Maths⏱ 5 min read
Income Effect vs Substitution Effect: Economics Made Clear
When prices change, consumers respond in two distinct ways that economics separates carefully. Understanding the income and substitution effects explains behaviour that otherwise seems contradictory.
The income effect and substitution effect are the two components of any consumer response to a price change. Every price change affects you in both ways simultaneously — understanding how they work individually reveals why demand behaves the way it does.
The Substitution Effect
When the price of good A rises relative to good B, consumers substitute away from A toward B — regardless of whether they feel richer or poorer overall. It's a pure relative price response.
Example: Bus fares rise 30%, train fares unchanged.
Substitution effect: some bus commuters switch to train.
This happens even if their total income is unaffected.
The substitution effect always works in the expected direction:
Higher price of A → less A consumed (substitute for B)
Lower price of A → more A consumed (substitute away from B)
Direction is always "downward sloping" — no exceptions.
The Income Effect
When the price of something you buy rises, your real purchasing power falls — you're effectively poorer, even if your nominal income hasn't changed. This change in real income affects how much you buy.
Example: Energy bills rise 40%.
Income effect: households have less real income to spend overall.
If energy is a normal good: buy less of it (demand falls)
If energy is an inferior good: buy more of it (demand rises — see below)
Normal good: income rises → you buy more (e.g. restaurant meals)
Inferior good: income rises → you buy less (e.g. instant noodles)
Giffen good: income falls due to price rise → buy MORE (very rare)
Combining the Two Effects
Total effect of a price change = Substitution effect + Income effect
For normal goods (most goods):
Price rises → Substitution effect: buy less ✓
→ Income effect (feel poorer): buy less ✓
→ Total: buy significantly less ✓
For inferior goods:
Price rises → Substitution effect: buy less ✓
→ Income effect (feel poorer): buy MORE ✗
→ Total: depends on which effect dominates
Giffen Goods: When the Law of Demand Appears to Break
A Giffen good is one where a price rise leads to higher consumption — the income effect dominates the substitution effect for an inferior good. Classic example: staple foods (bread, rice) for very poor households.
Very poor household spends 60% of income on bread:
Bread price rises → income effect very large (big drop in real income)
As real income falls, household cuts luxury foods and buys MORE bread
(because they're now too poor to afford the alternatives)
This is genuinely rare — most inferior goods don't have large enough
income effects to overcome the substitution effect.
Practical Applications
ScenarioSubstitution EffectIncome Effect
Petrol price risesDrive less, use public transport moreFeel poorer, cut other spending
Wages riseWork more (leisure becomes more expensive)Feel richer, may work LESS for same income
Mortgage rate risesSubstitute toward rentingLess real income, cut other spending