AQA A-Level Economics · Theme 2
Aggregate
Demand
Components, the multiplier, and AD curve shifts
📘 20 slides + 8 questions
⏱ 25 min
🎯 Theme 2: National Economy
Learning Objectives
By the end of this lesson you will be able to…
Define aggregate demand and state its four components: AD = C + I + G + (X – M)
Explain the determinants of each component and what causes the AD curve to shift
Calculate and apply the Keynesian multiplier; explain the accelerator effect
Evaluate the effectiveness of demand-management policies and the limitations of AD analysis
What is Aggregate Demand?
What is Aggregate Demand?
Core Definition
Aggregate Demand (AD): the total demand for goods and services in an economy at a given price level in a given time period. AD = C + I + G + (X – M). The AD curve slopes downward: higher price level → lower real wealth (wealth effect), higher interest rates needed to control inflation, less competitive exports → lower quantity demanded.
SHAPE OF AD CURVE
Downward sloping — NOT the same mechanism as the microeconomic demand curve. Three effects explain the slope: wealth effect (higher prices erode real value of savings), interest rate effect (higher prices → central bank raises rates → less borrowing), international substitution (domestic goods become less competitive).
SHIFTS vs MOVEMENTS
Movement along AD = change in price level. Shift of AD = change in any component (C, I, G, X–M) — represents an increase or decrease in total spending at every price level.
UK AD COMPOSITION (approx)
C ≈ 65%; I ≈ 17%; G ≈ 20%; Net exports ≈ −2% (UK runs a trade deficit). Consumer spending is by far the dominant component.
Component 1: Consumption
Consumption (C)
Definition
Consumption: household spending on goods and services. Largest component of UK AD (~65% of GDP). Keynes: consumption is primarily determined by current income — the Consumption Function: C = a + bY, where a = autonomous consumption, b = marginal propensity to consume (MPC), Y = income.
DETERMINANTS OF C
Real income (most important); wealth effects (house prices, stock market — Bank of England estimates £1 of extra wealth raises C by 3–5p); consumer confidence; real interest rates (cost of borrowing/reward for saving); credit availability; inflation expectations.
MARGINAL PROPENSITY TO CONSUME (MPC)
Fraction of extra income spent. MPC = ΔC/ΔY. If MPC = 0.8, an extra £1 of income generates 80p of extra spending. Lower-income households have higher MPC (spend more of each extra pound).
SAVINGS RATIO & COVID
In 2020 UK savings ratio spiked to 27% (highest since 1960s) as COVID lockdowns prevented spending and furlough maintained incomes. "Forced saving" created pent-up demand which fuelled the 2021 inflationary surge as restrictions lifted.
Component 2: Investment
Investment (I)
Definition
Investment: firm spending on capital goods (machinery, buildings, equipment) and household spending on new housing. Typically ~17% of UK GDP. Most volatile component of AD — investment expectations are highly sensitive to confidence, interest rates, and economic outlook.
DETERMINANTS OF I
Rate of interest (cost of borrowing — lower rates → more investment); business confidence and expectations (Keynes: "animal spirits"); corporate profitability; accelerator effect (rate of change of GDP); technological change; government policy (investment tax credits, capital allowances).
THE ACCELERATOR EFFECT
Investment depends on the rate of change of output, not just its level. If GDP grows 3% → firms need 3% more capital → high investment. If GDP growth slows to 1% → less new capital needed → investment falls sharply, even though GDP is still growing. Amplifies the economic cycle.
UK INVESTMENT WEAKNESS
UK business investment has been chronically weak — among lowest in G7 (≈17% of GDP vs OECD average ~22%). Post-Brexit uncertainty suppressed investment 2016–2019. Short-termism of UK financial system (focus on dividends, share buybacks) may partly explain the "investment puzzle."
Components 3 & 4: G and (X – M)
Government Spending (G) & Net Exports (X – M)
GOVERNMENT SPENDING (G)
Public sector spending on goods and services (education, NHS, infrastructure, defence). Does NOT include transfer payments (benefits, pensions) — these affect C when spent by recipients, not G directly. G ≈ 20% of UK GDP. Determined by fiscal policy stance and automatic stabilisers.
