Long-Run Aggregate Supply

AP Macroeconomics· AP Macroeconomics CED — National Income and Price Determination· 14 min read

1. Definition and Shape of LRAS★★☆☆☆⏱ 4 min

Long-Run Aggregate Supply (LRAS) represents the relationship between the aggregate price level and the total quantity of real GDP supplied by all firms in an economy after all nominal input prices (including wages, raw material contracts, and rental rates) are fully flexible and able to adjust to market shocks. Unlike the short run, where nominal prices are often sticky, the long run allows full adjustment to shocks, so changes in the aggregate price level do not change the overall quantity of output the economy can produce at full employment.

📘 Definition

Potential Output (Full-Employment Output)

$Y_{p}$

The output level produced when unemployment equals the natural rate of unemployment ( $u_{n}$ ); LRAS is always anchored at this level.

LRAS is a core concept in AP Macroeconomics Unit 3, which accounts for 10–15% of total AP exam weight. It appears regularly on both multiple-choice and free-response sections, and forms the foundation for all long-run analysis of growth, recessions, and inflation.

The standard LRAS curve expected on the AP exam is perfectly vertical at potential output. This shape follows from the core long-run assumption that all nominal prices and wages are fully flexible. If aggregate demand increases, pushing up the aggregate price level, nominal wages will eventually rise proportionally to match the higher price level, restoring real wages to their original level. Firms have no incentive to increase output beyond potential, because their production costs rise proportionally with output prices. If aggregate demand decreases, pushing the price level down, nominal wages eventually fall proportionally, so firms have no incentive to cut output below potential. Early Keynesian theory proposed a horizontal LRAS in deep depressions, but this rare extreme case is not tested on the AP exam.

📐 Worked Example

An economy is initially at long-run equilibrium. The government increases deficit spending on government services, shifting aggregate demand to the right. What happens to real GDP and the aggregate price level in the long run? What does this imply about the shape of LRAS?

1
In the short run, higher aggregate demand increases both the price level and real GDP above potential output, creating an inflationary gap.
2
In the long run, nominal input prices (especially wages) adjust to the higher price level: workers negotiate higher nominal wages to restore their original real wage, increasing production costs for all firms.
3
As production costs rise, short-run aggregate supply shifts left, bringing real GDP back to the original potential output level, while the price level rises permanently.
4
This demonstrates that changes in the aggregate price level do not change long-run real output, so LRAS is vertical at potential output.

Exam tip:

On AP FRQs, always draw LRAS as a perfectly vertical line; explicitly state it is vertical because full-employment output is independent of the aggregate price level in the long run to earn full points.

2. Shifts of the LRAS Curve★★★☆☆⏱ 4 min

LRAS shifts only when potential output $Y_{p}$ changes, which occurs when the economy's long-run productive capacity changes. Anything that changes the quantity or quality of factors of production (labor, physical capital, human capital, technology, natural resources) or institutions that support production (property rights, regulatory efficiency) will shift LRAS. Increases in productive capacity shift LRAS right; decreases shift LRAS left.

A key AP exam rule: any permanent change that shifts short-run aggregate supply (SRAS) will also shift LRAS. Only temporary supply shocks (e.g., a one-year drought that raises grain prices, a temporary oil price cut) shift SRAS without shifting LRAS. For example, a permanent improvement in agricultural technology that permanently lowers food production costs shifts both SRAS and LRAS right, while a temporary bumper corn crop only shifts SRAS right.

📐 Worked Example

A country reforms its tax code to cut taxes on business investment, leading to a 15% permanent increase in the country's capital stock. Ceteris paribus, how does this change affect LRAS? Explain.

1
Potential output depends on the quantity and quality of factors of production available to the economy at full employment.
2
A permanent increase in the capital stock (one of the core factors of production) raises the total output the economy can produce at full employment, increasing potential output $Y_{p}$ .
3
Because the change is permanent, the entire LRAS curve shifts rightward from its original position at $Y_{p 1}$ to a new position at $Y_{p 2} > Y_{p 1}$ .
4
$Y_{p2} > Y_{p1}$
5
Short-run aggregate supply also shifts right along with LRAS, because the permanent increase in productive capacity lowers long-run production costs for firms.

