How do AD and AS set the price level and output, and why does an injection raise income by more than itself?
Determine macroeconomic equilibrium using AD-AS and explain the multiplier process and the size of the multiplier
A focused answer to the H2 Economics learning outcome on macroeconomic equilibrium and the multiplier. How AD and AS set output and the price level, and why an injection raises national income by a multiple through the multiplier process.
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What this dot point is asking
SEAB wants you to determine macroeconomic equilibrium using the AD-AS model and to explain the multiplier process and what sets the size of the multiplier. The central insight is that AD and AS together fix output and the price level, and that any change in spending is amplified through the economy by the multiplier, with the amplification limited by leakages.
The answer
Macroeconomic equilibrium
Macroeconomic equilibrium is where aggregate demand equals aggregate supply, fixing the equilibrium real output and price level. If AD exceeds AS at the current price level, the price level and output are bid up; if AS exceeds AD, they fall, until the two are equal.
Why the effect of an AD shift depends on the AS curve
The same rightward shift in AD has very different effects depending on where the economy is on the AS curve:
- With large spare capacity (the flat, Keynesian range): a rise in AD raises real output substantially with little inflation, because idle resources can be brought into use without bidding up prices.
- Near full capacity (the steep or vertical range): a rise in AD mostly raises the price level (demand-pull inflation) with little extra output, because resources are scarce.
This is why demand-side policy is powerful in a deep recession but inflationary near full employment.
The multiplier process
The mechanism, round by round: an injection (say investment) is income for those who supply the goods. They spend a fraction of it (their marginal propensity to consume), which becomes income for others, who spend a fraction again. Each round is smaller because some income leaks out (to saving, tax and imports), so the series converges to a finite total greater than the initial injection.
The size of the multiplier
The smaller the leakages (the smaller MPW), the larger the multiplier; the larger the leakages, the smaller it is. So a high propensity to save, to tax, or to import gives a small multiplier.
Examples in context
Example 1. Why fiscal stimulus is muted in Singapore. With a very high marginal propensity to import, much of any extra domestic spending leaks abroad, so Singapore's expenditure multiplier is small. This is one reason the government relies more on the exchange rate and supply-side measures than on large discretionary spending to manage the economy.
Example 2. Stimulus in a recession versus a boom. A spending boost during a deep recession, when the economy sits on the flat range of AS with idle resources, raises output strongly with little inflation. The same boost during a boom, near full capacity, mostly raises prices. The AD-AS model explains why the timing of demand-side policy is decisive.
Try this
Q1. Define the multiplier. [2 marks]
- Cue. The ratio of the final change in national income to the initial change in injection that caused it; an injection raises income by a multiple of itself.
Q2. An economy has MPW of . Calculate the multiplier. [2 marks]
- Cue. .
Q3. Explain why an open economy with a high propensity to import has a small multiplier. [3 marks]
- Cue. A high marginal propensity to import means a large fraction of each round of extra spending leaks abroad on imports, raising MPW; since , a larger leakage gives a smaller multiplier.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original10 marksExplain how an increase in aggregate demand affects output and the price level, and why the effect depends on the position on the aggregate supply curve.Show worked answer →
A 10 mark question rewards the AD-AS equilibrium, the spare-capacity-versus-full-capacity contrast, and the multiplier.
- Equilibrium
- Macroeconomic equilibrium is where AD equals AS, setting the equilibrium real output and price level.
- AD rise with spare capacity
- Starting with a large output gap (Keynesian flat range), a rightward AD shift raises real output a lot with little price increase, because idle resources can be employed without bidding up prices.
- AD rise near full capacity
- Near or at full employment (steep or vertical range), the same AD shift mostly raises the price level (demand-pull inflation) with little extra output, because resources are scarce.
- Multiplier
- The initial AD rise sets off further rounds of spending, so the final increase in output exceeds the initial injection.
Markers reward the AD = AS equilibrium, the contrast between the flat and steep ranges of AS, and the multiplier amplifying the initial change.
Original9 marksExplain the multiplier process, and what determines the size of the multiplier.Show worked answer →
A 9 mark question rewards the round-by-round mechanism, the formula, and the leakage logic.
- The process
- An injection (say investment) becomes income for those who supply the goods. They spend part of it, which becomes income for others, who spend part again, and so on. Each round is smaller because some income leaks out, but the total rise in income exceeds the initial injection.
- The formula
- The multiplier , where MPW is the marginal propensity to withdraw (the fraction of extra income that leaks to saving, tax and imports). Equivalently using the marginal propensity to consume domestic output.
- Size
- The smaller the leakages (the smaller MPW), the larger the multiplier; the larger the leakages, the smaller it is. So a high saving, tax or import propensity gives a small multiplier.
Markers reward the round-by-round spending chain, the multiplier formula in terms of withdrawals, and the conclusion that larger leakages mean a smaller multiplier.
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