U3.3 Short-Run Aggregate Supply
Master AP Macro topic 3.3: what short-run aggregate supply is, why SRAS slopes upward, its shifters, and movement vs. shift.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on U3.3 Short-Run Aggregate Supply, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
When you see the price level rise and firms respond by pumping out more real output, you're watching short-run aggregate supply (SRAS) in action. This topic explains the upward-sloping SRAS curve — why it slopes up, what makes the whole curve move, and how to tell a shift apart from a slide along the curve.
These distinctions matter because Unit 3 free-response questions constantly ask you to shift SRAS correctly and predict what happens to output and the price level. Nail the mechanics here and the later equilibrium topics become routine.
These distinctions matter because Unit 3 free-response questions constantly ask you to shift SRAS correctly and predict what happens to output and the price level. Nail the mechanics here and the later equilibrium topics become routine.
What SRAS Is and Why It Slopes Upward
Short-run aggregate supply shows the positive relationship between the aggregate price level and the quantity of real GDP that firms are willing to produce, holding input prices fixed. The horizontal axis is real output () and the vertical axis is the price level (). Because the curve slopes upward, a higher price level is associated with more real output supplied in the short run.
The key word is short run, defined in macro as the period during which some input prices — especially nominal wages — have not yet adjusted to changes in the overall price level. Three standard explanations support the upward slope:
Sticky wages: Nominal wages are locked in by contracts and norms. If output prices rise but wages stay fixed, each unit sold becomes more profitable, so firms hire and produce more.
Sticky prices: Some firms cannot or do not change their menu prices immediately, so higher demand raises output rather than price for those firms.
Misperceptions: Producers may mistake a rise in the general price level for a rise in the relative price of their own good and expand production, temporarily raising output.
On the exam, you rarely have to argue all three — but you should be able to name at least one reason SRAS is upward sloping, most commonly sticky wages.
The key word is short run, defined in macro as the period during which some input prices — especially nominal wages — have not yet adjusted to changes in the overall price level. Three standard explanations support the upward slope:
Sticky wages: Nominal wages are locked in by contracts and norms. If output prices rise but wages stay fixed, each unit sold becomes more profitable, so firms hire and produce more.
Sticky prices: Some firms cannot or do not change their menu prices immediately, so higher demand raises output rather than price for those firms.
Misperceptions: Producers may mistake a rise in the general price level for a rise in the relative price of their own good and expand production, temporarily raising output.
On the exam, you rarely have to argue all three — but you should be able to name at least one reason SRAS is upward sloping, most commonly sticky wages.
SRAS Shifters: What Moves the Whole Curve
A shift of SRAS means that at every price level firms now supply a different quantity of real output. Anything that changes production costs or productivity economy-wide shifts the curve. A rightward (increase) shift raises output for any price level; a leftward (decrease) shift reduces it.
A memory aid is that anything raising per-unit production costs shifts SRAS left, and anything lowering those costs shifts it right. Note that expectations work through wages: if workers expect higher inflation, they demand higher nominal wages now, raising costs and shifting SRAS left. Be careful — input prices are an SRAS shifter, but the overall price level is not; a change in causes movement along the curve.
| Shifter | Rightward shift (SRAS increases) | Leftward shift (SRAS decreases) |
|---|---|---|
| Input/resource prices | Nominal wages or oil prices fall | Nominal wages or oil prices rise |
| Productivity | Technology or worker skill improves | Productivity declines |
| Business taxes/subsidies | Subsidies rise or business taxes fall | Business taxes rise or subsidies fall |
| Expected future price level | Expected inflation falls (lower wage demands) | Expected inflation rises |
| Supply shocks | Favorable shock (good harvest) | Adverse shock (natural disaster) |
Movement Along vs. Shift of SRAS
This is the single most tested distinction in the topic, and students lose points by confusing the two.
A movement along SRAS happens when the aggregate price level changes while the curve stays put. This is caused by something on the demand side — a shift in aggregate demand. When AD increases, the economy slides up along SRAS to a higher price level and higher real output; when AD decreases, it slides down to a lower price level and lower output.
A shift of SRAS happens when a non-price determinant (input prices, productivity, taxes/subsidies, expectations, supply shocks) changes. The whole curve moves, changing the quantity supplied at every price level.
That last row is the exam's favorite trick. When SRAS shifts, the price level and real output move in opposite directions (an adverse shock raises but lowers — stagflation). When you slide along SRAS because AD moved, and move in the same direction. Always ask: did a cost/productivity factor change (shift) or did demand change (movement)?
A movement along SRAS happens when the aggregate price level changes while the curve stays put. This is caused by something on the demand side — a shift in aggregate demand. When AD increases, the economy slides up along SRAS to a higher price level and higher real output; when AD decreases, it slides down to a lower price level and lower output.
A shift of SRAS happens when a non-price determinant (input prices, productivity, taxes/subsidies, expectations, supply shocks) changes. The whole curve moves, changing the quantity supplied at every price level.
| Feature | Movement along SRAS | Shift of SRAS |
|---|---|---|
| Cause | Change in aggregate price level (from AD) | Change in a cost or productivity determinant |
| Curve position | Unchanged | Moves left or right |
| Effect on PL and Y | Move in same direction | PL and Y move in opposite directions |
Key terms
- Short-run aggregate supply (SRAS).
