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Aggregate Supply (AS) is the total quantity of goods and services that producers in an economy are willing and able to supply at each price level over a given period. If aggregate demand is the spending side of the macroeconomy, aggregate supply is the production side — and it is where the two great traditions of macroeconomics part company. The distinction between Short-Run Aggregate Supply (SRAS) and Long-Run Aggregate Supply (LRAS) is one of the most important analytical frameworks at A-Level, and the shape of the long-run curve — vertical for the classical/monetarist school descended from Jean-Baptiste Say (1803) and Milton Friedman (1968), L-shaped for the Keynesian tradition founded by John Maynard Keynes (1936) — is the single biggest fault line in the whole subject. Master AS and you can explain why the same increase in AD raises real output in one economy and merely raises prices in another, why a supply shock causes stagflation, and why classical and Keynesian economists prescribe opposite policies from the same diagram.
This lesson maps to AQA 7136 section 4.2.2 — How the macroeconomy works: aggregate supply in the short run and long run, and feeds directly into 4.2.4 (the determination of equilibrium output) and 4.2.7 (supply-side policies). It is examined principally in Paper 2 (National and international economy) through multiple-choice, data-response and 25-mark essays, and is synoptic with Paper 3, where AS reasoning underpins case studies of shocks and growth, and with Paper 1, since the productivity of individual markets aggregates into the position of LRAS. All four assessment objectives apply: AO1 for the definitions and the reasons SRAS slopes up while classical LRAS is vertical; AO2 for applying cost shocks and supply-side improvements to real UK data; AO3 for chains of reasoning from a cost change to a shift to a new equilibrium; and AO4 for evaluating the classical-versus-Keynesian dispute and judging which model better fits a given context.
It pays to be exact about the three terms before drawing anything, because AQA penalises loose definitions and rewards the precise distinction between the short run (some costs fixed) and the long run (all costs adjusted).
| Concept | Definition |
|---|---|
| Aggregate Supply (AS) | The total output all producers are willing and able to supply at each price level in a given period |
| Short-Run Aggregate Supply (SRAS) | AS when at least some factor prices — above all nominal wages — are fixed or "sticky"; conventionally a horizon of up to about two years |
| Long-Run Aggregate Supply (LRAS) | AS when all factor prices have fully adjusted to the price level; it represents the economy's full productive capacity and is determined by the quantity and quality of the factors of production, not by the price level |
The crucial conceptual move is that the short run and long run are not calendar periods but states of adjustment. The short run is defined by sticky costs; the long run is defined by fully flexible costs. This is exactly why the two curves have different shapes and why a shock can do one thing now and another thing later.
Exam Tip: Never define the long run as "a long time". Define it as the period in which all factor prices have fully adjusted (especially wages). Examiners reward the adjustment-of-costs definition because it explains why LRAS behaves differently from SRAS.
The SRAS curve slopes upward from left to right: a higher price level is associated with a greater quantity of real output supplied. Note the parallel structure with AD — price level on the vertical axis, real GDP on the horizontal — so that AS and AD can be drawn on the same axes.
The upward slope is a movement along the curve driven by a change in the price level, and it rests on the assumption that some costs — especially nominal wages — are fixed in the short run. Each explanation deserves to be understood as a chain of reasoning, because examiners reward the mechanism, not the label.
| Reason | Chain of reasoning |
|---|---|
| Sticky (fixed) wages | Nominal wages are fixed by contracts in the short run → a rise in the price level cuts the real wage (labour is cheaper in real terms) → firms' profit margins on each unit widen → it pays to hire more and produce more → output rises. Keynes (1936) stressed this. |
| Fixed input prices | Raw-material, energy and rent contracts are fixed in the short run → when output prices rise but these input costs do not, the margin on each unit rises → firms expand production. |
| Imperfect information | A firm sees its own output price rise → it may misread a general price rise as a rise in demand for its product specifically → it raises output. Robert Lucas (1972) formalised this "misperceptions" idea. |
| Menu costs | Some firms are slow to reprice (the cost of changing price lists, menus, catalogues) → when the general price level rises, their relative prices temporarily fall → they sell more. |
The unifying theme is that in the short run output prices can rise faster than costs, so higher prices mean fatter margins and more production. The moment costs catch up — which is the long run — that incentive disappears.
It is worth pausing on why the upward slope is a short-run phenomenon, because this is the hinge on which the whole classical case turns. In the short run, the rise in the price level outpaces the rise in costs precisely because wages and input prices are locked in by contracts and have not yet adjusted. But contracts expire, expectations update, and workers eventually notice that their real wages have fallen; they then bargain for higher nominal pay to restore their living standards. As nominal wages rise, firms' unit costs rise, SRAS drifts back to the left, and the temporary output gain unwinds. This is exactly the adjustment that, in the classical model, returns the economy to the vertical LRAS — and it is why classical economists insist the upward-sloping SRAS describes only a transitional state, not a lasting relationship between prices and output. A Keynesian, by contrast, argues that because wages are sticky downwards, this adjustment is slow and asymmetric: it happens readily when prices rise (workers demand catch-up pay) but very reluctantly when prices fall (workers resist nominal cuts), so the economy does not slide smoothly back to full employment after a negative shock.