Exam tip:

When asked what shifts LRAS, remember: only permanent changes in productive capacity shift LRAS. Temporary price changes or demand shocks never shift LRAS, even if they shift SRAS.

3. Long-Run Macroeconomic Equilibrium★★★☆☆⏱ 4 min

Long-run macroeconomic equilibrium occurs when aggregate demand (AD), short-run aggregate supply (SRAS), and LRAS all intersect at the same point. At this equilibrium, real GDP equals potential output ( $Y = Y_{p}$ ) and unemployment equals the natural rate ( $u = u_{n}$ ).

If the economy is in short-run equilibrium but not long-run equilibrium, an output gap exists: $Output Gap = Y - Y_{p}$ . A positive output gap ( $Y > Y_{p}$ ) is an inflationary gap, with unemployment below the natural rate. A negative output gap ( $Y < Y_{p}$ ) is a recessionary gap, with unemployment above the natural rate. In the classical self-correcting mechanism, the economy automatically adjusts back to long-run equilibrium at LRAS without government intervention: input prices adjust, shifting SRAS until output returns to $Y_{p}$ . LRAS itself does not shift during this adjustment process, because productive capacity has not changed.

📐 Worked Example

An economy is initially at long-run equilibrium. A drop in business confidence reduces investment spending, shifting AD left. Identify the short-run output gap, explain how the economy self-corrects to long-run equilibrium, and state whether LRAS shifts.

1
After AD shifts left, short-run equilibrium occurs at $Y_{1} < Y_{p}$ , so this is a recessionary output gap. Unemployment is above the natural rate.
2
High unemployment creates downward pressure on nominal wages and other input prices, because workers are willing to accept lower nominal wages to secure jobs.
3
Lower input prices reduce production costs for firms, so the short-run aggregate supply curve shifts right. This continues until SRAS intersects the new AD at the original LRAS level of $Y_{p}$ , resulting in a lower long-run equilibrium price level.
4
LRAS does not shift, because the productive capacity of the economy did not change; only aggregate demand fell.

Exam tip:

Never shift LRAS in the self-correction process after a demand shock; only SRAS shifts to bring output back to the existing LRAS level of potential output.

4. AP-Style Concept Check★★★★☆⏱ 2 min

✓ Quick check

Test your understanding of core LRAS concepts:

Which of the following will most likely cause the long-run aggregate supply curve to shift rightward?
- A temporary decrease in the price of imported crude oil
- A broad-based improvement in technology that permanently increases productivity
- An increase in the aggregate price level caused by higher consumer spending
- An increase in the minimum wage that raises nominal wages for all low-skilled workers
Reveal answer
1 —
Correct. Only permanent increases in productive capacity shift LRAS right. Temporary shocks only shift SRAS, price level changes move along existing LRAS, and one-time wage changes do not shift LRAS.

📐 Worked Example

Japan’s working-age population has declined by an average of 1.2% per year for 20 years, due to an aging population and low birth rates. Initial potential output in 2003 was ¥500 trillion. Holding all other factors constant, calculate the approximate potential output in 2023 and explain how this change affects LRAS.

1
A permanent decline in the size of the working-age labor force, holding all other factors constant, reduces Japan's total long-run productive capacity.
2
This reduces potential output, so the LRAS curve shifts leftward relative to what it would be with a stable working-age population.
3
Calculate potential output after 20 years using compound growth:
4
$Y_{2023} = Y_{2003} \times (1 - 0.012)^{20} \approx 500 \times 0.786 = 393 \text{ trillion yen}$
5
This persistent demographic change slows long-run economic growth by reducing the size of the full-employment labor force, leading to a permanently lower level of potential output.

