The components of aggregate demand, the determinants of consumption, investment, government spending and net trade, and the shape and shifts of the AD curve.

What this dot point is asking

Edexcel wants you to define aggregate demand and its four components, explain what determines each component, and explain why the AD curve slopes down and what shifts it.

The components of aggregate demand

Consumption is the largest component (around 60 per cent of UK AD). The marginal propensity to consume (MPC) is the fraction of any extra income that is spent; the marginal propensity to save (MPS) is the fraction saved, and $MPC + MPS = 1$ in a closed economy with no tax.

What determines each component

• Consumption depends on disposable income, interest rates, consumer confidence, wealth effects (such as house prices and share values) and the availability of credit.
• Investment depends on interest rates, business confidence ("animal spirits"), expected demand and profit, corporation tax, the cost of capital goods and the accelerator effect.
• Government spending is set by fiscal policy and the position in the economic cycle; automatic stabilisers raise it in a downturn.
• Net trade depends on UK and overseas incomes, the exchange rate, and the relative price and quality of UK goods.

Why the largest components move first

Because consumption is around 60 per cent of UK AD, even a small percentage change in $C$ shifts the AD curve more than a similar change in $G$ or $X - M$. Investment is the most volatile component: it is forward-looking, so it reacts sharply to changes in expected demand and confidence even before income itself has changed. This is why investment swings amplify the business cycle, while consumption tends to be smoothed by households drawing on savings and credit. When you analyse a shock, identify which component moves, by how much, and then trace the multiplier effect through the rest of the flow.

The marginal propensity to consume in practice

The MPC is not a fixed number: it tends to be higher for low-income households, who spend a larger share of any extra income, and lower for the wealthy, who save more at the margin. This matters for policy. A tax cut targeted at low earners injects more spending into AD than the same sum returned to high earners, because the average MPC of the recipients is larger. It also means the consumption response to a shock is uneven across the income distribution: the 2022 cost-of-living squeeze hit lower-income households hardest precisely because they spend most of their income on energy and food, where prices rose fastest, leaving little room to absorb the shock by cutting saving.

The AD curve

Evaluating the size of an AD shift

When you assess any change in AD, weigh three things. Magnitude: how large is the affected component? A confidence shock to consumption (60 per cent of AD) matters far more than the same percentage move in net trade. Time lags: monetary policy works with a lag of up to 18 months to two years, so a rate cut today does not lift AD immediately, while fiscal changes can act faster. Ceteris paribus: the four components rarely move alone. A sterling depreciation that helps net trade also raises import prices, squeezing real incomes and consumption, so the net effect on AD is ambiguous and depends on the price elasticity of demand for exports and imports (the Marshall-Lerner condition). A strong answer reaches a judgement on whether the AD shift is likely to be large and lasting or small and temporary.

Examples in context

• COVID-19 shock (2020). UK consumption collapsed as lockdowns shut shops and confidence fell, cutting AD sharply; the furlough scheme and emergency rate cuts to $0.1\%$ aimed to offset it. UK GDP fell by around 11 per cent in 2020, the deepest annual fall in over three centuries, before rebounding as restrictions eased.
• House prices and the wealth effect. Rising UK house prices in the 2010s lifted homeowner wealth and consumption, illustrating the wealth determinant of C. The reverse effect appeared in 2023 as higher mortgage rates cooled the housing market and dampened spending.
• Sterling depreciation (2016). After the Brexit referendum the pound fell around 10 per cent, making UK exports cheaper and imports dearer, supporting net trade over time, but also pushing CPI inflation above 3 per cent by late 2017 as import costs rose.
• Business investment and uncertainty. UK business investment stagnated from 2016 to 2019 as Brexit uncertainty dampened "animal spirits", showing how confidence drives I and how policy uncertainty can hold back AD even when interest rates are low.
• Cost-of-living squeeze (2022). Energy and food price spikes after the Ukraine invasion cut real disposable incomes, weakening consumption despite a tight labour market.
• Fiscal stimulus and government spending (2008 to 2009). During the global financial crisis the UK temporarily cut VAT from 17.5 per cent to 15 per cent and brought forward capital spending, a deliberate use of the $G$ and $C$ components to prop up AD after private demand collapsed. The chain of analysis runs: lower VAT raises real disposable income, which lifts consumption, which raises firms' revenue and expected demand, which supports investment, with the multiplier amplifying the total effect on national income. The episode shows fiscal policy directly targeting the AD components when monetary policy alone (rates already near their floor) had limited room to act.

Try this

Q1. State the four components of aggregate demand. [2 marks]

• Cue. Consumption, investment, government spending and net trade (exports minus imports).

Q2. Explain two factors that could increase consumption in an economy. [4 marks]

• Cue. Lower interest rates, rising confidence, higher wealth (house prices) or easier credit each raise consumer spending.