Aggregate demand

IB Economics HL· Unit 3: Macroeconomics, Topic 2: Aggregate Demand· 15 min read

1. Definition and Components of Aggregate Demand★★☆☆☆⏱ 4 min

📘 Definition

Aggregate Demand

AD

The total quantity of finished goods and services demanded by all sectors of an economy at a given average price level, in a given time period.

Example:

At an average price level of 100, aggregate demand in Economy A is \$2.1 trillion of real output.

Consumption (C): Total spending by households on durable goods, non-durable goods and services.
Investment (I): Total spending by firms on capital goods, plus inventory changes and residential construction.
Government Spending (G): Spending by government on new public goods and services (excludes transfer payments).
Net Exports ( $N X$ ): Value of exports ( $X$ ) minus value of imports ( $M$ ), so $N X = X - M$ .

AD = C + I + G + (X - M)

📐 Worked Example

Calculate aggregate demand for an economy with: Consumption = \ $1200 bn, I n v es t m e n t =$ 350bn, Government spending = \ $400 bn, E x p or t s =$ 280bn, Imports = \$320bn

1
Recall the standard AD formula:
2
$AD = C + I + G + X - M$
3
Substitute the given values into the formula:
4
$AD = 1200 + 350 + 400 + 280 - 320$
5
Calculate the final value of aggregate demand:
6
$AD = \\$1910\ \text{bn}$

Exam tip:

Never count transfer payments (unemployment benefits, pensions) in G — they only redistribute income, not purchase new output.

2. Why the Aggregate Demand Curve Slopes Downward★★★☆☆⏱ 5 min

Unlike a microeconomic demand curve for a single good, the downward slope of AD is explained by three distinct macroeconomic effects, not individual substitution effects.

Wealth effect: A fall in the price level increases the real value of household nominal assets (cash, bonds), raising real wealth and encouraging higher consumption.
Interest rate effect: A lower price level reduces demand for money, which lowers interest rates. Lower rates encourage more investment and interest-sensitive consumption.
Exchange rate effect: Lower interest rates from a falling price level cause domestic currency depreciation, increasing net exports.

📐 Worked Example

A country experiences a fall in its average price level, ceteris paribus. Explain how the interest rate effect leads to higher real output demanded.

1
When the average price level falls, households and firms need less money to purchase the same quantity of goods, so demand for money falls.
2
Lower demand for money reduces the equilibrium interest rate in the money market.
3
Lower interest rates reduce borrowing costs for firms, encouraging more investment spending on capital goods. Lower rates also cut borrowing costs for households, encouraging consumption of big-ticket items like housing and cars.
4
Since consumption (C) and investment (I) are core components of AD, higher C and I increase the total quantity of output demanded at the lower price level.

3. Movements vs Shifts of the AD Curve★★★☆☆⏱ 4 min

A movement along the existing AD curve is only caused by changes in the domestic average price level. Any change to spending from non-price factors will shift the entire AD curve left (decrease AD) or right (increase AD).

Shifters for C: Changes in consumer confidence, income taxes, household wealth, interest rates
Shifters for I: Changes in business confidence, corporate taxes, technology, interest rates
Shifters for G: Changes in government spending on public services and infrastructure
Shifters for NX: Changes in foreign income, exchange rates, trade barriers, global price levels

📐 Worked Example

The central bank raises interest rates to reduce high inflation. Ceteris paribus, how does this impact the AD curve?

1
Higher interest rates increase borrowing costs for both households and firms, and this change is not caused by a change in the current price level.
2
Higher rates reduce interest-sensitive consumption (new cars, housing) so C falls, and reduce planned business investment so I falls.
3
C and I are components of AD, so for every possible price level, total output demanded is now lower than before.
4
The entire AD curve shifts leftward, there is no movement along the original curve.

Exam tip:

Always label your axes correctly when drawing AD: vertical axis = Average Price Level (PL), horizontal axis = Real GDP (Y)

4. Applying AD Analysis to Shocks★★★★☆⏱ 2 min

✓ Quick check

Test your understanding of AD shifts:

The government increases spending on new national infrastructure, ceteris paribus. What happens to AD?
- AD shifts right
- AD shifts left
- Movement up along AD
- No change
Reveal answer
AD shifts right —
Government spending on infrastructure is a direct increase in the G component of AD, so AD increases and shifts right.
The average domestic price level rises due to a global energy price shock. What happens to AD?
- AD shifts right
- AD shifts left
- Movement up along AD
- No change
Reveal answer
Movement up along AD —
Changes in the domestic average price level only cause a movement along the existing AD curve, not a shift of the entire curve.

5. Common Pitfalls

Wrong move:

Counting transfer payments in the government spending (G) component of AD

Why:

Transfer payments only redistribute existing income, they do not represent spending on new goods and services

Correct move:

Exclude transfer payments from G, only count direct government purchases of new output

Wrong move:

Confusing movements along the AD curve with shifts of the curve

Why:

Only changes in the domestic average price level cause movement along the AD curve

Correct move:

Attribute non-price changes in spending to a full shift of the AD curve, not a movement

Wrong move:

Explaining the downward slope of AD using microeconomic substitution effects

Why:

AD describes total output in the economy, so the slope relies on three distinct macroeconomic effects, not individual substitution between goods

Correct move:

Use the wealth, interest rate and exchange rate effects to explain why AD slopes downward

Wrong move:

Defining investment as financial investment in stocks and bonds

Why:

In macroeconomics, investment only refers to physical investment in new capital goods

Correct move:

Count only physical capital investment and residential construction in the I component of AD

Wrong move:

Assuming currency depreciation reduces AD

Why:

Depreciation makes exports cheaper and imports more expensive, which increases net exports

Correct move:

Currency depreciation increases NX, shifting AD right ceteris paribus

6. Quick Reference Cheatsheet

Component	Symbol	Content	Shifts AD Right When:
Consumption	C	Household spending on goods/services	Consumer confidence rises, taxes fall
Investment	I	Capital goods + residential construction	Interest rates fall, business confidence rises
Government Spending	G	Government purchases of output	Expansionary fiscal policy, infrastructure spending
Net Exports	NX	Exports minus Imports	Currency depreciates, foreign income rises

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 1
AD shift from expansionary fiscal policy
2022 · 2
Draw AD shift after interest rate rise
2021 · 1
Explain why AD slopes downward

Going deeper

syllabusIB Economics HL SyllabusCheck for latest curriculum updates

What's Next

Aggregate demand is the foundation of the AD-AS model, the core framework for all macroeconomic analysis in IB Economics HL. After mastering AD, you will add aggregate supply to the model to determine equilibrium price level and real output, and analyze how the economy adjusts to short-run and long-run shocks. Understanding AD shifts is also critical for evaluating the impact of fiscal and monetary policy, two of the most heavily tested topics in both Paper 1 and Paper 2. This knowledge will also help you assess the causes of and policy responses to common macroeconomic problems including recession, inflation and unemployment.