Aggregate Supply

SRAS vs LRAS, supply-side shocks, the Keynesian–Classical debate, and how to use AS diagrams for top marks

SPEC 2.5 • THE UK ECONOMY

Key Definitions

These must be word-perfect in the exam. Learn them exactly as written below.

Aggregate Supply (AS)

The total output of goods and services that producers in an economy are willing and able to supply at a given price level.

Short-Run Aggregate Supply (SRAS)

The total output firms are willing to supply at each price level, with wages and other input costs fixed. SRAS slopes upward — higher prices make production more profitable at given costs, so firms supply more.

Long-Run Aggregate Supply (LRAS)

The total output the economy can produce when all factors of production are fully and efficiently employed — the economy's productive capacity. Drawn as a vertical line at potential (full employment) output Y_f.

Why SRAS Slopes Upward

In the short run, wages and input costs are fixed by contracts. When the general price level rises, firms receive higher revenue per unit while costs stay the same. This makes production more profitable, so firms increase output.

The Logic

Price level rises→Revenue per unit ↑→Costs fixed (short run)→Profit margin ↑→Firms supply more

Exam Tip

"Short run" in macro means the period during which at least one factor price (usually wages) is fixed. This is different from the micro definition. Always state this assumption when explaining the SRAS slope.

The SRAS Curve

The SRAS is an upward-sloping curve. A movement along SRAS occurs when the price level changes. A shift of SRAS occurs when input costs or productivity change.

SRAS Shifters — All Six Required

Any change in the costs of production (other than caused by a change in the price level) will shift SRAS.

FactorSRAS Left ←SRAS Right →

1. Wage costsWages riseWages fall

2. Raw material / commodity pricesOil price spikeOil price fall

3. Exchange rateDepreciation (import costs ↑)Appreciation (import costs ↓)

4. Indirect taxesVAT riseVAT cut

5. ProductivityProductivity fallsTechnology / better working practices

6. Government subsidiesSubsidies removedSubsidies to producers

Exam Tip

Learn the mnemonic: W-R-E-T-P-S (Wages, Raw materials, Exchange rate, Taxes, Productivity, Subsidies). This covers every SRAS shift the examiner can ask about.

Why LRAS Is Vertical

In the long run, all input prices (including wages) fully adjust to changes in the price level. If the price level doubles, wages and costs eventually double too. Real profitability is unchanged, so firms have no incentive to produce more or less.

Classical View

Productive capacity depends on real factors — the quantity and quality of land, labour, capital, and enterprise — not on the price level. Therefore LRAS is vertical at potential output (Y_f), the level of output achievable when all resources are fully and efficiently employed.

Potential Output (Y_f)

The maximum sustainable level of real output the economy can produce when all factors of production are fully and efficiently employed. Also called full employment output. The economy can temporarily exceed Y_f (positive output gap) but this is unsustainable.

The Classical LRAS Curve

LRAS is a vertical line at Y_f. A shift of LRAS occurs only when the economy's productive capacity changes.

Key Point

LRAS shifting right = economic growth in potential output. This is what supply-side policies aim to achieve. The price level does not affect where LRAS sits — only real factors do.

LRAS Shifters — Six Factors

These change the economy's productive capacity — the maximum sustainable output.

#FactorHow It Shifts LRAS Right

1TechnologyNew methods allow more output from same inputs

2Quantity & quality of capitalMore / better machinery, equipment, infrastructure

3Working populationImmigration, higher participation, later retirement

4Education & skillsHigher human capital raises labour productivity

5Institutional improvementsBetter property rights, rule of law, less corruption

6Natural resourcesDiscovery of new resource deposits

Examiner Alert

Students often confuse SRAS and LRAS shifters. A rise in oil prices shifts SRAS left (cost of production), not LRAS. LRAS only changes when the economy's capacity changes. If in doubt, ask: "Does this change the maximum the economy can produce?" If yes → LRAS. If it changes the cost of current production → SRAS.

The Great Debate: Classical vs Keynesian LRAS

This debate underpins every macroeconomic policy essay. Use it as an AO4 evaluation framework.

Classical View

LRAS is vertical at Y_f. Markets clear in the long run — wages and prices adjust to restore full employment. Government intervention is unnecessary and may simply cause inflation.

Policy implication: Only supply-side policy can increase output. Demand-side policy just changes the price level.

