Ch2. Consumer Theory — Utility, Indifference Curves, Budget Constraints, and Optimal Choice

What Is Consumer Theory?

Consumer theory analyzes how a rational consumer makes choices.

Core assumption: consumers seek to maximize their own satisfaction (utility) subject to a budget constraint.

These two elements — preferences (indifference curves) and constraints (budget line) — are the foundation of consumer choice theory.

Utility

Utility is the subjective satisfaction gained from consuming a good or service. What matters is relative comparison, not an absolute number.

Marginal Utility

Marginal utility = the increase in total utility from consuming one additional unit

The Law of Diminishing Marginal Utility: As consumption of the same good continues, the satisfaction from each additional unit decreases.

First slice of pizza when hungry: very satisfying
Second slice: still enjoyable
Third and fourth: satisfaction falls
Fifth: dissatisfaction (negative utility)

This law is the theoretical foundation of the downward-sloping demand curve. Because the utility from additional units falls, consumers are willing to pay less for each extra unit.

Consumer Equilibrium Condition (Marginal Utility View)

The consumer’s optimal choice is reached when the marginal utility per dollar is equal for all goods:

MUx / Px = MUy / Py

If MUx/Px > MUy/Py, the consumer should buy more of X and less of Y.

Indifference Curves

An indifference curve connects all combinations of goods X and Y that give the consumer the same level of utility.

Four Properties of Indifference Curves

Downward-sloping: To get more of X, you must give up Y (opportunity cost)
Higher curves represent higher utility: More consumption yields more satisfaction
Cannot cross: Crossing curves produce logical contradictions
Convex to the origin: Reflects the law of diminishing marginal rate of substitution

Marginal Rate of Substitution (MRS)

MRS = the amount of Y a consumer is willing to give up to gain one additional unit of X

The MRS is the absolute value of the slope of the indifference curve. The convex shape means that as the consumer holds more X, they are willing to give up less Y to acquire even more X (diminishing MRS).

The Budget Line

The budget line is a straight line showing all combinations of goods X and Y that the consumer can afford with a given income.

Budget line equation: Px·X + Py·Y = I (I = income)

Slope = -Px/Py: the amount of Y that must be sacrificed to buy one more unit of X (relative price)

Shifts in the Budget Line

Change	Budget Line Shift
Income increases	Parallel outward shift
Income decreases	Parallel inward shift
Px increases	X-intercept shifts inward (steeper slope)
Py increases	Y-intercept shifts inward (flatter slope)

Consumer Equilibrium: Optimal Choice

The consumer’s optimal choice occurs at the tangency point between the budget line and an indifference curve.

At this tangency:

MRS = Px/Py
The consumer’s subjective exchange rate (Y given up) equals the market’s objective relative price

Intuition: Optimal choice is reached when the consumer’s subjective valuation of the trade-off matches the market’s exchange ratio.

Decomposing the Effects of a Price Change

When Px falls, how does the consumer respond? The effect can be decomposed into two parts:

Substitution Effect

When Px falls, X becomes relatively cheaper than Y. The consumer substitutes away from Y toward X. Always in the opposite direction to the price change (price falls → demand increases).

Income Effect

When Px falls, real purchasing power rises (the consumer can buy more with the same income). The change in demand due to this real income gain:

Normal good: income rises → demand for X rises (same direction as substitution effect)
Inferior good: income rises → demand for X falls (opposite to substitution effect)
Giffen Good: income effect dominates substitution effect, so when price falls, demand actually decreases (extreme theoretical case)

Revealed Preference Theory

Revealed Preference Theory identifies consumer preferences from actual observed choices rather than directly measuring utility.

Core principle: “If a consumer chose A over B when both were affordable, A is revealed to be preferred to B.”

This approach validates demand theory using observable behavior rather than subjective utility.

Consumer Surplus Revisited

The difference between the maximum price a consumer is willing to pay and the price actually paid.

Consumer surplus = Area under the demand curve − actual expenditure

Used to analyze how government policy or price changes affect consumer welfare:

Tax imposed → price rises → consumer surplus decreases
Price ceiling → consumer surplus increases, but supply reduction creates allocation problems

Learning Checklist

Explain marginal utility and the law of diminishing marginal utility
Explain the meaning of the equilibrium condition MUx/Px = MUy/Py
Describe the four properties of indifference curves with a graph
Explain what the slope of the budget line represents
Explain the consumer’s optimal choice condition (MRS = Px/Py)
Apply substitution and income effects to normal goods and inferior goods

Key Concept Cards

Price Elasticity of Demand (PED) ★★★★★ : PED = |% Change in Qty Demanded / % Change in Price|. Elastic (PED>1): price ↑ → total revenue ↓. Inelastic (PED<1): price ↑ → total revenue ↑. Unit elastic (PED=1): total revenue unchanged. Memory tip: Necessities (medications) → inelastic. Luxuries (designer goods) → elastic.

Income Elasticity of Demand (YED) ★★★★ : YED = %ΔQd / %ΔY. Normal good: YED>0. Luxury good: YED>1. Necessity: 0<YED<1. Inferior good: YED<0.

Determinants of Supply Elasticity ★★★ : 1) Production period (short run < long run), 2) Spare capacity, 3) Storability of output, 4) Ease of sourcing inputs. Supply is more elastic in the long run.

Practice Quiz

Q. If PED = 2 and price rises by 10%, what happens to total revenue?

Quantity demanded falls 20% → total revenue decreases. Elastic demand (PED>1) → price ↑ causes TR to fall.

Q. What shape is a perfectly inelastic demand curve?

A vertical line (PED = 0). Like essential prescription drugs — quantity demanded is fixed regardless of price.

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