- The positive relationship between the aggregate price level and real GDP supplied while input prices, especially nominal wages, remain fixed.
- Sticky wages.
- Nominal wages that are slow to adjust due to contracts and norms, making higher output prices more profitable and explaining the upward SRAS slope.
- Misperceptions.
- When producers mistake a general price-level increase for a rise in their good's relative price and temporarily expand output.
- Input (resource) prices.
- Costs of factors like labor and raw materials; a change shifts SRAS because it alters per-unit production cost.
- Productivity.
- Output produced per unit of input; higher productivity lowers per-unit cost and shifts SRAS right.
- Supply shock.
- An unexpected event that changes production costs economy-wide, shifting SRAS (adverse shocks shift it left).
- Movement along SRAS.
- A change in quantity of real output supplied caused by a change in the price level, typically from a shift in aggregate demand.
Worked example
An economy relies heavily on imported oil, and the world price of oil suddenly doubles. Using the AD-AS model, explain what happens to the SRAS curve, the aggregate price level, and real output in the short run.
Step 1: Identify what changed. Oil is an input used across the economy, so this is a change in input prices — a non-price determinant of SRAS, not a change in the price level. Therefore SRAS shifts rather than the economy sliding along it.
Step 2: Determine direction. Higher oil prices raise per-unit production costs. Higher costs mean firms supply less real output at every price level, so SRAS shifts left (decreases).
Step 3: Read the new equilibrium. Holding AD fixed, the leftward SRAS shift moves equilibrium up and to the left along AD. The aggregate price level rises and real output falls.
Step 4: Name the outcome. Rising combined with falling is stagflation — the classic signature of an adverse supply shock. Notice that and move in opposite directions, which confirms this was an SRAS shift rather than a movement along the curve caused by AD.
Step 2: Determine direction. Higher oil prices raise per-unit production costs. Higher costs mean firms supply less real output at every price level, so SRAS shifts left (decreases).
Step 3: Read the new equilibrium. Holding AD fixed, the leftward SRAS shift moves equilibrium up and to the left along AD. The aggregate price level rises and real output falls.
Step 4: Name the outcome. Rising combined with falling is stagflation — the classic signature of an adverse supply shock. Notice that and move in opposite directions, which confirms this was an SRAS shift rather than a movement along the curve caused by AD.
Practice questions
Which of the following would cause the short-run aggregate supply curve to shift to the right?
- An increase in nominal wages negotiated in new labor contracts
- A technological advance that raises worker productivity
- An increase in the aggregate price level
- An increase in expected future inflation
Answer: A technological advance that raises worker productivity
A productivity increase lowers per-unit production costs, so firms supply more real output at every price level, shifting SRAS right. Higher nominal wages and higher expected inflation both raise costs and shift SRAS left. A change in the aggregate price level causes movement along SRAS, not a shift.
Explain the difference between a movement along the SRAS curve and a shift of the SRAS curve, and give one cause of each.
Answer: A movement along SRAS is caused by a change in the aggregate price level (from a shift in AD), while a shift of SRAS is caused by a non-price determinant such as input prices, productivity, taxes/subsidies, or expectations.
A movement along SRAS occurs when the price level changes and the economy slides up or down a fixed curve; a common cause is an increase in aggregate demand, which raises both and . A shift of SRAS occurs when a cost or productivity factor changes the entire curve's position; for example, falling input prices shift SRAS right. The tell-tale sign of a shift is that and move in opposite directions, whereas along a movement they move together.
A country's government grants a large subsidy to manufacturers. In the short run, what happens to the SRAS curve and to real output, assuming AD is unchanged?
Answer: SRAS shifts right and real output rises while the price level falls.
A subsidy lowers firms' per-unit production costs, so they are willing to supply more output at every price level, shifting SRAS to the right. Moving down the fixed AD curve to the new equilibrium, real output increases and the aggregate price level decreases.
FAQ
- Why does SRAS slope upward but LRAS is vertical?
- SRAS slopes upward because in the short run some input prices — especially nominal wages — are sticky, so a higher price level raises profit margins and encourages more production. In the long run all prices and wages fully adjust, so output returns to its potential level regardless of the price level, making LRAS vertical. Topic 3.4 covers LRAS in detail.
- Is a change in wages a movement along SRAS or a shift?
- A change in nominal wages is a change in input prices, which is a non-price determinant, so it shifts the entire SRAS curve. Higher wages shift SRAS left; lower wages shift it right. Only a change in the aggregate price level itself causes a movement along the curve.
- How do I know if the price level and output move together or in opposite directions?
- Ask what caused the change. If aggregate demand shifted, you move along SRAS and and move in the same direction. If SRAS itself shifted, and move in opposite directions — for example, an adverse supply shock raises the price level while lowering output (stagflation).
- What are the main SRAS shifters I need to memorize for the exam?
- Focus on input/resource prices (like wages and oil), productivity, business taxes and subsidies, expected future price level or inflation, and supply shocks. A helpful rule: anything that raises per-unit production costs shifts SRAS left, and anything that lowers them shifts it right.
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