# Aggregate demand

> IB Economics SL · IB Economics 2025 Syllabus
> Source: https://www.owlsprep.com/study/ib-economics-sl-u3-aggregate-demand/

This module covers the definition, components and shape of aggregate demand (AD), the core building block of the AD-AS macroeconomic model, used to analyse economic fluctuations and the impact of government policy.

**Prerequisites:** [National income accounting basics](https://www.owlsprep.com/study/ib-economics-sl-u3-national-income-accounting/)

## Learning objectives

- Define aggregate demand and its four components
- Explain why the AD curve is downward sloping
- Distinguish between movements along and shifts of the AD curve
- Calculate aggregate demand from national income data

## Definition and Components of AD

**Aggregate Demand** — The total demand for all final goods and services produced domestically in an economy, at a given average price level, over a specific time period.

*Notation:* AD

*Example:* At an average price level of 100, AD in Country Z is $2 trillion.

Aggregate demand is calculated as the sum of four expenditure components from national income accounting:

- $C$ = Consumption: Spending by households on durable goods (cars, appliances), non-durable goods (food, clothing) and services (healthcare, education)
- $I$ = Investment: Spending by firms on capital goods, plus residential investment by households
- $G$ = Government Spending: Government spending on new public goods and services (excludes transfer payments)
- $(X-M)$ = Net Exports: Total export revenue minus total import spending

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

**Worked example:** Calculate AD from the following data (all values in $bn): Consumption = 310, Investment = 80, Government spending on goods = 90, Transfer payments = 40, Exports = 60, Imports = 50

1. Recall that transfer payments are not spending on new goods, so we exclude them. Use the AD identity:
2. $$AD = C + I + G + (X-M)$$
3. Substitute the valid values:
4. $$AD = 310 + 80 + 90 + (60 - 50)$$
5. Calculate the final value:
6. $$AD = 480 + 10 = 490$$
7. Final aggregate demand = $490 billion.

> **Exam tip:** Always remember to exclude transfer payments from G when calculating AD in data response questions.

## Why the AD Curve is Downward Sloping

The AD curve plots the total quantity of output demanded against the average price level. It slopes downward for three macroeconomic reasons, completely separate from the reasoning for a microeconomic demand curve:

1. **Wealth effect**: A fall in the price level increases the real purchasing power of household wealth (savings, bonds), encouraging higher consumption, increasing output demanded.
2. **Interest rate effect**: A fall in the price level reduces money demand, which lowers interest rates. Lower rates encourage borrowing for consumption and investment, increasing output demanded.
3. **International trade effect**: A fall in the domestic price level makes exports cheaper for foreign buyers and imports more expensive for domestic buyers, increasing net exports, increasing output demanded.

> **note**
>
> All three effects work in reverse when the price level rises: higher prices reduce the total quantity of output demanded, giving the curve its downward slope.

**Worked example:** Explain how a rise in the average price level reduces output demanded via the interest rate effect.

1. 1. When the average price level rises, households and firms need more cash to make routine purchases, so total demand for money increases.
2. 2. Higher money demand pushes up the market interest rate, which is the cost of borrowing.
3. 3. Higher interest rates reduce firm investment spending (on capital goods) and household consumption spending (on big-ticket items like houses and cars).
4. 4. Lower C and I reduce the total quantity of output demanded at the higher price level, which is consistent with a downward sloping AD curve.

## Movements Along vs Shifts of the AD Curve

It is critical to distinguish between changes in quantity demanded along an existing AD curve, and shifts of the entire AD curve:

**Movement along the AD curve** — A change in the quantity of output demanded caused *only* by a change in the average domestic price level, with all other factors held constant.

A shift of the entire AD curve occurs when any non-price determinant of AD changes, meaning more or less output is demanded at every possible price level. A rightward shift means an increase in AD; a leftward shift means a decrease in AD.

**Worked example:** State whether each change causes a movement along or a shift of the AD curve: (a) Average consumer prices rise by 10%; (b) The government cuts income taxes to stimulate spending.

