# Consumer behaviour and demand

> Economics · Edexcel IAL
> Source: https://www.owlsprep.com/study/edexcel-ial-economics-u1-consumer-behaviour-and-demand/

This guide covers core Edexcel IAL Unit 1 microeconomics content on consumer rationality, demand curve analysis, and the three core elasticity measures (PED, YED, XED), including exam-focused worked examples and common pitfalls.

**Prerequisites:** [Basic economic problem and price mechanism](https://www.owlsprep.com/study/edexcel-ial-economics-u1-price-mechanism/)

## Learning objectives

- Explain rational consumer choice and key limitations to rationality from introductory behavioural economics
- Distinguish between movements along and shifts in the demand curve, and identify non-price shift factors
- Link diminishing marginal utility to the downward slope of the demand curve
- Define, calculate and interpret values for PED, YED and XED
- Apply the PED-total revenue relationship to real-world firm decision-making
- Analyse the significance of elasticity measures for consumers, firms and governments

## Rational Consumer Behaviour and Limits to Rationality

The standard neoclassical model assumes consumers are rational utility maximisers: they allocate their limited income to buy the combination of goods that delivers the highest possible total satisfaction (utility). However, behavioural economics identifies several common reasons consumers may not act rationally.

**Utility** — The total satisfaction a consumer gains from consuming a good or service, measured subjectively per individual.

- **Herding**: Following the choices of other consumers rather than making independent utility calculations, e.g. buying a viral social media product
- **Habit/inertia**: Sticking to existing brands or routines even if cheaper, higher-quality alternatives exist
- **Poor computational ability**: Being unable to calculate the expected utility of complex purchasing choices
- **Framing & bias**: Choices being affected by how options are presented, e.g. preferring meat labelled '90% fat free' over '10% fat'

**Worked example:** A fast food chain offers a loyalty card that gives 1 free meal after 9 purchased meals. Explain why this may lead consumers to make non-rational choices.

1. Identify the non-rational driver: inertia and habit formation.
2. Explain the logical chain: Consumers may continue buying from the chain even if a nearby competitor sells cheaper, better quality meals, to accumulate points for the free meal, even if the total cost of 9 meals at the chain is higher than buying 10 meals at the cheaper competitor.

> **Exam tip:** When explaining limits to rationality in 4-mark questions, always link the factor to a real-world context to get full marks for application.

## Demand Curve Analysis

**Demand** — The quantity of a good or service consumers are willing and able to buy at each given price level over a given time period.

The demand curve slopes downwards due to the law of diminishing marginal utility: as a consumer consumes more units of a good, the additional satisfaction (marginal utility) from each extra unit falls, so consumers will only purchase more units if the price decreases.

It is critical to distinguish between two types of demand change: <br>1. **Movement along the curve (extension/contraction)**: Only caused by a change in the *own price* of the good, leading to a change in quantity demanded. <br>2. **Shift of the entire curve**: Caused by non-price factors, leading to a change in demand at every price point. Shift factors include: prices of substitute/complement goods, real consumer income, tastes/preferences, population size/demographics, and advertising.

**Worked example:** Draw a fully labelled diagram to show the impact of a rise in the price of petrol on the demand for electric cars.

1. Draw axes: Y-axis = Price of electric cars, X-axis = Quantity of electric cars demanded.
2. Draw an original downward sloping demand curve labelled D, then draw a second demand curve D1 shifted to the right of D, with an arrow showing the shift from D to D1.
3. Explain the link: Petrol and electric cars are substitutes. A rise in petrol prices makes running petrol cars more expensive, so consumers demand more electric cars at every price point, shifting demand right from D to D1.

> **Exam tip:** Always label axes clearly on demand diagrams, and explicitly link your diagram to your written analysis to access all 4 marks for diagram questions.

## Price Elasticity of Demand (PED) and Total Revenue Relationship

**Price Elasticity of Demand (PED)** — The responsiveness of quantity demanded of a good to a change in its own price.

*Notation:* PED = \frac{\%\Delta Q_d}{\%\Delta P}

- |PED| > 1 = **Elastic**: Quantity demanded is highly responsive to price changes
- |PED| < 1 = **Inelastic**: Quantity demanded is not very responsive to price changes
- |PED| = 1 = **Unitary elastic**: Percentage change in quantity demanded equals percentage change in price

Along a straight-line demand curve, PED varies: it is elastic in the top half of the curve, unitary at the midpoint, and inelastic in the bottom half of the curve. The PED value directly links to total revenue (TR = price × quantity sold): <br>- If demand is elastic: A price fall increases TR, a price rise reduces TR <br>- If demand is inelastic: A price rise increases TR, a price fall reduces TR <br>- If demand is unitary elastic: TR is maximised.

**Worked example:** A supermarket increases the price of milk from £1 to £1.10, and quantity demanded falls from 300 litres to 270 litres per day. Calculate PED and explain whether the price rise will increase or decrease the supermarket's total revenue from milk.

1. Calculate percentage change in quantity demanded: \(\frac{270 - 300}{300} \times 100 = -10\%\)
2. Calculate percentage change in price: \(\frac{1.10 - 1}{1} \times 100 = +10\%\)
3. Calculate PED: \(\frac{-10\%}{+10\%} = -1\)
4. Interpret: PED is unitary elastic, so total revenue will stay the same. Original TR = 300 × £1 = £300, new TR = 270 × £1.10 = £297 (small difference due to rounding, accepted as unitary for exam purposes).

