How do aggregate demand and supply determine national output and the price level, and how does a change ripple through the economy?
The aggregate demand and supply model: the components of aggregate demand, short-run and long-run aggregate supply, macroeconomic equilibrium, and the multiplier and accelerator effects of a change in spending.
An SQA Advanced Higher Economics answer on the aggregate demand and supply model: the components of aggregate demand, the shapes of short-run and long-run aggregate supply, macroeconomic equilibrium, and how the multiplier and accelerator magnify a change in spending through the circular flow.
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What this key area is asking
The aggregate demand and supply (AD/AS) model is the central diagram of the macroeconomic area, the macro counterpart of demand and supply. You must know the components of aggregate demand, the shapes and shifts of short-run and long-run aggregate supply, how they combine to set macroeconomic equilibrium (national output and the price level), and how a change in spending is magnified through the multiplier and accelerator. This machinery is then used to analyse every macro policy.
Aggregate demand and its components
The AD curve slopes down against the price level (a higher price level reduces real wealth, raises interest rates and worsens competitiveness, all cutting spending). It shifts when any component changes for a reason other than the price level: consumer confidence and income (C), interest rates and business confidence (I), fiscal policy (G), and the exchange rate and world income (X and M).
Aggregate supply: short run and long run
- Short-run aggregate supply (SRAS) slopes up: with money wages and some costs fixed, a higher price level widens profit margins, so firms supply more. It shifts with costs of production (wages, raw materials, the exchange rate, taxes).
- Long-run aggregate supply (LRAS) reflects the economy's productive capacity and is independent of the price level. It is commonly drawn vertical at the full-employment (potential) level of output. It shifts right with potential growth: more or better factors of production, investment, technology and productivity.
Macroeconomic equilibrium
Equilibrium real output and the price level are set where aggregate demand meets aggregate supply:
The effect of a shift in AD depends on where the economy sits on the AS curve:
- Spare capacity (elastic SRAS): a rightward AD shift mainly raises real output with little price rise.
- Near full capacity (steep SRAS or vertical LRAS): the same shift mainly raises the price level (demand-pull inflation) with little extra output.
This is why the output gap matters so much for policy: stimulus works on output when there is slack but just raises prices when there is not.
The multiplier
The multiplier connects to the circular flow of income: injections (investment, government spending, exports) add to the flow, withdrawals/leakages (saving, taxation, imports) drain it. National income settles where planned injections equal planned withdrawals; a change in injections is multiplied because withdrawals are only a fraction of each round.
The accelerator
The accelerator links investment to the rate of change of output: firms invest in new capital when demand and output are rising quickly, and cut investment when growth slows. Combined with the multiplier, the accelerator can amplify the business cycle: rising output prompts more investment (an injection), which the multiplier magnifies into still higher income, and the reverse on the way down.
Worked example: a multiplied rise in government spending
Why this matters
AD/AS is the workhorse diagram of the whole macroeconomic area: every fiscal, monetary and supply-side policy is analysed as a shift of one of these curves. The multiplier explains why a modest change in government spending or investment can have a much larger effect on national income, which is central to evaluating fiscal policy, and the accelerator helps explain the volatility of the business cycle. Master this model and the policy topics become applications of it.
Try this
Q1. Name the four components of aggregate demand. [2 marks]
- Cue. Consumption (C), investment (I), government spending (G) and net exports (exports minus imports, ).
Q2. If the marginal propensity to withdraw is 0.25, calculate the multiplier and state what it means. [2 marks]
- Cue. : an initial injection leads to a final rise in national income four times as large, because only a quarter of each round's income leaks out.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH (style)10 marksUsing an aggregate demand and supply diagram, explain how an increase in investment affects the level of national output and the price level.Show worked answer →
Worth 10 marks: the AD shift (about 4 marks), the AS interaction (about 4 marks) and the multiplier (about 2 marks).