1. Only changes in the average price level cause movements along the AD curve. All other changes cause shifts.
2. Case (a): The change is to the average price level, so this causes an upward (leftward) movement along the existing AD curve.
3. Case (b): Lower income taxes increase household disposable income, leading to higher consumption at every price level. This is not caused by a change in the price level, so it causes a rightward shift of the entire AD curve.

**Common factors that shift AD right:**

- Higher consumer/business confidence
- Lower interest rates
- Higher government spending on goods and services
- Lower income/business taxes
- Weaker domestic exchange rate
- Higher foreign income (increases export demand)

> **Exam tip:** Examiners frequently test this distinction, so always check if the trigger is a price level change first.

## AD Components in Practice

In most high-income economies, the relative size and volatility of AD components follows a consistent pattern:

| Component | Typical share of AD (US example) | Volatility |
| --- | --- | --- |
| Consumption (C) | ~68% | Low |
| Investment (I) | ~17% | High |
| Government Spending (G) | ~18% | Medium |
| Net Exports (X-M) | ~-3% | Medium |

> **tip**
>
> Investment is by far the most volatile component of AD, which is why changes in business confidence are a common cause of large AD shifts.

**Worked example:** A collapse in housing prices reduces household wealth. How does this impact AD?

1. Lower household wealth reduces consumer confidence and discretionary consumption spending by households.
2. Consumption is a major component of AD, so total AD falls at every price level.
3. This causes a leftward shift of the entire AD curve, which reduces real output in the short run.

## Common pitfalls

- **Wrong:** Counting transfer payments as part of government spending in AD calculations
  - Why it fails: Transfer payments are just transfers of income, not spending on new goods and services
  - Correct: Exclude all transfer payments from G when calculating AD
- **Wrong:** Confusing movements along the AD curve with shifts of the curve
  - Why it fails: Only changes in the average price level cause movements along the curve
  - Correct: Check first if the change is to the price level: if yes, movement; if no, shift
- **Wrong:** Using microeconomic substitution effects to explain why AD slopes down
  - Why it fails: IB examiners specifically test that you know macroeconomic reasoning for the AD slope
  - Correct: Use the wealth effect, interest rate effect and international trade effect to explain the downward slope
- **Wrong:** Counting just exports instead of net exports in the AD identity
  - Why it fails: Imports are spending on foreign output, not domestic output, so they must be subtracted
  - Correct: Always calculate net exports as $X - M$ for the AD identity
- **Wrong:** Treating all government spending as part of AD
  - Why it fails: Only spending on new goods and services is counted; transfer payments and debt interest are not
  - Correct: Only include government expenditure on final goods and services in AD

## Cheatsheet

| Concept | Key Summary |
| --- | --- |
| AD Identity | $AD = C + I + G + (X-M)$ |
| Downward Slope Reasons | Wealth effect, Interest rate effect, Trade effect |
| Right AD Shift (increase) | Lower rates, higher confidence, higher G, lower taxes, weaker currency |
| Left AD Shift (decrease) | Higher rates, lower confidence, lower G, higher taxes, stronger currency |
| Movement along AD | Only caused by change in average price level |

## What's next

Aggregate demand is the foundation of the AD-AS model, the core framework for analysing all macroeconomic issues including recessions, inflation, unemployment, and the impact of fiscal and monetary policy. Mastery of AD concepts is required for almost every IB SL macroeconomics exam question, from multiple choice to 10-mark paper 1 responses. Next, we build out the other half of the model to understand how aggregate supply interacts with AD to determine equilibrium output and price levels.

- [Aggregate Supply](https://www.owlsprep.com/study/ib-economics-sl-u3-aggregate-supply/)
- [Macroeconomic equilibrium](https://www.owlsprep.com/study/ib-economics-sl-u3-macroeconomic-equilibrium/)
- [Unemployment and inflation](https://www.owlsprep.com/study/ib-economics-sl-u3-unemployment-and-inflation/)

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