> **Exam tip:** Always show your full working for elasticity calculations to get method marks even if your final answer is wrong. You do not need to include the negative sign for PED unless explicitly asked.

## Income Elasticity of Demand (YED) and Cross Elasticity of Demand (XED)

**Income Elasticity of Demand (YED)** — The responsiveness of quantity demanded of a good to a change in real consumer income (Y).

*Notation:* YED = \frac{\%\Delta Q_d}{\%\Delta Y}

**Cross Elasticity of Demand (XED)** — The responsiveness of quantity demanded of Good A to a change in the price of Good B.

*Notation:* XED = \frac{\%\Delta Q_{dA}}{\%\Delta P_B}

- YED > 0 = **Normal good**: Demand rises as income rises. YED > 1 = luxury good, 0 < YED < 1 = necessity good
- YED < 0 = **Inferior good**: Demand falls as income rises
- XED > 0 = **Substitutes**: Demand for Good A rises when price of Good B rises
- XED < 0 = **Complements**: Demand for Good A falls when price of Good B rises
- XED = 0 = **Unrelated goods**: No relationship between price of B and demand for A

**Worked example:** Following a 6% rise in consumer incomes, demand for value bus tickets falls by 12%, while demand for train tickets rises by 9%. Calculate YED for both goods and classify each.

1. YED for value bus tickets: \(\frac{-12\%}{+6\%} = -2\). Negative YED means value bus tickets are an inferior good.
2. YED for train tickets: \(\frac{+9\%}{+6\%} = +1.5\). Positive YED > 1 means train tickets are a normal luxury good.

**Exam command terms**

Common command words for this topic:

- **Calculate** — Show all working, include units if required, 2-4 marks *(Calculate the XED for Good A if a 10% rise in price of Good B leads to 5% fall in demand for A)*

- **Analyse** — Build a logical chain of reasoning, use context, 6 marks *(Analyse how a recession (fall in average incomes) affects demand for inferior goods)*

- **Evaluate** — Include KAA plus judgement, consider magnitude/limitations, 20 marks *(Evaluate the extent to which knowledge of PED helps governments set indirect tax rates)*

## Common pitfalls

- **Wrong:** Confusing a movement along the demand curve with a shift when the good's own price changes, saying 'demand increases' when price falls.
  - Why it fails: Students mix up 'demand' (entire curve) and 'quantity demanded' (single point on curve).
  - Correct: Use 'quantity demanded' for movements caused by own price changes, and 'demand' for shifts caused by non-price factors, explicitly stating the cause of the change.
- **Wrong:** Assuming PED is constant along a straight-line demand curve, stating the entire curve is elastic or inelastic.
  - Why it fails: Only perfectly elastic or perfectly inelastic demand curves have constant PED. Standard linear curves have falling PED as you move down the curve.
  - Correct: Note that on a standard linear demand curve, PED is elastic at the top, unitary at the midpoint, and inelastic at the bottom.
- **Wrong:** Mixing up XED signs for substitutes and complements, assigning negative XED to substitutes.
  - Why it fails: Students incorrectly associate negative values with 'bad' rather than inverse relationships.
  - Correct: Remember: If price of Good B rises, demand for substitute Good A rises (positive XED), demand for complement Good A falls (negative XED).
- **Wrong:** Calculating percentage change as (old value / new value) × 100 instead of (change / old value) × 100 for elasticity formulas.
  - Why it fails: Simple arithmetic error leading to incorrect elasticity values.
  - Correct: Always use the formula: %Δ = (new value - old value) / old value × 100 for all elasticity calculations.
- **Wrong:** Stating elasticity values without linking them to real-world impacts in extended answers.
  - Why it fails: Students fail to build the logical chain of reasoning required for high KAA marks.
  - Correct: Always follow elasticity values with a chain: e.g. 'PED for cigarettes is -0.4, so a 10% tax rise will only reduce demand by 4%, meaning the tax will raise significant government revenue'.

## Cheatsheet

| Concept | Definition | Key Formula | Core Interpretation |
| --- | --- | --- | --- |
| Demand | Willingness/ability to buy at each price | N/A | Downward sloping due to diminishing marginal utility |
| PED | Qd responsiveness to own price change | \(\frac{\%\Delta Q_d}{\%\Delta P}\) | >1 elastic, <1 inelastic, =1 unitary |
| YED | Qd responsiveness to income change | \(\frac{\%\Delta Q_d}{\%\Delta Y}\) | >0 normal, <0 inferior |
| XED | Qd of A responsiveness to price of B | \(\frac{\%\Delta Q_{dA}}{\%\Delta P_B}\) | >0 substitutes, <0 complements |

## What's next

Now that you have mastered consumer behaviour and demand, you are ready to move to the next core Unit 1 topic: supply and price elasticity of supply (PES), where you will learn about producer decision-making and how supply interacts with demand to set market prices. You should also practice applying elasticity concepts to extended answer questions, which are common in 14 and 20-mark exam papers, and revise how to integrate demand diagrams into your analysis to access higher KAA marks. This content is also foundational for Unit 3 topics on firm revenue and pricing strategies that you will cover in IA2.

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