Keynesian View

LRAS has three sections: horizontal (spare capacity), upward-sloping (approaching capacity), then vertical (full capacity). Wages are sticky downward — the economy can get stuck in a low-output equilibrium.

Policy implication: Demand-side policy can increase real output when there is spare capacity, without causing inflation.

Exam Tip

For any 25-mark macro essay, you can gain AO4 marks by writing: "The extent to which this policy is effective depends on whether the Classical or Keynesian view of LRAS holds. If the economy has significant spare capacity (Keynesian view), demand-side policy can raise real output. If the economy is near full employment (Classical view), the same policy would primarily raise the price level."

Keynesian LRAS Diagram

The Keynesian LRAS has three distinct sections. Where AD intersects determines the effect of demand-side policy.

Three Zones

Horizontal: Deep recession — huge spare capacity. AD increase raises output with zero inflation.

Upward-sloping: Approaching capacity — AD increase raises output AND prices (trade-off begins).

Vertical: At full capacity — any further AD increase is purely inflationary (same as Classical).

Supply-Side Shock: Stagflation

A negative supply-side shock (e.g. oil price spike) shifts SRAS left. This produces the worst-case macro outcome: stagflation.

The Policy Dilemma

Stagflation creates a trade-off that demand-side policy cannot resolve. This is the key evaluation point.

Stagflation

A simultaneous occurrence of stagnation (falling output, rising unemployment) and inflation (rising price level). Caused by a leftward shift in SRAS.

Option A: Expand AD

Tackles unemployment (AD shifts right → output recovers) but worsens inflation (price level rises further to P₃).

Option B: Contract AD

Tackles inflation (AD shifts left → price level falls) but worsens unemployment (output falls further).

Option C: Supply-Side Policy

The only solution without a trade-off. Shift SRAS back right by reducing costs (e.g. subsidies, tax cuts on producers, productivity improvements). Output rises AND prices fall. But supply-side policy is slow — it takes years, not months.

LRAS Shift Right: The Supply-Side Ideal

When supply-side reform succeeds, LRAS shifts right. If SRAS also shifts right (lower costs), the economy gets the best possible outcome.

The Contrast With Demand-Side

Demand-side expansion raises output but also raises the price level. Supply-side is the only way to raise output without inflationary pressure. This is the core argument for supply-side policy in any essay.

AD Beyond LRAS: Overheating

When AD shifts right beyond LRAS, the economy temporarily exceeds potential output. This is a positive output gap.

AD shifts right past Y_f

Unemployment below natural rate

Wages bid up

SRAS shifts left

Return to Y_f at higher P

Classical Self-Correction

In the Classical view, the economy self-corrects back to Y_f. Excess demand bids up wages → firms' costs rise → SRAS shifts left → output returns to potential but the price level is permanently higher. The AD expansion was purely inflationary in the long run.

Exam Tip

This chain is the standard case for contractionary policy. If the economy is overheating (positive output gap), the appropriate response is to reduce AD — raise interest rates, cut government spending, or raise taxes.

Examiner Alert: Common AS Errors

These mistakes are called out repeatedly in Edexcel examiner reports. Avoid every one of them.

Examiner Alert

Confusing SRAS and LRAS shifters. Oil prices shift SRAS. Education shifts LRAS. Always ask: "Does this change costs or capacity?"

Drawing LRAS as upward-sloping. Classical LRAS is vertical. If you draw a Keynesian LRAS, you must explain why and label all three sections.

Not labelling Y_f. The LRAS must be labelled at potential output. Without Y_f, the examiner cannot assess whether you understand the concept.

Saying "supply shifts left" without specifying SRAS or LRAS. There is no generic "AS curve" on an AD/AS diagram. You must state which one.

Forgetting the policy dilemma with stagflation. If SRAS shifts left, always discuss why demand-side policy creates a trade-off.

Exam Tip

For any question requiring AD/AS analysis, a valid diagram is mandatory for Level 3+ KAA. Your diagram must show: correct shift direction, both old and new equilibria labelled (P₁Y₁ → P₂Y₂), and explicit reference to the diagram in your written analysis.

AO4 Evaluation Toolkit

Use these evaluative points in any AS-related essay. Each one is a potential paragraph.

1. Classical vs Keynesian

"The impact depends on whether the economy has spare capacity. In the Keynesian view, demand-side policy can raise real output. In the Classical view, the economy self-corrects and demand-side policy is inflationary."

