Substitution Effect
What is Substitution Effect?
The substitution effect is the change in quantity demanded when a price change makes a good relatively cheaper or pricier than its alternatives.
When a good's price falls, consumers substitute toward it and away from now relatively more expensive substitutes, raising quantity demanded. It always moves opposite to the price change. With the income effect, it explains the downward-sloping demand curve.
Substitution Effect: a worked example
A shopper buys 5 boxes of Brand A cereal at $4 and 4 boxes of Brand B at $6, spending (5 × $4) + (4 × $6) = $20 + $24 = $44. Brand A then drops to $3. In relative terms a box of A used to cost $4 ÷ $6, or two thirds of a box of B, and now costs $3 ÷ $6, or one half. To isolate the substitution effect, strip away the purchasing power gain by cutting the budget to whatever just buys the old bundle at the new prices: (5 × $3) + (4 × $6) = $15 + $24 = $39. Given $39, the shopper switches to 7 boxes of A and 3 of B, since (7 × $3) + (3 × $6) = $21 + $18 = $39. Brand A rose by 2 boxes purely because it became relatively cheaper.
The mistake students make with substitution effect
Students explain the substitution effect by saying the shopper has money left over after the price cut, which is the income effect wearing the wrong label. The substitution effect is measured with purchasing power deliberately held constant, so the only thing driving it is the new price ratio between the two goods. The other error is expecting the direction to flip for inferior goods. This effect always moves quantity demanded opposite to the price change, for normal and inferior goods alike, since a cheaper good is relatively more attractive whatever its income classification.
Substitution Effect questions
Does the substitution effect ever move in the same direction as price?
The substitution effect always moves opposite to the price change. A price increase makes a good relatively more expensive than its alternatives, so the response with purchasing power held constant is to buy less of it and more of the substitutes. That holds for normal goods, inferior goods, and even Giffen goods, where the income effect is what reverses the total response. Because this effect never flips sign, it is the part of the law of demand with no exceptions.
What is a simple example of the substitution effect?
Two competing lunch spots make the cleanest example. When one raises its sandwich price while the other holds steady, some customers walk to the cheaper counter even though their budgets did not change at all. Nothing about their hunger shifted and nothing about their income shifted. Only the relative price moved, and the quantity bought at the pricier spot fell. The same reasoning covers switching between brands, fuels, or streaming services.
Which is bigger, the substitution effect or the income effect?
Relative size depends on the good. For goods absorbing a small share of spending, the substitution effect dominates, because the purchasing power freed by the price change is trivial. For goods eating a large share of the budget, such as housing, the income effect grows big enough to matter. Textbook Giffen goods are the extreme case, where a strongly negative income effect on an inferior staple outweighs the substitution effect and quantity demanded rises with price.
Related terms
Common comparisons
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