Intermediatemacroeconomics
Aggregate Supply: What the Economy Can Produce
Overview
Aggregate supply (AS) describes how much output firms will produce at each price level — the production side of the macroeconomy. This lesson explains why short-run aggregate supply slopes upward, why long-run aggregate supply is vertical at potential output, and which forces shift each curve.
Learning objectives
- Define aggregate supply and distinguish SRAS from LRAS.
- Explain the sticky-wage, sticky-price, and misperception reasons for upward-sloping SRAS.
- Explain why LRAS is vertical at potential (full-employment) output.
- List the shifters of SRAS and LRAS.
- Predict how a supply shock moves output and the price level.
The Supply Side in One Picture
Aggregate supply relates the price level to the quantity of real GDP firms are willing to produce. Economists split it into two curves: short-run aggregate supply (SRAS), which slopes upward, and long-run aggregate supply (LRAS), which is vertical. The split reflects a deep idea: some prices and wages adjust slowly (the short run), but eventually all adjust fully (the long run). Together with aggregate demand, these curves determine output and prices in both horizons.
Why SRAS Slopes Upward
Three stories explain the upward slope, and all involve sluggish adjustment. Sticky wages: when the price level rises but nominal wages are locked in contracts, labor becomes cheaper in real terms, so firms hire more and produce more. Sticky prices: some firms' prices are fixed by menus or contracts, so a rising price level means their relative prices fall and they sell more. Misperceptions: producers may mistake a general price rise for higher demand for their own product and expand. In each story, higher prices coax out more output only because some other price hasn't caught up yet.
LRAS: Vertical at Potential Output
In the long run, wages, prices, and expectations fully adjust, so the price level no longer affects how much the economy produces. LRAS is therefore vertical at potential output (also called full-employment or natural output) — the GDP the economy produces with labor, capital, and technology fully employed at their sustainable rates. Money is neutral in the long run: doubling the money supply eventually doubles prices but leaves real output unchanged. The vertical LRAS is the graphical expression of the classical idea that real factors determine real output.
LRAS is vertical at potential output, set by labor, capital, and technology — not by the price level or demand.
What Shifts the Curves
SRAS shifts when production costs change: higher oil or wage costs shift it left (stagflation territory); falling input prices or productivity gains shift it right. Supply shocks — natural disasters, wars, breakthroughs — are SRAS shifters. LRAS shifts only when potential output itself changes: more workers, more capital, better technology, or better institutions shift it right. Note the asymmetry: demand policies shift AD, but lasting rightward LRAS shifts come from the supply side — education, investment, innovation. Confusing SRAS and LRAS shifters is a classic error.
Supply Shocks in Action
Suppose an energy crisis doubles fuel costs: SRAS shifts left, and the economy moves to higher prices with lower output — stagflation from the supply side. Contrast a productivity breakthrough in manufacturing: SRAS shifts right, lowering prices while raising output — the benign 'good' supply shock. If the productivity gain is permanent, LRAS shifts right too, raising the economy's long-run capacity. Reading the news through SRAS/LRAS shifts turns vague 'supply issues' talk into precise predictions about prices and output.
Key points
- SRAS slopes upward: sticky wages, sticky prices, and misperceptions let higher prices coax out more output short-run.
- LRAS is vertical at potential output, determined by labor, capital, and technology.
- In the long run, money is neutral — demand changes affect prices, not real output.
- SRAS shifters: input costs, productivity, supply shocks, expectations.
- LRAS shifters: labor force, capital stock, technology, institutions.
- Leftward SRAS shifts cause stagflation; rightward shifts ease prices and raise output.
Common misconceptions
✓ Truth: Individual supply curves slope up because of rising marginal cost. SRAS slopes up because of sluggish wage/price adjustment — a macroeconomic, not microeconomic, mechanism.
✓ Truth: LRAS shifts right as labor, capital, and technology grow — that rightward shift IS long-run economic growth. Vertical only means the price level doesn't determine output.
✓ Truth: Temporary cost changes shift SRAS only. LRAS moves only when the economy's productive capacity itself changes — a stricter test.
Practice questions
Q1. The SRAS curve slopes upward mainly because…
- of rising marginal costs at each firm
- wages and prices adjust sluggishly in the short run
- of the wealth effect
- governments set prices
Sticky wages, sticky prices, and misperceptions mean a higher price level temporarily makes production more profitable — a macro adjustment-lag story, not a micro cost story.
Q2. LRAS is vertical because…
- prices never change
- in the long run all prices adjust, so output depends only on real factors
- the government fixes output
- demand is constant
Once wages, prices, and expectations fully adjust, the price level can't affect real production — output is set by labor, capital, and technology.
Q3. A permanent improvement in production technology would…
- shift SRAS left only
- shift both SRAS and LRAS right
- shift AD left
- have no effect on output
Better technology lowers costs (SRAS right) and raises the economy's productive capacity permanently (LRAS right).
Q4. A sudden spike in oil prices shifts…
- SRAS left, causing higher prices and lower output
- LRAS right
- AD right
- SRAS right
Higher input costs shift short-run aggregate supply left: firms produce less at each price level, so prices rise while output falls — stagflation.
Click a question to reveal the answer and explanation.
Frequently asked questions
What is potential output, exactly?
The real GDP an economy produces when labor and capital are employed at sustainable rates — unemployment at its natural rate, no overheating. It is estimated, not observed, and it grows as inputs and technology grow.
How long is the 'short run'?
There is no fixed calendar length — it is the period during which some wages and prices haven't fully adjusted, often thought of as a year or two. The long run is when adjustment is complete.
Can LRAS shift left?
Yes — wars, natural disasters, or institutional collapse can destroy capital, shrink the workforce, or wreck productivity, reducing potential output. It is rare in stable economies but devastating when it happens.
Why do economists disagree about SRAS's slope?
Keynesians see flat SRAS (lots of slack, prices sticky); classicals see steep SRAS (fast adjustment). The truth varies with circumstances — deep recessions look Keynesian, overheated booms look classical.