Consumer Behaviour and Utility

Consumer Behaviour and Utility

Consumer behaviour theory explains how consumers make choices to maximise satisfaction given limited income. Understanding what drives consumer decisions helps businesses design products, set prices, and target marketing.

Utility Concept

Utility is the satisfaction from consuming a good. Total utility (TU): total satisfaction from all units consumed. Marginal utility (MU): additional satisfaction from one more unit: MU = ΔTU/ΔQ. Utility is subjective — a glass of water has high utility in a desert but low utility after drinking five glasses. Cardinal utility assumes measurement in numerical units (utils); ordinal utility assumes consumers can only rank preferences.

Law of Diminishing Marginal Utility

As a consumer consumes more units, additional satisfaction from each successive unit decreases (though total utility may still increase). The first slice of pizza gives great satisfaction, the second somewhat less, eventually reaching zero (satiation) then negative (dissatisfaction). This law explains the downward-sloping demand curve — consumers only buy more at lower prices because each additional unit gives less satisfaction.

Consumer Equilibrium (Cardinal)

Consumer maximises utility when: MUx/Px = MUy/Py = ... = MU of money. If MUx/Px > MUy/Py, buy more of X and less of Y until equality is restored. This equi-marginal principle guides rational allocation of limited budget.

Indifference Curves (Ordinal)

An indifference curve shows combinations of two goods giving equal satisfaction. Properties: downward-sloping, convex to origin (diminishing MRS), higher curves = higher utility, curves never intersect. Marginal Rate of Substitution (MRS) = ΔY/ΔX = MUx/MUy — rate of trading one good for another at same satisfaction. MRS diminishes along the curve.

Budget Constraint

The budget line: Px·X + Py·Y = Income. Slope = −Px/Py (price ratio). Income increase shifts line outward (parallel). Price change rotates the line.

Consumer Equilibrium (Ordinal)

Maximise utility where budget line is tangent to highest indifference curve: MRS = Px/Py. Consumer’s willingness to trade (MRS) equals the market’s exchange rate (price ratio).

Income and Substitution Effects

Substitution effect: good becomes relatively cheaper, consumer switches to it (always opposite to price change). Income effect: price change affects real purchasing power. For normal goods, reinforces substitution effect. For inferior goods, works against it. For Giffen goods, income effect overwhelms substitution effect, violating the law of demand.

Summary

Consumer behaviour theory — utility analysis, indifference curves, budget constraints, and equilibrium — explains rational consumer choices. The equi-marginal principle and MRS = price ratio guide optimal decisions.

Worked Example: Consumer Equilibrium (Cardinal Approach)

A consumer has Rs 100 to spend on two goods: Momo (Rs 20 per plate) and Tea (Rs 10 per cup). The marginal utilities are:

Solution: Equilibrium where MU/P is equal for both goods. At 3 Momos and 4 Teas: MU/P(Momo) = 3 and MU/P(Tea) = 3. Budget check: (3 × 20) + (4 × 10) = 60 + 40 = Rs 100 ✔️

Interpretation: The consumer maximises satisfaction by buying 3 plates of momo and 4 cups of tea. The last rupee spent on each good gives equal marginal utility (3 utils per rupee). Any other combination would give less total satisfaction.

Indifference Curve Properties — Visual Guide

Income Effect vs Substitution Effect

Exam Tips

Tip 1: Equi-marginal principle (MUx/Px = MUy/Py) numerical problems are very commonly tested — practice the table method shown above. Tip 2: Know all 4 properties of indifference curves with reasons (not just statements). Tip 3: Consumer equilibrium (ordinal): MRS = Px/Py at tangent point of IC and budget line. Tip 4: Income and substitution effects for normal, inferior, and Giffen goods — the table above is the fastest way to remember.