Consumer Behaviour

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Consumer behaviour is the study of how individuals, groups, or organisations select, buy, use, and dispose of goods, services, or experiences to satisfy needs (biological) and wants (psychological). The foundation rests on utility — the satisfaction derived from consumption.

Total Utility (TU) = ΣMU; Marginal Utility (MU) = TU_n − TU_(n−1).
Consumer equilibrium: MU_x / P_x = MU_y / P_y (and MU of money constant).
Law of Diminishing Marginal Utility: as more units of a good are consumed, MU falls — the basis of the downward-sloping demand curve.
Indifference curve shows combinations yielding equal satisfaction; the budget line shows affordability: P_x·X + P_y·Y = M.
ICAN must-knows: equi-marginal condition, MRS = MU_x / MU_y, and the substitution vs income effect split.
High-yield traps: confusing Giffen with inferior goods, and treating MU as negative once it merely starts declining.

🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Cardinal (Utility) Approach

Utility is treated as measurable in utils. A rational consumer maximises satisfaction subject to income. The decision rule is the Law of Equi-Marginal Utility: a consumer is in equilibrium when the last naira spent on each good yields the same marginal utility, i.e. MU_x / P_x = MU_y / P_y = MU_z / P_z. If MU_x/P_x > MU_y/P_y, the consumer shifts spending from Y to X until equality holds.

Ordinal (Indifference Curve) Approach

Utility is ranked, not measured. An indifference curve (IC) plots bundles giving equal satisfaction; an indifference map is a family of such curves. Standard assumptions yield four properties: (1) downward-sloping, (2) convex to the origin (diminishing MRS), (3) higher curves = higher utility, (4) curves never intersect. The Marginal Rate of Substitution (MRS) = MU_x / MU_y = slope of the IC.

Consumer Equilibrium (IC Approach)

Equilibrium occurs where the budget line is tangent to the highest attainable indifference curve, so: MRS_xy = P_x / P_y, equivalently MU_x / P_y = MU_y / P_x.

Income and Substitution Effects

A price fall increases real income and changes relative prices. Substitution effect = pure change in quantity due to relative price change (always opposite to price change). Income effect = change due to higher real purchasing power (negative for inferior goods, positive for normal goods). Giffen goods are a subset of inferior goods where the income effect is large enough to outweigh the substitution effect, making demand slope upward.

Typical ICAN Question Patterns

Compute equilibrium from a utility table.
Sketch IC, budget line, and show tangency.
Decompose a price change into income + substitution effects (Hicks or Slutsky).
State and apply Engel’s Law and Engel’s Coefficient (food share of expenditure falls as income rises).

🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Hicks vs Slutsky decomposition: Hicks holds real income (utility) constant; Slutsky holds the original bundle affordable. For exam answers, name the method explicitly.
Giffen paradox: empirically rare (classically, the Irish potato famine); requires the good to be a large share of a poor consumer’s budget and strongly inferior.
Revealed Preference Theory (Samuelson): consumers’ choices reveal preferences — if bundle A is chosen over B when both are affordable, A is preferred to B.
Consumer Surplus = area between the demand curve and the price line, up to the quantity bought — used in cost-benefit analysis and welfare statements.

Connections to Adjacent Topics

Consumer theory is the micro-foundation of the demand curve; shift in preferences shifts the curve, movement along reflects price change.
Links to market structures (monopolist’s price discrimination depends on differing elasticities from consumer heterogeneity).
Tax incidence and subsidy incidence on consumers depend on price elasticity of demand, derived from consumer choice.

Common Mistakes

Equating MRS = P_x/P_y (slope of budget line) — true only at the tangency point, not everywhere.
Drawing ICs that intersect (violates transitivity) or are concave (violates diminishing MRS assumption).
Treating diminishing MU as automatically yielding negative MU — it merely means MU is falling, not yet negative.
Confusing movement along a demand curve (price change) with a shift (taste/income change).

Worked Mini-Example

A consumer has MU_x = 20 − X, MU_y = 30 − 2Y, P_x = ₦4, P_y = ₦5, income M = ₦40. At equilibrium: (20 − X)/4 = (30 − 2Y)/5 = λ. From tangency: 5(20 − X) = 4(30 − 2Y) → 100 − 5X = 120 − 8Y → 8Y − 5X = 20. Budget: 4X + 5Y = 40. Solving: X = 4, Y = 4.8. Plug back: MU_x/4 = 4 → MU_x = 16; MU_y/5 = 4 → MU_y = 20. Check: 4(4) + 5(4.8) = 16 + 24 = ₦40 ✓.

Practice Prompts

With a utility schedule for two goods, derive the consumer’s equilibrium bundle given P_x, P_y, and M, and verify the equi-marginal condition.
Using a labelled diagram, decompose a fall in the price of good X into substitution and income effects, and identify whether X is normal, inferior, or Giffen.

Content adapted based on your selected roadmap duration. Switch tiers using the selector above.

Consumer Behaviour