Lesson preview · Aggregate Demand
Aggregate Demand
~14 min · Free to read
In late 2008, frightened American households slammed their wallets shut. Retail sales fell, so factories cut production, so workers were laid off — and those workers then spent even less. In 2020 the government ran the tape backwards: it mailed cheques to millions of households, spending surged, restaurants filled up, and firms started hiring again. Same economy, opposite directions.
What moved in both stories was aggregate demand — the total spending on everything a country produces. The whole short-run story of booms and recessions is a story about aggregate demand rising and falling. So we need to pin down exactly what it is and how to draw it.
Key Term
Aggregate demand — who is doing the buying
Aggregate demand (AD) is the total amount of a country’s output that everyone plans to buy at a given price level. “Everyone” is four groups of buyers:
- Consumers (C) — households buying goods and services. The biggest chunk, around two-thirds of spending.
- Investment (I) — firms buying machines, factories, and buildings. (Not stocks and shares — real capital.)
- Government (G) — the state buying roads, defence, salaries.
- Net exports (NX) — what foreigners buy from us (exports) minus what we buy from them (imports).
Aggregate demand is the sum of consumption, investment, government spending, and net exports — the same four components that make up GDP on the spending side.
Key Term
The aggregate demand curve
Plot the overall price level (a price index for the whole economy, like the CPI) up the vertical axis, and real GDP (the total quantity of output demanded, with inflation stripped out) along the horizontal. The line you get — AD — slopes downward: the lower the price level, the more output buyers want.
It looks like an ordinary demand curve for a single product, but do not be fooled. It slopes down for completely different reasons.
Key Term
Three reasons AD slopes down
Wealth effect. A lower price level makes the money sitting in your bank account worth more — it buys more. Households feel richer, so they spend more (C rises).
Interest-rate effect. With lower prices, people need less cash for day-to-day spending. The spare money gets lent out, which pushes interest rates down, so firms borrow and invest more (I rises).
Net-export effect. When our prices fall relative to other countries’, our goods look cheap abroad. Exports rise and imports fall (NX rises).
All three push the same way: a lower price level means more total spending. That is the whole reason AD slopes down.
That is the takeaway: a lower price level lifts spending through all three channels at once, which is exactly why aggregate demand slopes downward. Notice what did not appear — nobody “switched to a cheaper substitute,” because when the whole price level falls there is no other economy to switch to.
Trap · Common Misconception
"AD slopes down for the same reason a normal demand curve does"
A single product’s demand curve slopes down because when its price rises, buyers switch to substitutes and feel a little poorer. Neither reason works for the whole economy. When the overall price level rises, there is no “other product” to switch to — everything is in the index. And one person’s higher price is another person’s higher income, so the economy as a whole isn’t straightforwardly poorer. The AD curve slopes down only because of the wealth, interest-rate, and net-export effects — three genuinely macro forces.
Key Term
Movement along vs a shift of AD
Keep two different moves apart:
- A change in the price level moves you along a fixed AD curve (the dot slides up or down the line). This is what the widget shows.
- A change in spending at every price level — because C, I, G, or NX changed for some other reason — shifts the whole curve left or right.
More spending at every price level shifts AD right; less spending shifts it left.
Tip
What shifts aggregate demand
Work through the four components — anything that changes one of them (other than the price level) shifts AD:
- C — consumer confidence, household wealth, income taxes, expectations of the future.
- I — business confidence, interest rates set by the central bank, new technology, corporate taxes.
- G — government choosing to spend more or less (fiscal policy).
- NX — foreign incomes, the exchange rate, and tariffs.
The 2008 confidence collapse (C down) shifted AD left; the 2020 stimulus cheques (C up, via G) shifted it right.
Check yourself · no marks
A central bank raises interest rates to cool the economy. Borrowing gets more expensive. Does this move the economy along the AD curve, or shift the whole curve — and which way?
Commit to one first. Guessing and being wrong beats reading the answer cold.
Worked Example
An economy’s spending components are: consumption 70, investment 22, government 20, and net exports 8 (all in trillion dollars). (a) Find aggregate demand. (b) A recession overseas then cuts demand for this country’s exports, lowering net exports from 8 to 4. Find the new aggregate demand, and say whether the AD curve shifts or the economy simply moves along it — and in which direction.
Practice · 1 / 4
The aggregate demand curve slopes downward. Which set of reasons explains this?
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