The AD shift (about 4 marks). Investment (I) is a component of aggregate demand (), so a rise in investment shifts the AD curve to the right. At the original price level, total planned spending now exceeds output.
The AS interaction (about 4 marks). The effect on output and prices depends on where the economy is on the aggregate supply curve. With spare capacity (the elastic part of short-run AS), the rightward AD shift raises real output with little rise in the price level. Near full capacity (the steep part), the same shift mainly raises the price level with little extra output, causing demand-pull inflation.
The multiplier (about 2 marks). The initial rise in investment has a multiplied effect: the spending becomes income for others, who spend a fraction of it, and so on, so the final rise in AD and output exceeds the initial injection. Note the accelerator may then raise investment further.
SQA AH (style)10 marksExplain the multiplier effect, and how the size of the multiplier depends on the marginal propensities to withdraw from the circular flow.Show worked answer →
Worth 10 marks: the mechanism (about 5 marks) and the size (about 5 marks).
The mechanism (about 5 marks). The multiplier is the process by which an initial change in injections (investment, government spending or exports) leads to a larger final change in national income. The initial spending becomes income for households; they spend a fraction of it, which becomes income for others, who spend a fraction again, and so on. Each round is smaller, so the total settles at a finite multiple of the initial change.
The size (about 5 marks). The multiplier , where is the marginal propensity to withdraw (save, tax and import out of each extra pound of income), . The larger the withdrawals, the smaller the multiplier, because less of each round's income is passed on. For example, with the multiplier is 2; with it is 4. A high marginal propensity to consume (low withdrawals) gives a large multiplier.
Related dot points
- Economic growth and the business cycle: measuring real GDP and output gaps, the phases and causes of the cycle, the distinction between actual and potential growth, and the benefits and costs of growth including sustainability.
An SQA Advanced Higher Economics answer on economic growth and the business cycle: measuring growth with real GDP, the difference between actual and potential growth, output gaps, the phases and causes of the business cycle, and a balanced evaluation of the benefits and costs of growth including sustainability.
- Inflation and unemployment: their measurement, types and causes, their economic costs, and the Phillips curve relationship between them in the short run and long run.
An SQA Advanced Higher Economics answer on inflation and unemployment: how each is measured, the types and causes of inflation (demand-pull and cost-push) and unemployment, their economic costs, and the Phillips curve relationship showing a short-run trade-off but a vertical long-run curve at the natural rate.
- Fiscal and monetary policy: government spending, taxation and the budget; the role of the central bank, interest rates and quantitative easing; how each affects aggregate demand; and an evaluation of their effectiveness.
An SQA Advanced Higher Economics answer on demand-side policy: fiscal policy (government spending, taxation, the budget balance and the national debt), monetary policy (the central bank, interest rates and quantitative easing), how each shifts aggregate demand, and a balanced evaluation of their effectiveness and limitations.
- Supply-side policy and the Scottish economy: market-based and interventionist supply-side measures and their effects on long-run aggregate supply, and the division of economic powers between the Scottish and UK governments under devolution and the fiscal framework.
An SQA Advanced Higher Economics answer on supply-side policy and the Scottish economy: market-based and interventionist supply-side measures and how they shift long-run aggregate supply, plus the division of devolved and reserved economic powers between the Scottish and UK governments and the Scottish fiscal framework.
- Exchange rates and the balance of payments: the structure of the balance of payments, floating and fixed exchange rate systems, how a floating rate is determined and what causes appreciation and depreciation, and the effects of a changing exchange rate on the current account.
An SQA Advanced Higher Economics answer on exchange rates and the balance of payments: the structure of the balance of payments, fixed and floating exchange rate systems, how a floating rate is determined and the causes of appreciation and depreciation, and how a changing exchange rate affects the current account, including the Marshall-Lerner condition and the J-curve.
Sources & how we know this
- Advanced Higher Economics Course Specification — SQA (Qualifications Scotland) (2024)