Lecture 5: Consumer Behaviour and Budget Constraints in Microeconomics

Consumer Behaviour

Introduction to the Buyer's Problem

Consumer behaviour in microeconomics examines how individuals make choices about what to purchase, given their preferences, prices, and budget constraints. The buyer's problem is central to understanding demand and market outcomes.

• The Buyer's Problem: Consists of three main components: preferences (what you like), prices of goods and services, and your budget (how much money you have to spend).

• Optimizing Decisions: Buyers make decisions at the margin, meaning they consider the additional benefit and cost of consuming one more unit of a good.

• Individual Demand Curve: Reflects both the ability and willingness to pay for a good or service, showing the relationship between price and quantity demanded.

Consumer Surplus

Consumer surplus is a key concept for measuring the benefit consumers receive from market transactions.

• Definition: The difference between what a buyer is willing to pay for a good and what the buyer actually pays.

• Significance: It is a meaningful measure of changes in consumer well-being, especially when prices change.

Elasticity

Elasticity measures how responsive one variable is to changes in another variable, such as how quantity demanded responds to price changes.

• Price Elasticity of Demand: Indicates the percentage change in quantity demanded resulting from a percentage change in price.

• Formula: $\text{Price Elasticity of Demand} = \frac{\% \text{ change in quantity demanded}}{\% \text{ change in price}}$

Consumers and Incentives

The Demand Curve and Its Slope

The demand curve typically has a negative slope, reflecting the law of demand: as price increases, quantity demanded decreases.

• Example Demand Function: $Q_d = 350 - 50P$

• Inverse Demand Function: $P = 7 - \frac{Q_d}{50}$

• Graphical Representation: The demand curve plots price against quantity demanded, showing the negative relationship.

Price (per gallon)	Quantity Demanded (gallons/year)
7	0
6	50
5	100
4	150
3	200
2	250
1	300

What You Like: Tastes and Preferences

Role of Preferences in Consumer Choice

Preferences determine what consumers choose to buy. While tastes vary, two assumptions are commonly made in microeconomics:

• Biggest Bang for the Buck: Consumers aim to maximize their utility given their budget.

• Revealed Preferences: Actual purchases reflect underlying tastes and preferences.

Prices of Goods and Services

Assumptions about Prices

Prices play a crucial role in consumer decision-making. Microeconomic models often assume:

• Fixed Prices: Prices are set and not subject to negotiation.

• Perfectly Elastic Supply: Consumers can buy as much as they want at the given price without affecting the price.

How Much Money You Have to Spend: The Budget Set

Budget Constraints and Consumer Choice

The budget set represents all possible combinations of goods a consumer can afford, given their income and the prices of goods.

• No Saving or Borrowing: Consumers spend all their income on goods.

• Discrete Choices: Purchases are made in whole units, even though the budget line is drawn as a continuous line.

Budget Constraint Equation

Given jeans cost $50 each, sweaters cost $25 each, and a budget of $300, the budget constraint is:

• Equation: $50j + 25s = 300$

Graphing the Budget Set

The budget line shows all combinations of jeans and sweaters that exhaust the budget.

• Solving for Sweaters: $s = \frac{300 - 50j}{25}$ or $s = 12 - 2j$

• General Form: If $P_y Q_y + P_x Q_x = B$, then $Q_y = \frac{B}{P_y} - \frac{P_x}{P_y} Q_x$

Bundle	Quantity of Sweaters	Quantity of Jeans
A	12	0
B	10	1
C	8	2
D	6	3

Interpreting the Slope of the Budget Line

The slope of the budget line represents the rate at which one good can be traded for another, given their prices.

• Slope: $-\frac{P_j}{P_s}$ (for jeans and sweaters)

• Example: $s = 12 - 2j$; the slope is -2, meaning each additional jean purchased requires giving up 2 sweaters.

Opportunity Cost

Opportunity cost is the value of the next best alternative forgone when making a choice.

• Opportunity Cost of Jeans: $\text{Loss in sweaters} / \text{Gain in jeans} = 2 / 1$

• Opportunity Cost of Sweaters: $\text{Loss in jeans} / \text{Gain in sweaters} = 1 / 2$

• General Formula: $\text{Opportunity cost of something} = \frac{\text{Loss of units of other stuff}}{\text{Gain in units of something}}$

Example Application

Suppose you have $300 to spend, jeans cost $50, and sweaters cost $25. The budget constraint is $50j + 25s = 300$. If you buy 4 jeans, you can buy 4 sweaters ($50*4 + 25*4 = 200 + 100 = 300$). The opportunity cost of buying one more jean is giving up 2 sweaters.

Additional info: The notes above are expanded with definitions, formulas, and examples to provide a comprehensive overview suitable for microeconomics students.