Perfect substitutes
Topic: Consumer Behaviour, Demand and Elasticity · NCERT: Class 12, Ch 2 "Theory of Consumer Behaviour"
Meaning
Perfect substitutes are goods that can replace each other completely and give exactly the same utility. The consumer always swaps them at a fixed rate, so the marginal rate of substitution (MRS) stays constant. MRS is the amount of one good she gives up for one more unit of the other. Because of this constant MRS, the indifference curve is a straight line, not a curve bowed towards the origin.
Example
₹5 notes and ₹5 coins are perfect substitutes. Class 12 Table 2.3 shows the bundles (1, 8), (2, 7), (3, 6) and (4, 5), all giving the same utility. Each extra note replaces exactly one coin, so the MRS is 1:1 throughout.
Don't confuse with
- Perfect complements: goods used only in a fixed ratio, such as left and right shoes. An extra left shoe adds nothing on its own, so the indifference curve is L-shaped.
- Substitute goods in general: tea and coffee are substitutes but not perfect ones. Their indifference curve is still convex.
Related concepts
- Ordinal utility analysis
- Indifference curve
- Marginal rate of substitution
- Law of diminishing marginal rate of substitution
- Convexity of indifference curve
- Monotonic preferences
- Indifference map
- Properties of indifference curves
- Utility function
- Revealed preference