Aggregate Demand (AD): Study Guide (Macroeconomics 3.1)

What aggregate demand means

In Unit 1 you studied demand for one good. In Unit 3 the question is bigger: how much of everything the economy produces do buyers want, and how does that depend on the general level of prices?

Aggregate demand (AD) is the total quantity of final goods and services (real GDP) that all buyers in the economy want to purchase at each possible price level, in a given period, with other influences held constant.

There are four groups of buyers. They are the same four parts you met in the expenditure approach to GDP (Topic 2.1):

Buyer	Component of spending	Symbol
Households	Consumption	C
Firms	Investment (new capital goods, new buildings, additions to inventories)	I
Government	Government spending on goods and services	G
The rest of the world	Net exports (exports − imports)	NX

So, at any price level:

Quantity of real GDP demanded = C + I + G + NX

Two terms you must keep apart, just as in Unit 1:

• Quantity of real GDP demanded: the total amount buyers want at one price level.
• Aggregate demand: the whole relationship between the price level and the quantity of real GDP demanded, shown as a schedule or a curve.

The axes

The vertical axis shows the price level (PL), an index such as the CPI or GDP deflator. The horizontal axis shows real GDP (Y), total output measured at constant prices. Using “Price” and “Quantity” here is a common exam error: it confuses the whole-economy model with a single market.

Why the AD curve slopes downward

When the price level rises, the quantity of real GDP demanded falls. When it falls, the quantity demanded rises. There are three reasons, and each works through a different component of spending.

1. Real wealth effect (works through C). Households hold money, savings accounts and other assets with a fixed money value. A higher price level reduces what that wealth can buy. Households feel poorer, so they cut consumption. A lower price level does the opposite.

2. Interest rate effect (works mainly through I). At a higher price level, people and firms need more money to buy the same goods, so the demand for money rises. This pushes interest rates up. Borrowing becomes more expensive, so firms cut investment and households cut interest-sensitive purchases such as cars and houses.

3. Exchange rate effect (works through NX). The higher interest rates also attract foreign savers, who buy domestic financial assets. To do this they need the domestic currency, so the currency appreciates (rises in value). Domestic goods now cost more for foreign buyers and imports are cheaper for domestic buyers. Exports fall and imports rise, so net exports fall. You can also see this through relative prices: if domestic prices rise while foreign prices do not, domestic goods are relatively dearer, which again lowers net exports.

Notice that government spending (G) does not respond to the price level in this model. The three effects come from C, I and NX.

Why not “people switch to other goods”? The demand curve for one good slopes down partly because buyers substitute towards other, now relatively cheaper, goods. On the AD curve all prices are rising together, so that reason does not apply. Use the three effects instead.

What shifts the AD curve

Any change in C, I, G or NX that happens for a reason other than a change in the price level shifts the whole AD curve. More spending at every price level shifts AD right (an increase in AD). Less spending at every price level shifts AD left (a decrease in AD).

Component	Things that shift AD right (the opposite shifts it left)
Consumption (C)	Higher household wealth from rising share or house prices; higher consumer confidence; lower personal income taxes or higher transfer payments (more disposable income); less household debt
Investment (I)	Lower interest rates set by the central bank; more optimistic business expectations; new technology that makes new capital profitable; lower business taxes
Government spending (G)	The government decides to spend more on goods and services, such as roads, schools or defence
Net exports (NX)	Higher incomes in trading-partner countries; a depreciation of the domestic currency for reasons other than the price level; stronger foreign tastes for the country’s goods

Two points need care:

• Taxes and transfers work through C (or I). A cut in personal income tax does not shift AD by itself. It raises disposable income, households spend more, and that shifts AD.
• Interest rates can cause a movement or a shift. If interest rates rise because the price level rose, that is the interest rate effect: a movement along AD. If the central bank raises interest rates as a policy decision, investment falls at every price level: AD shifts left. Ask what caused the change.

Worked example 1: which component, which direction?

Valdoria is the fictional country used across the Marlbridge macroeconomics pages. Its currency is the valda (VD).

Question. For each Valdorian news headline, name the component of aggregate demand affected and say whether AD shifts right, shifts left or does not shift.

