📌 “When prices change, your buying behavior shifts for two distinct reasons.” Understanding these two forces is key to mastering consumer theory in microeconomics.
When the price of a good changes, the total change in quantity demanded is split into two parts: the Substitution Effect and the Income Effect. Separating them helps economists predict how consumers react to price hikes or discounts.
1. The Substitution Effect
The substitution effect occurs when consumers replace a more expensive item with a cheaper alternative. It isolates the change in relative prices, assuming purchasing power stays constant.
2. The Income Effect
The income effect describes how a price change alters your real purchasing power. If prices drop, you feel richer; if prices rise, you feel poorer. This changes how much you can afford to buy.
3. Comparison at a Glance
| Feature | Substitution Effect | Income Effect |
|---|---|---|
| Driver | Change in relative prices | Change in real purchasing power |
| Direction | Always opposite to price change | Depends on the type of good |
| Focus | Switching between products | Changing total consumption ability |
| Assumption | Utility remains constant | Relative prices remain constant |
⚠️ Common Pitfall: Normal vs. Inferior Goods
- Normal Goods: Income effect reinforces substitution effect. (Price ↓ → Buy More).
- Inferior Goods: Income effect opposes substitution effect. (Price ↓ → Feel Richer → Buy Less of the cheap good).
- Giffen Goods: A rare case where the income effect is so strong it overpowers the substitution effect, leading to higher demand when prices rise.