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Income Effect, Substitution Effect and Diminishing Marginal Utility
IB DP Economics — Unit 2: Microeconomics | HL | 2.1 Demand [HL Extensions]
📋 What You Will Learn
- Define marginal utility, the law of diminishing marginal utility, and explain how it underpins the demand curve
- Distinguish between the income effect and the substitution effect of a price change
- Explain how the combined income and substitution effects produce the downward-sloping demand curve for normal and inferior goods
- Analyse why the demand curve for inferior goods still slopes downward despite a negative income effect
- Apply these concepts to real-world consumer behaviour and evaluate their limitations as explanations of demand
🔍 Guiding Question Why do consumers buy less of a good when its price rises — and does this always hold true for every type of good?
🌍 Why This Matters In 2023, as UK food prices rose by over 19% — the highest rate in 45 years — consumers sharply cut back on branded goods and switched to supermarket own-label products. Some households simultaneously reduced their consumption of premium goods and increased purchases of cheaper staples. This is not random behaviour: it reflects two distinct economic forces — the substitution effect and the income effect — operating simultaneously on every consumer decision.
1. Utility and the Foundation of Demand
To explain why demand curves slope downward, economists begin with a simple idea: consumers derive satisfaction from consuming goods and services. This satisfaction is called utility. The key question is: what happens to that satisfaction as a consumer buys more and more of the same good?
📖 Definition — Utility Utility is the satisfaction or benefit a consumer derives from consuming a good or service. It is a subjective measure of wellbeing, not directly observable, but used as a theoretical tool to explain consumer choices.
📖 Definition — Marginal Utility (MU) Marginal utility is the additional satisfaction gained from consuming one more unit of a good or service, holding all other consumption constant. It is the change in total utility from a one-unit increase in consumption: \( MU = \frac{\Delta TU}{\Delta Q} \).
Total Utility vs Marginal Utility
- Total utility (TU): the cumulative satisfaction from all units consumed — rises as consumption increases (up to a point)
- Marginal utility (MU): the extra satisfaction from the last unit consumed — typically falls as consumption increases
- TU is maximised when MU = 0 (consuming one more unit adds nothing to satisfaction)
- Beyond that point, MU becomes negative (consuming more actually reduces total satisfaction)
2. The Law of Diminishing Marginal Utility
The most important insight from utility theory is that marginal utility declines as consumption increases. This is not a mathematical assumption — it reflects a genuine feature of human psychology and preference.
📖 Definition — Law of Diminishing Marginal Utility The law of diminishing marginal utility states that as a consumer increases consumption of a good, ceteris paribus, the marginal utility derived from each additional unit consumed will eventually decline.
🍕 Intuitive Hook — The Pizza Analogy Imagine eating slices of pizza when hungry. The first slice is deeply satisfying (high MU). The second is still good. By the fourth or fifth slice, the additional