1. “Share prices rise 20%; households feel richer”
2. “Valdoria’s main export market slides into recession”
3. “Firms expect record sales and order new machinery”
4. “Parliament raises personal income tax rates”
5. “Valdoria’s price level rises 4% this year”
6. “Central bank cuts interest rates to support borrowing”

Answers.

1. C rises. Higher household wealth that is not caused by the price level raises consumption at every price level. AD shifts right.
2. NX falls. Foreign incomes fall, so foreigners buy fewer Valdorian exports. AD shifts left.
3. I rises. Better business expectations raise investment at every price level. AD shifts right.
4. C falls. Higher taxes reduce disposable income, so households spend less. AD shifts left.
5. No shift. A change in the price level causes a movement up along AD: the quantity of real GDP demanded falls through the three effects.
6. I rises (and interest-sensitive C). The central bank’s decision lowers borrowing costs at every price level. AD shifts right.

Check. Headlines 5 and 6 both involve interest rates or prices, but only 6 shifts AD. In 5 the price level itself changed; in 6 something other than the price level changed.

Worked example 2: reading an AD schedule

Question. The table shows Valdoria’s planned spending by each sector at five price levels (VD billion, at base-year prices; all data fictional).

Price level	C	I	G	NX	Real GDP demanded
90	615	160	200	55	?
100	600	150	200	50	?
110	585	140	200	45	?
120	570	130	200	40	?
130	555	120	200	35	?

(a) Complete the last column. (b) The price level rises from 110 to 120. By how much does the quantity of real GDP demanded change? Link the change in each component to one of the three effects. (c) Business confidence then falls sharply. Investment drops by VD 10 billion at every price level, and after the knock-on rounds of spending (Topic 3.2) the quantity of real GDP demanded is VD 50 billion lower at every price level. Describe the new schedule and what happens to the AD curve.

(a) Add C + I + G + NX in each row: 615 + 160 + 200 + 55 = 1,030; 600 + 150 + 200 + 50 = 1,000; 970; 940; 910.

(b) Real GDP demanded falls from 970 to 940, a change of −VD 30 billion (about −3.09%). This is a movement up along the AD curve. The parts:

• C falls by 15 (585 → 570): the real wealth effect.
• I falls by 10 (140 → 130): the interest rate effect.
• NX falls by 5 (45 → 40): the exchange rate effect.
• G does not change (200).

Check: −15 − 10 − 5 + 0 = −30. ✓

(c) New schedule (AD₂): 980 at PL 90, 950 at 100, 920 at 110, 890 at 120 and 860 at 130. Every value is 50 lower, so the whole curve shifts left by VD 50 billion. The price level did not cause this change; falling business confidence did.

Interpretation. On AD₂, if the price level then fell from 120 to 110, the quantity demanded would rise from 890 to 920. That is a movement along AD₂. Shifts and movements can happen one after the other; keep them separate in your explanation.

Common misconceptions

• “A rise in the price level decreases aggregate demand.” No. It decreases the quantity of real GDP demanded: a movement along AD. Only non-price-level changes shift AD.
• Labelling the axes “Price” and “Quantity”. Use Price level and Real GDP. The model is about the whole economy.
• Explaining the slope with substitution between goods. That reason belongs to single-market demand. For AD use the real wealth, interest rate and exchange rate effects.
• Mixing up the three effects. Wealth effect → consumption. Interest rate effect → investment. Exchange rate effect → net exports.
• “Any rise in interest rates is just the interest rate effect.” Only if the price level caused it. A policy rise in interest rates shifts AD left.
• “A tax cut shifts AD because the government spends more.” A tax cut does not change G. It raises disposable income, so C rises.
• “Higher imports shift AD right because people are buying more.” Imports are subtracted in NX. More spending on imports, other things equal, lowers NX and shifts AD left.
• “A currency appreciation shifts AD right because the currency is stronger.” It makes exports dearer and imports cheaper, so NX falls and AD shifts left.

Where this leads

Next, Topic 3.2, Multipliers, shows why a change in spending shifts AD by more than the first change, and how to calculate the size of the shift. Later in Unit 3 you will add aggregate supply to find equilibrium. Try the practice questions now, then use the revision notes and the checklist to consolidate.