NET EXPORTS (X – M)
Exports minus imports. UK runs a persistent current account deficit (imports > exports) — net exports are typically negative. Determinants: exchange rate (depreciation → cheaper exports, dearer imports); relative inflation rates; income in partner countries (higher overseas income → more demand for UK exports).
MARSHALL-LERNER & J-CURVE
Depreciation improves net exports ONLY if the sum of price elasticities of demand for exports and imports exceeds 1 (Marshall-Lerner condition). Short-run J-curve: PEDs are inelastic — depreciation may worsen the current account initially. Long-run: demand becomes more elastic → current account improves.
AUTOMATIC STABILISERS
Government spending and taxation automatically move to stabilise AD without deliberate policy action. In recession: welfare spending rises (G↑), tax revenues fall (T↓) → fiscal deficit widens → supports AD. In boom: welfare falls (G↓), tax revenues rise (T↑) → surplus → dampens inflation. Reduces amplitude of the economic cycle without policy lags.
The Keynesian Multiplier
The Keynesian Multiplier
Core Concept
The Multiplier: an initial increase in injection (G, I, or X) leads to a larger final increase in GDP. Multiplier (k) = 1 / (1 – MPC) = 1 / MPS. If MPC = 0.8, k = 1 / 0.2 = 5. A £1bn increase in G generates £5bn of GDP. Mechanism: G paid to workers → workers spend 80p of every £1 → recipients spend 80% of that → chain of successive rounds of spending.
LEAKAGES
The multiplier is reduced by savings (MPS), taxation (MPT), and imports (MPM). Full multiplier: k = 1 / (MPS + MPT + MPM). In open economies with high taxation and imports, the multiplier is typically much smaller. UK multiplier estimates: 0.6–1.5 depending on state of the economy.
SIZE OF MULTIPLIER
Largest when: economy has spare capacity (unemployed resources); MPC is high (lower incomes → more of each pound spent); leakages are low. Smallest in a boom (capacity constraints mean injection bids up prices, not output).
OBR & FISCAL MULTIPLIERS
OBR estimates UK government spending multiplier ≈ 0.6–1.0. Post-2008 austerity: IMF (Blanchard & Leigh, 2013) found multipliers were underestimated → fiscal consolidation caused deeper recessions than forecast. The "austerity debate" — multiplier size is politically contentious.
AD Curve Shifts
What Shifts the AD Curve?
FACTORS SHIFTING AD RIGHT (↑)
Increase in consumer confidence/wealth; lower interest rates (Bank of England cut); fiscal stimulus (higher G or tax cuts); exchange rate depreciation (more competitive exports); higher global income (more demand for UK exports).
FACTORS SHIFTING AD LEFT (↓)
Fall in confidence; higher interest rates; fiscal austerity; exchange rate appreciation; trading partner recessions; credit crunch (2008: banks stopped lending → I and C collapsed).
PRICE LEVEL EFFECTS ON AD
Along the curve: higher price level reduces real wealth (Pigou effect), requires higher nominal interest rates (Fisher effect), makes exports less competitive. All reduce quantity of AD demanded — a movement, not a shift.
DISTINCTION FROM LRAS SHIFTS
AD shifts affect short-run equilibrium price and output. Only supply-side factors (technology, capital accumulation, population, institutions) shift LRAS and determine long-run growth potential. AD management affects cyclical position, not trend growth.
Real-World Applications
AD in the Real World
COVID-19 FISCAL RESPONSE (2020)
UK government injected ~£400bn in emergency spending (furlough, business grants, NHS). Largest peacetime fiscal expansion. Multiplier effect supported incomes and limited demand collapse. Consumer spending recovered rapidly in 2021 as restrictions lifted.