2. Time Period

"Supply-side policies take years to feed through — education reform takes a generation. Demand-side policy acts within 12–18 months. The appropriate policy depends on urgency."

3. The Supply-Side Ideal vs Reality

"While a rightward LRAS shift is the ideal outcome (higher output, lower prices, lower unemployment), achieving this requires effective structural reform. Not all supply-side policies succeed — R&D may not yield innovation, training may not match employer needs."

4. Distribution Effects

"Market-based supply-side policies (tax cuts, benefit reductions) tend to widen income inequality. Income tax cuts primarily benefit higher earners. Interventionist supply-side policies (education, infrastructure) can reduce inequality but are more costly."

5. Nature of the Shock

"The policy response depends on whether the shock is temporary or permanent. A temporary oil price spike may not require policy intervention — SRAS will shift back. A permanent structural change (e.g. decline of an industry) requires supply-side reform."

Practice Question

Explain the difference between a movement along and a shift of the SRAS curve. Use a diagram in your answer. (5 marks)

1

Define SRAS: Short-run aggregate supply is the total output firms are willing to supply at each price level, with wages and other input costs fixed.

2

Movement along: Caused by a change in the general price level. Higher prices → higher revenue per unit at fixed costs → higher profit → firms supply more → movement up along SRAS.

3

Shift of SRAS: Caused by a change in input costs or productivity (e.g. rise in oil prices, fall in wages, improvement in technology). These change the cost of production at every price level, so the entire curve shifts.

4

Diagram: Draw SRAS with P on vertical axis and Real GDP on horizontal axis. Show movement along as an arrow tracing the curve (caused by ΔP). Show shift as SRAS₁ → SRAS₂ (caused by Δcosts).

5

Application: For example, a rise in global oil prices increases firms' production costs at every price level, shifting SRAS leftward from SRAS₁ to SRAS₂, raising the price level and reducing real output — a supply-side shock.

Practice Question 2

Using an AD/AS diagram, assess the likely macroeconomic effects of a significant and sustained rise in global energy prices on the UK economy. (8 marks)

1

KAA Point 1 (SRAS shift): Higher energy prices raise firms' production costs → SRAS shifts left from SRAS₁ to SRAS₂ → new equilibrium: higher price level (P₂) and lower real output (Y₂). This is cost-push inflation combined with falling output — stagflation. As shown in the diagram, the economy moves from E₁ to E₂ with a simultaneous rise in unemployment and inflation.

2

KAA Point 2 (AD effect): Higher energy costs also reduce real disposable income for households → consumption (C) falls → AD may also shift left, amplifying the output fall but moderating the price rise. UK firms using imported energy face higher costs → reduced international competitiveness → net exports may fall.

3

Evaluation: The extent of the impact depends on whether the energy price rise is temporary (SRAS shifts back) or permanent (structural adjustment needed). If the UK has a diversified energy mix or domestic supply (e.g. North Sea gas, renewables), the SRAS shift will be smaller. The policy dilemma is key: demand-side expansion would tackle unemployment but worsen inflation — only supply-side reform (e.g. investment in alternative energy) can resolve the trade-off without conflict.

Summary

Key Takeaways

ConceptKey Point

SRASUpward-sloping. Wages fixed. Shifted by changes in input costs (W-R-E-T-P-S).

LRAS (Classical)Vertical at Y_f. Shifted by changes in productive capacity (technology, labour, capital, institutions).

LRAS (Keynesian)Three sections: horizontal → upward → vertical. Allows demand-side policy when spare capacity exists.

StagflationSRAS shifts left → output falls + prices rise. Policy dilemma: only supply-side can resolve without trade-off.

Supply-side idealLRAS + SRAS shift right → output rises + prices fall. No inflationary trade-off.

SRAS vs LRAS — Quick Test

Does it change the cost of current production? → SRAS

Does it change the capacity of the economy? → LRAS

Could it do both? → Show both shifts.

Evaluation Framework

Classical vs Keynesian · Time period · Distribution effects · Temporary vs permanent shock · Supply-side ideal vs reality

Exam Tip

In any macro essay, always draw the AD/AS diagram first, label both equilibria, and reference it explicitly in your writing. Missing this caps you at Level 2 KAA. Use the Classical vs Keynesian debate as your go-to AO4 evaluation.