Consumer's Equilibrium: Ordinal Approach
Class 11 Micro Economics — Indifference curve and budget line approach to consumer equilibrium
Condition 1 — MRS = Price Ratio
Consumer equilibrium under the ordinal approach is the point of maximum satisfaction subject to the budget constraint. The consumer reaches equilibrium at the highest possible indifference curve that touches the budget line.
Equilibrium Condition
At equilibrium, the slope of the indifference curve equals the slope of the budget line. The consumer's willingness to substitute (MRS) matches the market's willingness to exchange (Price Ratio / MRE).
Tangency Explorer — MRS vs Price Ratio
Before
After
Key Insight
Equilibrium happens where MRS = Price Ratio. Left of E, you want more pizza. Right of E, you want less. Only AT E are you getting maximum satisfaction for your money.
If MRS > Price Ratio → the consumer values pizza more than the market. They will buy more pizza, and as pizza consumption rises, MRS falls (diminishing MRS). If MRS < Price Ratio → they will buy less pizza, and MRS rises. This process continues until MRS equals the Price Ratio — equilibrium is established.
Condition 2 — MRS Continuously Falls
Both Conditions Must Be Satisfied
MRS must be diminishing at the point of equilibrium. The indifference curve must be convex to the origin at point E. Unless MRS continuously falls, equilibrium cannot be established — the consumer would keep substituting forever. Both conditions (MRS = Price Ratio and Diminishing MRS) must be satisfied simultaneously for a stable equilibrium.
Diagrammatic Explanation
Consider the graph above with three indifference curves IC₁, IC₂, and IC₃ and the budget line AB.
- Budget Line ABshows all combinations that the consumer can afford with their income (₹200) at given prices (Pizza = ₹40, Burgers = ₹20). The consumer must choose a point on AB.
- IC₁ is below the budget line — the consumer could afford points on this curve, but they represent lower satisfaction. These points are attainable but not optimal.
- IC₂ touches the budget line at exactly one point — E. This is the highest IC the consumer can reach given their budget. At E, the slopes match: MRS = Price Ratio.
- IC₃ lies entirely above the budget line. Points on this curve would give even more satisfaction, but they are unattainable with the current income.
The consumer purchases OM quantity of Pizza and ON quantity of Burgers at equilibrium. All other points on the budget line lie on lower indifference curves, yielding lower satisfaction. The budget line can be tangent to only one indifference curve — satisfaction is maximized exclusively at E.
Key Takeaways
- At point E: MRS = Price Ratio (Condition 1 is satisfied).
- At point E: MRS is diminishing — IC₂ is convex to the origin (Condition 2 is satisfied).
- IC₂ is the highest attainable indifference curve given the budget constraint.
- The budget line is tangent to exactly one indifference curve at equilibrium.
Cardinal vs Ordinal Utility
| Aspect | Cardinal Utility | Ordinal Utility |
|---|---|---|
| Measurement | Utility measured in cardinal terms (1, 2, 3...) | Utility cannot be measured — only ranked |
| Unit | 'Util' developed as measurement unit | No unit of measurement |
| Example | Pizza = 20 utils, Burger = 15 utils (exact difference) | Pizza ranked 1st, Burger 2nd (only order matters) |
| Realism | Less realistic (cannot measure satisfaction absolutely) | More realistic (ranking is natural) |
| Key Contributors | Marshall | Hicks and Allen |