POST-2008 AUSTERITY
UK government cut G and raised taxes 2010–2016. AD growth suppressed. Critics: negative multiplier effects deepened and prolonged the recession. Defenders: without deficit reduction, markets would have required higher gilt yields → crowding out private investment.
HOUSE PRICES & WEALTH EFFECT
UK house prices rose 25% during 2020–2022 (COVID boom). Wealth effect: homeowners feel richer → increase consumption. Halifax estimates £1 rise in house wealth → ~£0.04 extra spending. But distributional effect: helps asset owners, not renters.
2022 ENERGY PRICE SHOCK
Gas/electricity bills rising 300%+ effectively reduced real disposable income (reducing C). Government response: Energy Price Guarantee (capped bills) maintained real incomes and supported AD at cost of ~£55bn. Without it, AD would have fallen sharply → likely recession.
Evaluation
Evaluating AD Management
For (AD management)
- Fiscal and monetary stimulus demonstrably effective for cyclical downturns (COVID response, New Deal)
- Multiplier means government spending has outsized impact on GDP when spare capacity exists
- Automatic stabilisers reduce cycle amplitude without implementation lags
- Real-world evidence: IMF found post-2008 multipliers were underestimated — austerity was more harmful than expected
Against (limitations)
- Multiplier may be smaller than expected (leakages, crowding out of private investment)
- AD expansion risks inflation if at or near full employment — stimulus generates price rises, not output
- Time lags (policy recognition → implementation → effect) reduce effectiveness
- Demand stimulus doesn't raise long-run productive potential — only supply-side policies shift LRAS
Essay Tip: "AQA examiners want you to distinguish between short-run (AD-side) and long-run (AS-side) effects. AD stimulus helps in a recession when there's spare capacity — the multiplier is larger and inflation risk is lower. Near full employment, the same stimulus generates inflation rather than output. Always contextualise your evaluation with the state of the economy."
Glossary
Key Terms
Aggregate Demand
Total demand for goods and services in an economy at a given price level in a given time period. AD = C + I + G + (X – M). The AD curve slopes downward due to the wealth, interest rate, and international substitution effects.
Marginal Propensity to Consume (MPC)
The fraction of additional income that is spent on consumption. MPC = ΔC/ΔY. Range: 0 to 1. Lower-income households tend to have a higher MPC. Determines the size of the Keynesian multiplier.
The Multiplier
k = 1/(1–MPC) = 1/MPS. An initial injection of spending leads to a larger final change in GDP through successive rounds of spending. Reduced by leakages: savings, taxation, and imports.
Accelerator Effect
Investment depends on the rate of change of GDP, not just its level. A slowdown in GDP growth causes a proportionally larger fall in investment. Amplifies economic cycles — investment is the most volatile component of AD.
Automatic Stabilisers
Fiscal mechanisms that automatically cushion AD without deliberate policy: welfare spending rises and tax revenues fall in a recession (supporting AD); both reverse in a boom (dampening inflation). Reduce the amplitude of the economic cycle.
Net Exports (X – M)
Exports minus imports. UK typically has negative net exports (trade deficit). Improved by exchange rate depreciation (if Marshall-Lerner condition holds), lower domestic inflation, or higher income in trading partner countries.
Question 1 of 8 · Aggregate Demand
Which of the following correctly states the formula for Aggregate Demand?
B
AD = C + I + G + (X – M)
C
AD = C + S + G + (X – M)
D
AD = C + I + T + (X – M)
Answer · Question 1
Which of the following correctly states the formula for Aggregate Demand?
B
AD = C + I + G + (X – M)
C
AD = C + S + G + (X – M)
D
AD = C + I + T + (X – M)
Correct: B. AD = Consumption + Investment + Government Spending + Net Exports (Exports minus Imports). Option A forgets to subtract imports. Option C substitutes savings (S) for investment — savings is a leakage, not a component of AD. Option D substitutes taxation (T) for investment. Net exports (X – M) can be negative if imports exceed exports, as is the case for the UK.
Question 2 of 8 · Aggregate Demand
If the marginal propensity to consume (MPC) is 0.75, what is the Keynesian multiplier?
