Lesson preview · Demand Shocks
Demand Shocks
~12 min · Free to read
In 1929 the American economy fell off a cliff. Stock prices crashed, banks failed, and frightened households and firms stopped spending almost overnight. By 1933 output had shrunk by roughly a quarter and one worker in four had no job. Prices did not climb during this misery — they collapsed too, falling around a quarter. That is the Great Depression, the largest fall in total spending the modern world has seen.
In the spring of 2020 it happened again, faster. Lockdowns shut shops and grounded planes, so spending plunged and unemployment spiked within weeks. Then governments mailed cheques and central banks cut rates, spending came roaring back, and the economy recovered. Both stories are about one thing: a sudden swing in total spending. Economists call such a swing a demand shock.
Key Term
Demand shock — a sudden shift of the AD curve
A demand shock is a sudden change in total planned spending that shifts the whole aggregate-demand (AD) curve — caused by a change in consumption (C), investment (I), government spending (G), or net exports (NX) for a reason other than the price level.
- A negative demand shock is a fall in spending. AD shifts left. Example: the confidence collapse of the Great Depression.
- A positive demand shock is a jump in spending. AD shifts right. Example: the stimulus cheques of 2020.
It is a shift of AD, not a slide along it, because the trigger is a non-price change in one of the four components.
Recall the picture you already have. Equilibrium sits where aggregate demand crosses short-run aggregate supply (SRAS) — the upward-sloping curve showing what firms produce while wages are still sticky. The vertical dashed line is long-run aggregate supply (LRAS), fixed at potential output, the amount the economy makes at full employment. A demand shock slides AD across that fixed SRAS, and the new crossing point tells us what happens to output and prices.
Here is the result that matters. Because SRAS slopes up, when AD shifts left the equilibrium slides down the SRAS curve — real GDP and the price level both fall. When AD shifts right the equilibrium slides up — both rise. A demand shock moves output and the price level in the same direction. Always.
That is the whole takeaway. Because the shock drags AD along an upward-sloping SRAS, output and the price level always move together — both down for a negative shock, both up for a positive one. That same-direction movement is the fingerprint of a demand shock.
Tip
Demand shock vs supply shock — the diagnostic
This same-direction result is how you tell the two kinds of shock apart on a diagram.
- Demand shock (AD moves): output and the price level go the same way. Recession with falling prices, or boom with rising prices.
- Supply shock (SRAS moves): output and the price level go opposite ways. The 1973 oil spike cut output while prices rose — stagflation.
So if you see output and prices both fall, suspect a demand shock. If you see them split, suspect a supply shock.
Trap · Common Misconception
"A recession always means falling output AND rising prices"
Many students picture every recession as output down, prices up. That is a supply-shock recession, like 1973. A demand-driven recession is the opposite: spending dries up, AD shifts left, and the economy slides down the SRAS — so the price level falls (or at least inflation slows) at the same time as output. The Great Depression brought roughly a quarter fall in prices alongside mass unemployment. Falling prices and falling output together is the normal signature of a negative demand shock, not a contradiction.
Now the two cases the lesson is built on. The Great Depression was a huge negative demand shock: confidence and credit evaporated, C and I collapsed, AD lurched left, and output, prices, and employment all sank together — deflation plus mass unemployment. The year 2020 was a sharp negative demand shock too — lockdowns crushed spending — but it was followed by a powerful positive shock as stimulus cheques and rate cuts pushed AD back to the right and spending recovered.
One short word on the long run. When a negative shock pushes output below potential, the gap between them — output sitting under the LRAS line — is called a recessionary gap. The economy can close it on its own: high unemployment eventually drags wages down, which shifts SRAS right and lifts output back to potential. Or policymakers can close it faster by shifting AD back to the right — a fiscal boost (more G or tax cuts) or a monetary boost (lower interest rates). Either way the destination is potential output.
Check yourself · no marks
After a positive demand shock, what happens to real GDP, the price level, and unemployment?
Commit to one first. Guessing and being wrong beats reading the answer cold.
Worked Example
An economy starts at potential output, which we set to 100, with the price level also at 100. A wave of pessimism then strikes: households and firms lose confidence and cut spending hard. This negative demand shock shifts aggregate demand left, and the new short-run equilibrium settles at output 90 and a price level of 95. (a) State the direction of real GDP, the price level, and unemployment. (b) Name the kind of gap this opens. (c) Briefly, what would a positive shock of similar size have done instead?
Picking which curve moves is the real skill here — most AD-AS mistakes come from shifting the wrong curve, not from shifting the right curve the wrong way.
Draw the shift, mark the new equilibrium, explain the gap — that is the complete exam recipe for any demand shock, and it runs in reverse for positive shocks.
Practice · 1 / 4
A negative demand shock hits an economy that was at potential output. In the short run, what happens to real GDP and the price level?
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