Long-Run Aggregate Supply — AP Macroeconomics Study Guide

For: AP Macroeconomics candidates sitting AP Macroeconomics.

Covers: This chapter covers the definition of long-run aggregate supply (LRAS), the vertical LRAS curve at potential output, shifts of the LRAS curve, long-run macro equilibrium, and the relationship between LRAS and the natural rate of unemployment.

You should already know: Short-run aggregate supply shape and determinants, aggregate demand fundamentals, the definition of the natural rate of unemployment.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Macroeconomics style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Long-Run Aggregate Supply?

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.

LRAS is always anchored at the level of potential output ($Y_p$), also called full-employment output: the output level when unemployment equals the natural rate of unemployment ($u_n$). According to the AP Macroeconomics CED, LRAS is a core concept in Unit 3 (National Income and Price Determination), which accounts for 10–15% of total AP exam weight. This topic appears regularly on both the multiple-choice (MCQ) and free-response (FRQ) sections of the exam, and it forms the foundation for almost all long-run analysis of growth, recessions, and inflation.

2. The Shape of the Long-Run Aggregate Supply Curve

The standard LRAS curve taught 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, where the economy has excess capacity and can increase output without raising prices, but this is a rare extreme case not tested on the AP exam. For all standard questions, AP expects a vertical LRAS.

Worked Example

Problem: 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? Solve in 4 steps.

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.

3. Shifts of the LRAS Curve

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

Problem: 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 $Y_{p1}$ to a new position at $Y_{p2}>Y_{p1}$.
4. 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.

4. LRAS and Long-Run Macroeconomic Equilibrium

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: $\text{Output Gap}=Y-Y_p$. 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

Problem: 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.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Shifting LRAS in response to a temporary increase in oil prices. Why: Students confuse temporary and permanent supply shocks, and often shift 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 the 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 the Keynesian extreme 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: Putting potential output to the right of full employment output on an AD-AS graph. Why: Students confuse output gaps: they think potential output is higher than full employment. 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., spending on infrastructure); transfer spending for consumption 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. Practice Questions (AP Macroeconomics Style)

Question 1 (Multiple Choice)

Which of the following will most likely cause the long-run aggregate supply curve to shift rightward? A) A temporary decrease in the price of imported crude oil B) A broad-based improvement in technology that permanently increases productivity C) An increase in the aggregate price level caused by higher consumer spending D) An increase in the minimum wage that raises nominal wages for all low-skilled workers

Worked Solution: First, recall that only permanent changes in an economy's productive capacity shift LRAS. Option A describes a temporary change, so it only shifts SRAS, not LRAS, so A is incorrect. Option C is a change in the aggregate price level from a demand shift, which moves the economy along the existing LRAS curve and does not shift it, so C is incorrect. Option D is a one-time increase in nominal wages, which shifts SRAS left but does not change long-run productive capacity, so D is incorrect. Only Option B describes a permanent increase in productivity, which raises potential output and shifts LRAS right. Correct answer: B.

Question 2 (Free Response)

Suppose the country of Grabbia is initially in long-run macroeconomic equilibrium. (a) Draw a correctly labeled AD-AS graph for Grabbia, showing LRAS, SRAS, AD, equilibrium output $Y_p$, and equilibrium price level $P_e$. (b) A major hurricane destroys 20% of Grabbia’s capital stock, causing a permanent decrease in Grabbia’s productive capacity. On your graph from part (a), show the effect of this change on LRAS, SRAS, equilibrium output, and the price level in the long run, assuming aggregate demand does not change. (c) How does the new long-run equilibrium unemployment rate compare to the original equilibrium unemployment rate? Explain your answer.

Worked Solution: (a) The graph has a vertical axis labeled "Aggregate Price Level (P)" and a horizontal axis labeled "Real GDP (Y)". LRAS is drawn as a vertical line at $Y_p$, SRAS as an upward-sloping line, and AD as a downward-sloping line. All three curves intersect at the equilibrium point ($Y_p$, $P_e$), which is labeled correctly. (b) A permanent decrease in productive capacity reduces potential output, so LRAS shifts left from $LRA{S}_1$ to $LRA{S}_2$ at a new lower potential output $Y_{p2}<Y_{p1}$. SRAS also shifts left along with LRAS, since productive capacity is permanently lower. With AD unchanged, the new long-run equilibrium occurs at $Y_{p2}$ (lower output) and a higher price level $P_2>P_e$. (c) The new long-run equilibrium unemployment rate is higher than the original. After the permanent decrease in productive capacity, the new natural rate of unemployment increases (fewer capital goods mean fewer jobs available at full employment), so unemployment equals the higher natural rate in the new long-run equilibrium.

Question 3 (Application / Real-World Style)

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

Worked Solution:

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 over time relative to what it would be with a stable working-age population.
3. Calculate potential output after 20 years using compound growth: $$Y_{2023}=Y_{2003}\times (1-0.012{)}^{20}\approx 500\times 0.786=393\text{ trillion yen}$$
4. In context, this persistent demographic change slows Japan's long-run economic growth by reducing the size of the full-employment labor force, leading to a permanently lower level of potential output than would occur with a stable working-age population.

7. Quick Reference Cheatsheet

8. 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. Understanding why potential output is independent of the aggregate price level is also critical for distinguishing between demand-side and supply-side economic policies.

AD-AS Model Equilibrium The Phillips Curve Long-Run Economic Growth Fiscal Policy Effects