Answer · Question 2
If the marginal propensity to consume (MPC) is 0.75, what is the Keynesian multiplier?
Correct: C. Multiplier k = 1 / (1 – MPC) = 1 / (1 – 0.75) = 1 / 0.25 = 4. MPS (marginal propensity to save) = 1 – MPC = 0.25. So k = 1/MPS = 1/0.25 = 4. This means a £1bn government spending injection ultimately generates £4bn of additional GDP. Option B (1.33) would result from using k = 1/(1–0.25) — incorrectly treating MPS as the denominator's complement.
Question 3 of 8 · Aggregate Demand
Which of the following would cause a rightward shift in the AD curve?
A
An increase in the general price level
B
A rise in interest rates set by the Bank of England
C
A fall in consumer confidence
D
An increase in government spending on infrastructure
Answer · Question 3
Which of the following would cause a rightward shift in the AD curve?
A
An increase in the general price level
B
A rise in interest rates set by the Bank of England
C
A fall in consumer confidence
D
An increase in government spending on infrastructure
Correct: D. Higher government spending (G) directly increases the G component of AD = C + I + G + (X–M), shifting the curve rightward at every price level. Option A is a movement along the AD curve (price level change), not a shift. Option B (higher interest rates) reduces C and I — shifting AD left. Option C (falling confidence) reduces consumer spending — also shifting AD left.
Question 4 of 8 · Aggregate Demand
According to the accelerator effect, what happens to investment if GDP is still growing but the rate of growth slows from 4% to 1%?
A
Investment rises because GDP is still growing
B
Investment is unchanged because the economy hasn't entered recession
C
Investment falls sharply because firms need less additional capital
D
Investment falls to zero because growth is expected to stop
Answer · Question 4
According to the accelerator effect, what happens to investment if GDP is still growing but the rate of growth slows from 4% to 1%?
A
Investment rises because GDP is still growing
B
Investment is unchanged because the economy hasn't entered recession
C
Investment falls sharply because firms need less additional capital
D
Investment falls to zero because growth is expected to stop
Correct: C. The accelerator effect links investment to the rate of change of output. When GDP growth was 4%, firms needed to expand their capital stock significantly. When growth slows to 1%, firms only need a fraction of the new capital they previously required — so investment falls sharply even though GDP is still rising. This is why investment is the most volatile component of AD and amplifies economic cycles — a slowdown in growth triggers a disproportionate fall in investment.
Question 5 of 8 · Aggregate Demand
How do automatic stabilisers help to stabilise aggregate demand during a recession?
A
The government automatically cuts interest rates to stimulate borrowing
B
Welfare spending rises and tax revenues fall, supporting incomes and AD without deliberate policy action
C
The central bank automatically increases the money supply to fund government deficits
D
Firms automatically invest more when consumer spending falls
Answer · Question 5
How do automatic stabilisers help to stabilise aggregate demand during a recession?
A
The government automatically cuts interest rates to stimulate borrowing
B
Welfare spending rises and tax revenues fall, supporting incomes and AD without deliberate policy action
C
The central bank automatically increases the money supply to fund government deficits
D
Firms automatically invest more when consumer spending falls
Correct: B. Automatic stabilisers are built-in fiscal mechanisms. In a recession: unemployment rises → welfare payments (Universal Credit, JSA) automatically increase (G↑); falling incomes reduce income tax and National Insurance revenues (T↓). Both effects widen the deficit but support household incomes and thus C component of AD — without requiring deliberate Parliamentary votes. This contrasts with discretionary fiscal policy (like a budget announcement) which faces implementation lags.
