Microeconomics Study Guide: Consumer Choice, Production, Costs, and Perfect Competition

Consumer Choice and Behavioral Economics

Budget Constraint

The budget constraint represents all combinations of goods a consumer can purchase given their income and the prices of those goods.

• Intercepts: Found by dividing income by the price of each good. For goods X and Y, the intercepts are $\frac{I}{P_X}$ and $\frac{I}{P_Y}$ respectively.

• Slope: The slope of the budget constraint is $-\frac{P_X}{P_Y}$, representing the rate at which the consumer can trade one good for another.

• Changes: An increase in income shifts the constraint outward; a change in price pivots the constraint.

Example: If income is $100, price of X is $10, and price of Y is $20, the intercepts are 10 units of X and 5 units of Y.

Utility and Marginal Utility

Utility measures satisfaction from consuming goods. Marginal Utility (MU) is the additional satisfaction from consuming one more unit.

• Law of Diminishing Marginal Utility: As more of a good is consumed, the additional utility from each extra unit decreases.

• Household Objective: Maximize total utility given budget constraints.

Example: Eating slices of pizza: the first slice gives high satisfaction, but each additional slice gives less.

Utility Maximization Rule

Consumers allocate their budget so that the last dollar spent on each good yields the same marginal utility per dollar.

• Formula: $\frac{MU_X}{P_X} = \frac{MU_Y}{P_Y}$

• If $\frac{MU_X}{P_X} > \frac{MU_Y}{P_Y}$, buy more X and less Y.

Example: If the marginal utility per dollar for X is higher than for Y, the consumer should buy more X.

Income and Substitution Effects

When the price of a good changes, two effects occur:

• Income Effect: Change in consumption due to change in purchasing power.

• Substitution Effect: Change in consumption as consumers substitute the cheaper good for the more expensive one.

• Normal Goods: Consumption increases with income.

• Inferior Goods: Consumption decreases with income.

Example: If the price of X falls, consumers buy more X (substitution effect) and may buy more of both goods if X is normal (income effect).

Downward Sloping Demand Curve

Demand curves slope downward due to the substitution and income effects: as price falls, quantity demanded rises.

Work-Leisure Model

This model analyzes the trade-off between leisure and labor (work).

• Optimum Bundle: The combination of leisure and consumption that maximizes utility.

• Labor Supply: Determined by the point where the marginal utility of leisure equals the marginal utility of income from working.

• Wage Changes: Higher wages increase the opportunity cost of leisure, leading to substitution and income effects.

Example: If wages rise, people may work more (substitution effect) or less (income effect, if leisure is a normal good).

The Costs of Production

Accounting Costs vs. Economic Costs

Accounting costs are explicit monetary expenses. Economic costs include both explicit and implicit costs (opportunity costs).

• Economic Profit: $\text{Economic Profit} = \text{Total Revenue} - \text{Economic Costs}$

• Positive economic profit: firm earns more than opportunity cost.

• Zero economic profit: firm earns exactly opportunity cost (normal profit).

• Economic loss: firm earns less than opportunity cost.

• Firm Objective: Maximize profit.

Short-Run vs. Long-Run

The short-run is a period where at least one input is fixed. The long-run is when all inputs can be varied.

Production Functions and Marginal Product

Total Product (TP): Total output produced. Marginal Product of Labor (MPL): Additional output from one more unit of labor.

• Law of Diminishing Returns: As more labor is added, MPL eventually decreases.

• Formula: $\text{MPL} = \frac{\Delta TP}{\Delta L}$

Example: If adding a worker increases output from 10 to 15, MPL = 5.

Cost Curves and Perfect Competition

Types of Costs and Formulas

Firms face various costs:

• Total Variable Cost (TVC): Costs that change with output.

• Total Fixed Cost (TFC): Costs that do not change with output.

• Marginal Cost (MC): Cost of producing one more unit.

• Average Variable Cost (AVC): $\text{AVC} = \frac{TVC}{Q}$

• Average Fixed Cost (AFC): $\text{AFC} = \frac{TFC}{Q}$

• Average Total Cost (ATC): $\text{ATC} = \frac{TC}{Q}$

Relationship: MC intersects AVC and ATC at their minimum points. AFC declines as output increases.

Efficient Scale: Output level where ATC is minimized.

Profit Maximization in Perfect Competition

Perfect competition is characterized by:

• Many firms

• Identical products

• No barriers to entry

For perfectly competitive firms:

• Market price is the firm's demand curve and marginal revenue (MR) curve.

• Profit maximization occurs where $MR = MC$.

Profit Calculation: Profit = (Price - ATC) × Quantity.

Example: If price is above ATC, firm earns profit; if below, firm incurs loss.

Short-Run and Long-Run Decisions

Shut-Down Point

In the short-run, a firm should shut down if price is below AVC.

• Shut-down point: Where MC intersects AVC.

Supply Curves

The individual supply curve for a perfectly competitive firm is its MC curve above AVC.

• Market Supply Curve: Sum of all individual supply curves.

Long-Run Average Cost (LRAC) and Returns to Scale

The LRAC curve shows the lowest possible cost per unit when all inputs are variable.

• Economies of Scale: LRAC decreases as output increases.

• Diseconomies of Scale: LRAC increases as output increases.

• Minimum Efficient Scale (MES): Lowest output at which LRAC is minimized.

• Returns to Scale: Describes how output changes as all inputs change.

Difference: Diminishing returns relate to variable input in short-run; returns to scale relate to all inputs in long-run.

Long-Run Market Adjustments

Long-run decisions depend on profits:

• Above normal profit: new firms enter, supply increases, price falls.

• Normal profit: no entry or exit, equilibrium.

• Below normal profit: firms exit, supply decreases, price rises.

In long-run equilibrium, firms operate at minimum LRAC.

Cost Industry Types and Long-Run Supply Curve

Industries can be:

• Constant Cost: LRAC does not change as industry expands.

• Increasing Cost: LRAC rises as industry expands.

• Decreasing Cost: LRAC falls as industry expands.

The slope of the long-run supply curve depends on how input prices and LRAC change with industry expansion.

Industry Type	LRAC Behavior	Long-Run Supply Curve Slope
Constant Cost	Flat	Horizontal
Increasing Cost	Upward	Upward-sloping
Decreasing Cost	Downward	Downward-sloping

Additional info: Academic context was added to clarify formulas, relationships, and examples for each concept.