How much output will an economy supply at each price level, and how does this differ in the short and long run?
Explain short-run and long-run aggregate supply, the shapes of the AS curve, and what shifts each
A focused answer to the H2 Economics learning outcome on aggregate supply. The upward-sloping short-run AS curve, the vertical or Keynesian long-run AS, and the determinants that shift each, including productive capacity.
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What this dot point is asking
SEAB wants you to explain short-run and long-run aggregate supply, the shapes of the AS curve, and what shifts each. The central insight is that in the short run a higher price level draws out more output, but in the long run output is set by the economy's productive capacity, so the two curves behave very differently and tell different policy stories.
The answer
Short-run aggregate supply (SRAS)
Short-run aggregate supply shows total output firms are willing to produce at each price level when input prices (especially wages) are sticky. It slopes upward: when the price level rises but wages have not yet adjusted, firms' profit margins widen, so they expand output. Conversely, a lower price level squeezes margins and output falls.
A movement along SRAS is caused by a change in the price level (typically from an AD shift). A shift of SRAS is caused by a change in costs of production at a given price level.
What shifts SRAS
- Wage rates and other input prices.
- Raw material and energy prices (an oil price spike shifts SRAS left).
- Indirect taxes and subsidies on production.
- Productivity (a rise shifts SRAS right).
- Supply shocks (natural disasters, supply-chain disruptions).
A leftward SRAS shift raises the price level and lowers output: cost-push inflation.
Long-run aggregate supply (LRAS)
Long-run aggregate supply shows output when all prices, including wages, have fully adjusted. There are two common depictions:
- Classical (vertical) LRAS. Vertical at the full-employment (potential) output, because in the long run wages and prices adjust fully, so output returns to capacity regardless of the price level. The price level cannot permanently raise real output.
- Keynesian LRAS. Horizontal at low output (spare capacity, so output can rise with no price pressure), then upward-sloping, then vertical at full capacity. This captures the idea that below full employment, demand can raise output without inflation.
What shifts LRAS (potential output)
LRAS shifts right when the economy's productive capacity grows:
- A larger or more skilled labour force (population growth, immigration, education and training).
- A larger capital stock (investment).
- Improved technology and higher productivity.
- Better institutions and incentives (efficient markets, infrastructure).
These are exactly the targets of supply-side policy.
Examples in context
Example 1. Supply shocks and stagflation. The oil shocks of the 1970s shifted SRAS left across importing economies, producing the rare combination of rising prices and falling output (stagflation) that demand-side policy struggles to fix, because boosting AD worsens inflation while restraining AD worsens output. The episode is the classic illustration of a cost-push, SRAS-driven problem.
Example 2. Singapore's capacity-building. Singapore's heavy investment in education, skills upgrading, infrastructure and attracting capital and talent is aimed squarely at shifting LRAS to the right, raising potential output. Because the economy is near full employment, growth must come from expanding capacity rather than simply boosting demand, which is why supply-side measures dominate its long-run strategy.
Try this
Q1. Explain why the short-run aggregate supply curve slopes upward. [3 marks]
- Cue. Wages and some input prices are sticky in the short run, so a higher price level widens firms' profit margins and they expand output, giving an upward slope.
Q2. State what determines the position of the long-run aggregate supply curve. [2 marks]
- Cue. The economy's productive capacity: the quantity and quality of factors of production - labour, capital, technology and productivity.
Q3. Explain the effect of a rise in oil prices on the SRAS curve. [2 marks]
- Cue. Oil is a key input, so dearer oil raises production costs at every price level, shifting SRAS to the left and causing cost-push inflation with lower output.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original10 marksDistinguish between short-run and long-run aggregate supply, and explain what determines the position of the long-run aggregate supply curve.Show worked answer →
A 10 mark question rewards the SRAS-LRAS distinction, the shapes, and the determinants of long-run capacity.
- Short-run AS
- SRAS slopes upward: in the short run, wages and some input prices are sticky, so a higher price level raises firms' profit margins and they expand output. Output can rise above or fall below the full-employment level temporarily.
- Long-run AS
- LRAS is determined by the economy's productive capacity, not the price level. In the classical view it is vertical at full-employment (potential) output, because in the long run wages and prices fully adjust, so output returns to capacity regardless of the price level.
- Determinants of LRAS
- The quantity and quality of factors of production: the size and skill of the labour force, the capital stock, technology, and productivity. An increase in any shifts LRAS right (raising potential output).
Markers reward the sticky-wage reasoning for an upward SRAS, the vertical (capacity-determined) LRAS, and the factor-based determinants that shift LRAS.
Original8 marksExplain the difference between a movement along the short-run aggregate supply curve and a shift of it.Show worked answer →
An 8 mark question rewards the price-level-versus-cost distinction.
- Movement along SRAS
- A change in the price level (caused by a shift in AD) causes a movement along the SRAS curve: firms supply more output as the price level rises because margins widen with sticky wages.
- Shift of SRAS
- A change in costs of production at a given price level shifts the whole SRAS curve. A rise in wages, raw material prices (such as oil) or indirect taxes raises costs and shifts SRAS left; a fall in costs or a rise in productivity shifts it right.
- Why it matters
- A leftward SRAS shift (a supply shock) raises the price level and lowers output (cost-push inflation), a different outcome from a demand change moving along the curve.
Markers reward the price-level movement versus the cost-driven shift, examples of cost shifters, and the supply-shock implication of a leftward shift.
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