounds.
  • (B) LRAS will shift left to match the lower level of actual output.
  • (C) SRAS will shift right as wages and input costs gradually fall.
  • (D) AD and SRAS will both shift right simultaneously.
  • (E) The price level will rise back to its original level.

โœ“ Correct answer: (C)

Self-correction is a supply-side process. During a recessionary gap, elevated unemployment puts downward pressure on wages. Workers face weaker bargaining power; suppliers cut prices to compete for shrinking demand. As input costs across the economy gradually fall, the SRAS curve shifts right. The shifting continues until the short-run equilibrium reaches LRAS again โ€” output returns to Yf, but at a lower price level than the original long-run equilibrium.

โš ๏ธ The "AD shifts back" trap: Option (A) is the single most common mistake on long-run FRQs. Students assume "what shifted out should shift back" โ€” but the AD shock is treated as permanent in the AD-AS model. Self-correction works through SRAS, not by reversing AD. This distinction is tested constantly.
Why the other options miss the mark
  • (A) AD does not automatically shift back during self-correction. AP exam treats AD shocks as permanent unless otherwise specified; the correction comes from SRAS.
  • (B) LRAS represents the economy's productive capacity โ€” it doesn't shift in response to demand shocks. LRAS only moves when something real (labor, capital, technology, resources) changes. A recessionary gap reflects underused capacity, not lost capacity.
  • (D) Only SRAS shifts during self-correction. AD stays in its post-shock position.
  • (E) The price level ends up lower than the original long-run equilibrium, not back at the original. Self-correction restores output to Yf, but the new long-run PL is permanently lower after a recessionary gap (and permanently higher after an inflationary gap).

๐Ÿ”— Review: Re-read the four-step "Self-Correction From a Recessionary Gap" walkthrough. The key insight is that wages adjust (sticky in short run, flexible in long run), and that adjustment shifts SRAS, not AD.

4. A major oil-exporting country imposes an embargo, causing global oil prices to triple. What is the most likely short-run effect on a typical oil-importing economy that started in long-run equilibrium?

  • (A) SRAS shifts left, causing a recessionary gap with higher prices (stagflation).
  • (B) AD shifts left, causing a recessionary gap with lower prices.
  • (C) SRAS shifts right, causing an inflationary gap with lower prices.
  • (D) LRAS shifts left, immediately moving the economy to a lower potential GDP.
  • (E) AD shifts right because consumers buy more oil at the higher price.

โœ“ Correct answer: (A)

An oil price spike is a classic negative supply shock โ€” higher input costs for producers across the economy. SRAS shifts left, moving the short-run equilibrium up and to the left along the AD curve. The result: output falls below Yf (recessionary gap) AND the price level rises. This unusual combination โ€” falling output with rising prices โ€” is stagflation. The 1970s oil shocks are the historical example the AP exam most often references.

โš ๏ธ The "recession = lower prices" trap: Students often think any recessionary gap must come with falling prices. But the price-level direction depends on what shifted. A leftward AD shift gives recession with falling prices (typical recession). A leftward SRAS shift gives recession with rising prices (stagflation). Always check which curve moved.
Why the other options miss the mark
  • (B) Oil prices affect costs of production, not aggregate demand directly. Higher oil prices don't shift AD left โ€” they raise costs for firms, shifting SRAS left. Wrong curve identified.
  • (C) Direction reversed. Higher input costs make production more expensive, shifting SRAS left, not right. Rising oil prices never shift SRAS right.
  • (D) LRAS represents long-run productive capacity. A short-term oil price spike doesn't destroy workers, factories, or technology โ€” so LRAS is unaffected (at least directly). LRAS shifts come from changes to real factors of production.
  • (E) Higher oil prices typically reduce overall consumption (households have less left to spend on other things). And even if oil demand stayed strong, that's a demand for one good โ€” not a shift in aggregate demand for all goods.

๐Ÿ”— Review: Cross-reference with Section 3.2's "Supply Shocks & Stagflation" subsection. The stagflation pattern (left SRAS shift โ†’ recession + inflation simultaneously) is one of the AP's favorite scenarios โ€” recognize it instantly.

5. An economy is currently in long-run equilibrium. The government passes a large stimulus package that significantly increases government purchases (G). In the long run, after all adjustments are complete, what is the most likely outcome?

  • (A) Real GDP is permanently higher; the price level is unchanged.
  • (B) Real GDP is permanently higher; the price level is also higher.
  • (C) Real GDP is permanently lower; the price level is higher.
  • (D) Real GDP returns to Yf; the price level returns to its original level.
  • (E) Real GDP returns to Yf; the price level is permanently higher.

โœ“ Correct answer: (E)

This is the textbook long-run AD shift story, and it's the AP exam's most-tested cross-section concept. The stimulus shifts AD right, creating an inflationary gap in the short run. Over time, wages and input costs rise (workers see higher prices and renegotiate), shifting SRAS left until the short-run equilibrium reaches LRAS again. The end result: Real GDP returns to Yf (since LRAS is vertical and didn't move), but the price level is permanently higher than before the stimulus. This is the central insight behind monetary neutrality and Rule 2 of the AD-AS golden rules.

โš ๏ธ The "stimulus boosts long-run GDP" trap: Options (A) and (B) capture the popular intuition that government spending grows the economy permanently. The AD-AS model says no โ€” AD shifts only have permanent effects on the price level, not output. To permanently raise Yf, you'd need to shift LRAS (workers, capital, technology, resources), and fiscal stimulus doesn't do that directly.
Why the other options miss the mark
  • (A) Demand-side stimulus doesn't permanently raise output (because LRAS is vertical) and doesn't leave the price level unchanged (because AD did shift right). Both halves are wrong.
  • (B) Half right โ€” the price level does end up higher. But Real GDP doesn't permanently rise; it returns to Yf. AD shifts can't beat LRAS in the long run.
  • (C) Real GDP doesn't fall โ€” it returns to Yf, the same level as before. And the price level rises, not falls. Wrong on both directions.
  • (D) Real GDP returns to Yf correctly, but the price level doesn't return to its original level. The AD shift permanently raises the long-run price level. This option captures half the picture but misses the lasting price-level effect.

๐Ÿ”— Review: Read "Three Golden Rules of AD-AS." Rule 2 โ€” "AD shifts change the price level in the long run, not output" โ€” is exactly what this question tests. This concept reappears constantly in Unit 4 (monetary policy long-run effects) and Unit 5 (Phillips Curve).

Ready for more? Take the full Unit 3 Practice Test โ†’

End of Section 3.4. Up next: 3.5 Fiscal Policy & the Multiplier Effect โ€” how the government deliberately shifts AD using spending and taxes, and why the spending multiplier amplifies every dollar.

โ† 3.3 Long-Run Aggregate Supply 3.5 Fiscal Policy & Multiplier โ†’
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