Exam Tip: The upward slope of SRAS reflects a movement along the curve caused by a change in the price level, with costs assumed fixed. A change in costs themselves — wages, energy, taxes, the exchange rate — shifts the whole SRAS curve. Confusing the two is the commonest SRAS error.
Of all the determinants of SRAS, the one AQA returns to most often is unit labour costs — the wage cost of producing one unit of output. This depends not on wages alone but on wages relative to productivity:
Unit labour cost=Productivity (output per worker)Wage rate
The implication is subtle and worth stating explicitly in answers: a pay rise does not necessarily raise costs if productivity rises by as much or more. If wages rise 4% but output per worker also rises 4%, unit labour costs are unchanged and SRAS does not shift. It is only when wages outpace productivity that unit labour costs rise and SRAS shifts left. This is why governments and the Bank of England watch the gap between earnings growth and productivity growth so closely — it is the proximate signal of domestically generated (wage-push) cost-push inflation. It is also why supply-side measures that raise productivity (training, investment, technology) are doubly powerful: they shift LRAS right and, by lowering unit costs, shift SRAS right too.
SRAS shifts when the costs of production change at every price level. A rise in costs shifts SRAS left (less supplied at each price); a fall in costs shifts it right.
| Factor | Direction of shift | Mechanism and UK illustration |
|---|---|---|
| Rise in raw-material or energy costs | SRAS left | Higher input costs cut margins — e.g. the oil price quadrupling of 1973 and the gas-price surge of 2022 |
| Fall in raw-material or energy costs | SRAS right | Lower input costs widen margins — e.g. the oil-price collapse of 2014–15 |
| Wage increases above productivity | SRAS left | Higher unit labour costs — wages rising faster than output per worker |
| Sterling depreciation | SRAS left | Imported components and materials become dearer in sterling, raising costs |
| Sterling appreciation | SRAS right | Imported inputs become cheaper |
| Changes in indirect taxes / subsidies | Either way | A VAT rise or higher employer National Insurance shifts SRAS left; a cut or a subsidy shifts it right |
| One-off supply shocks | Either way | Pandemics, wars and natural disasters shift SRAS left; a technological breakthrough shifts it right |
Exam Tip: When you analyse an SRAS shift, name the specific cost change. "SRAS shifts left because costs rise" is too vague — state whether it is energy, wages, imported inputs, indirect taxes or the exchange rate, and the AO2/AO3 marks follow.
It pays to rehearse the full chain on a concrete (hypothetical) example, because data-response questions reward the sequence, not the conclusion. Suppose world energy prices double and, at the same time, sterling depreciates by around 10%. Two cost pressures hit at once: energy is dearer directly, and the weaker pound raises the sterling price of every imported input — components, raw materials, fuel. At each and every price level, firms now face higher unit costs, so they are willing and able to supply less output: SRAS shifts left. Read off the consequences on the diagram: the equilibrium price level rises (cost-push inflation) while equilibrium real output falls. If the economy was near full capacity to begin with, the price effect dominates and the output loss is modest; if it had ample spare capacity, more of the adjustment shows up as lost output. Notice, too, the synoptic twist that catches weaker candidates: the same depreciation that pushes SRAS left also tends to push AD right (cheaper exports, dearer imports improving net trade), so the net effect on output is ambiguous and depends on which force is larger — a perfect opening for evaluation. Spelling out this chain — trigger → which costs rise → SRAS shifts left → price level up, output down → the offsetting AD effect — is the disciplined application that separates a top answer from a vague one.
In the long run all factor prices, including wages, have fully adjusted to the price level. Classical, new-classical and monetarist economists argue that the LRAS curve is therefore vertical at the full-employment (or "natural-rate") level of output, often labelled YFE or Yn.
| Key feature | Explanation |
|---|---|
| Vertical at YFE | Long-run output is fixed by the quantity and quality of the factors of production, not by the price level. Higher AD therefore raises only prices, not output. |
| Built on Say's Law | Jean-Baptiste Say (1803): "supply creates its own demand" — production generates the incomes that buy it, so general overproduction is impossible and the economy gravitates to full employment. |
| Self-correcting | If AD falls, unemployment puts downward pressure on wages; lower wages cut costs, shifting SRAS right until output returns to YFE — without any government action. |
| Natural rate of unemployment | Friedman (1968) and Edmund Phelps (1968) argued (independently) that there is a natural rate of unemployment consistent with stable inflation; LRAS sits at the output produced when unemployment is at this rate. |
The vertical LRAS embodies the classical dichotomy: in the long run, real variables (output, employment) are set by the supply side, while nominal variables (the price level) are set by demand. This is why monetarists insist that demand management cannot permanently raise output — only supply-side reform can.
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