Question 6 of 8 · Aggregate Demand
The Marshall-Lerner condition states that a depreciation of the exchange rate will improve the current account (net exports) only if:
A
The price of exports falls by more than the price of imports rises
B
The sum of the price elasticities of demand for exports and imports is greater than 1
C
The exchange rate falls by at least 10% in real terms
D
Trading partners also depreciate their currencies simultaneously
Answer · Question 6
The Marshall-Lerner condition states that a depreciation of the exchange rate will improve the current account (net exports) only if:
A
The price of exports falls by more than the price of imports rises
B
The sum of the price elasticities of demand for exports and imports is greater than 1
C
The exchange rate falls by at least 10% in real terms
D
Trading partners also depreciate their currencies simultaneously
Correct: B. The Marshall-Lerner condition: PED(exports) + PED(imports) > 1. If demand for both exports and imports is sufficiently elastic, a depreciation boosts export volumes and reduces import volumes enough to improve net exports. If combined PED < 1 (inelastic demand), the volume response is insufficient to offset the price effects and net exports worsen. In the short-run (J-curve), elasticities tend to be low — so the current account initially worsens before improving as contracts adjust.
Question 7 of 8 · Aggregate Demand
Consumer spending (C) accounts for approximately what percentage of UK aggregate demand?
Answer · Question 7
Consumer spending (C) accounts for approximately what percentage of UK aggregate demand?
Correct: C. UK consumer spending (C) is approximately 65% of GDP — by far the largest component of AD. This is why consumer confidence, house prices, interest rates, and real wages are so important: changes in C have a major impact on total AD. For comparison: Investment (I) ≈ 17%, Government spending (G) ≈ 20%, Net exports ≈ −2% (the UK runs a persistent trade deficit). AQA examiners often ask students to use these approximate figures in data response questions.
Question 8 of 8 · Aggregate Demand
Why is the net exports component (X – M) typically negative for the UK?
A
UK exports have declined to zero because of Brexit trade barriers
B
The UK imports more goods and services than it exports, running a persistent trade deficit
C
The UK government taxes exports but subsidises imports, distorting the balance
D
UK consumers prefer foreign goods for quality reasons regardless of price
Answer · Question 8
Why is the net exports component (X – M) typically negative for the UK?
A
UK exports have declined to zero because of Brexit trade barriers
B
The UK imports more goods and services than it exports, running a persistent trade deficit
C
The UK government taxes exports but subsidises imports, distorting the balance
D
UK consumers prefer foreign goods for quality reasons regardless of price
Correct: B. The UK has run a persistent current account deficit for decades — imports of goods and services consistently exceed exports, making net exports (X – M) negative (approximately −2% of GDP). This is primarily a structural trade deficit in goods (especially manufactured goods), partly offset by a surplus in services (finance, insurance, education). A negative net exports figure acts as a drag on AD. The UK's relatively low savings rate and high consumption of imported goods are key drivers of this persistent deficit.
Summary
Key Takeaways: Aggregate Demand
THE FORMULA
AD = C + I + G + (X – M). UK composition: C ≈ 65%, I ≈ 17%, G ≈ 20%, Net X ≈ −2%. The AD curve slopes downward due to wealth, interest rate, and international substitution effects. Shifts in AD occur when any component changes at every price level.
THE MULTIPLIER
k = 1 / (1 – MPC) = 1 / MPS. Reduced by leakages (savings, tax, imports): k = 1 / (MPS + MPT + MPM). Largest when spare capacity exists, MPC is high, and leakages are low. UK estimates: 0.6–1.5. The austerity debate centres on whether the multiplier was underestimated post-2008.
ACCELERATOR & VOLATILITY
Investment is the most volatile component. The accelerator links investment to the rate of change of GDP — a slowdown in growth triggers a disproportionate fall in investment. Animal spirits (Keynes) means investment is also driven by expectations and confidence.
AQA EXAM FOCUS
Distinguish movement along vs shift of AD curve. Know that G excludes transfer payments. Apply Marshall-Lerner and J-curve to exchange rate questions. Always evaluate multiplier size relative to state of economy. AD only affects cyclical output — LRAS determines trend growth potential.
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Lesson Complete
You've covered aggregate demand components (C, I, G, X–M), the Keynesian multiplier and accelerator effect, AD curve shifts, real-world policy applications, and evaluated demand-management effectiveness.