5. Common Pitfalls

Wrong move:

Shifting LRAS in response to a temporary increase in oil prices.

Why:

Students confuse temporary and permanent supply shocks, often shifting LRAS any time SRAS shifts.

Correct move:

Only shift LRAS if the shock permanently changes the economy's productive capacity; temporary input price changes only shift SRAS, not LRAS.

Wrong move:

Drawing LRAS as upward-sloping like short-run aggregate supply.

Why:

Students mix up short-run and long-run assumptions of price/wage stickiness vs flexibility.

Correct move:

Always draw LRAS as a perfectly vertical line at potential output for standard AP questions, unless explicitly asked to describe an extreme Keynesian case.

Wrong move:

Claiming that an increase in aggregate demand increases long-run real GDP.

Why:

Students confuse short-run and long-run effects of demand policy.

Correct move:

Always state that demand shifts only change the price level, not long-run real GDP, because output returns to potential output after full price adjustment.

Wrong move:

Labeling potential output to the right of full-employment output on an AD-AS graph.

Why:

Students confuse output gap definitions, incorrectly thinking potential output is higher than full employment output.

Correct move:

Always label LRAS at potential output, which is the full-employment output level by definition.

Wrong move:

Shifting LRAS when the government increases deficit-financed transfer spending (a demand-side policy).

Why:

Students confuse demand-side fiscal policy with supply-side policies that change productive capacity.

Correct move:

Only shift LRAS for government policy if the policy permanently changes productive capacity (e.g., infrastructure spending); consumption-focused transfer spending does not shift LRAS.

Wrong move:

Claiming LRAS shifts left when the natural rate of unemployment falls.

Why:

Students mix up the relationship between natural unemployment and potential output.

Correct move:

A lower natural rate of unemployment means more workers are employed at full employment, so potential output increases, shifting LRAS right.

6. Quick Reference Cheatsheet

Category	Rule / Formula	Notes
Output Gap	$Output Gap = Y - Y_{p}$	Negative = recessionary gap, positive = inflationary gap
Standard LRAS Shape	Vertical at $Y_{p}$	Default for AP questions; assumes fully flexible wages/prices
Shifts LRAS Right	Any increase in quantity/quality of factors of production	Examples: more labor, more capital, better technology, lower natural unemployment
Shifts LRAS Left	Any decrease in quantity/quality of factors of production	Examples: natural disaster destroying capital, permanent labor force decline, higher natural unemployment
Long-Run Equilibrium Condition	$A D = S R A S = L R A S$	$Y = Y_{p}$ , $u = u_{n}$ (unemployment equals natural rate)
Effect of Demand Shock on Long-Run Output	$Δ Y_{p} = 0$	Only price level changes; output returns to $Y_{p}$ via SRAS adjustment
Temporary vs Permanent Supply Shocks	Permanent shocks shift LRAS; temporary do not	Temporary oil price changes only shift SRAS, not LRAS
Self-Correction: Recessionary Gap	SRAS shifts right, no LRAS shift	Lower wages reduce production costs, return output to $Y_{p}$
Self-Correction: Inflationary Gap	SRAS shifts left, no LRAS shift	Higher wages increase production costs, return output to $Y_{p}$

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · Multiple Choice
LRAS shift determinants
2024 · FRQ Q1
LRAS and equilibrium adjustment

What's Next

Mastering Long-Run Aggregate Supply is the foundation for all subsequent long-run macroeconomic analysis in AP Macroeconomics. Next, you will use the vertical LRAS framework to analyze the effects of fiscal and monetary policy on the economy in the long run, including crowding out, the long-run Phillips curve, and sustained economic growth. Without a solid understanding of what shifts LRAS and why it is vertical, you will not be able to correctly analyze policy trade-offs or long-run inflation dynamics on FRQs. LRAS connects core concepts across the course, linking the AD-AS model to the Phillips curve, growth theory, and open-economy macroeconomics.