CHAPTER 9: Perfect Competition: Theory, Firm Behavior, and Market Dynamics

Perfect Competition

Definition and Characteristics

A perfectly competitive market is defined by the presence of many buyers and sellers trading identical products, such that each participant is a price taker. No single buyer or seller can influence the market price due to their relatively small market share.

• Identical Products: Goods offered by different firms are indistinguishable from one another.

• Free Entry and Exit: Firms can enter or leave the market without restriction.

• Price Taker: Each firm and consumer accepts the market price as given.

Example: The market for milk, where no individual buyer or seller can affect the price due to the large number of participants.

Profit Maximization in Perfect Competition

Firms in a perfectly competitive market aim to maximize profit, defined as:

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

• Profit Maximization Rule: Firms choose the output level where marginal revenue equals marginal cost.

Marginal Revenue: The change in total revenue from selling one additional unit of output. In perfect competition, marginal revenue equals price ($MR = P$).

• Profit Maximization Condition: $P = MC$ (where $P$ is price and $MC$ is marginal cost)

Short-Run Firm Decisions

Shut-Down Decision Rule

Firms must decide whether to continue operating or shut down in the short run based on their ability to cover costs.

• Stay Open: If total revenue exceeds total variable cost, the firm should continue operating, as it can cover variable costs and contribute to fixed costs.

• Shut Down: If the firm cannot cover variable costs, it should shut down to avoid further losses.

Shut-Down Rule:

• A firm should shut down if $P < \text{minimum average variable cost}$.

• A firm should continue to operate if $P > \text{minimum average variable cost}$.

• If $P = \text{minimum average variable cost}$, the firm is indifferent between operating and shutting down (the shut-down price).

Long-Run Equilibrium and Market Supply

Entry and Exit Dynamics

In the long run, firms can freely enter or exit the market:

• Positive Economic Profit: Attracts new firms, increasing market supply and driving down price.

• Losses: Cause firms to exit, reducing market supply and increasing price.

• Long-Run Equilibrium: Each firm earns zero economic profit ($\text{Economic Profit} = 0$).

Long-Run Market Supply Curve

The shape of the long-run market supply curve depends on industry cost structure:

• Increasing Cost Industry: Supply curve is positively sloped; average cost rises as output increases.

• Constant Cost Industry: Supply curve is horizontal; average cost remains unchanged as output increases.

Market Response to Demand Shifts

Short-Run and Long-Run Effects

When demand increases, the market adjusts in two stages:

Short-run:

• Initial equilibrium: $P_1$, $Q_1$, $q_1$, zero economic profit.

• New equilibrium: $P_2$, $Q_2$, $q_2$, positive economic profit, same number of firms.

Long-run:

• New firms enter due to positive profit.

• Market supply increases, price falls (rightward shift in supply).

• New equilibrium: $P_1$, $Q_3$, $q_1$, zero economic profit, more firms.

Increasing Cost Industry Adjustment

• Final long-run equilibrium price ($P_3$) is higher than initial price ($P_1$) due to increased average costs.

Applications and Exercises

Case Studies

Case	Industry	Input Supply	Short-Run Price Effect	Long-Run Price Effect
1	Housing	Limited (lumber)	Price rises	Price remains higher (increasing cost)
2	Ice Cream Stands	Plentiful (labor, milk)	Price rises	Price returns to original (constant cost)

Economic Puzzle: Butter Market

When demand for butter dropped, price fell in the short run, causing firms to lose money. Over time, firms exited the market, reducing supply and causing price to rise again, even though demand remained unchanged.

• Short-run: Price falls due to decreased demand.

• Long-run: Firms exit, supply decreases, price rises.

Additional info: In a constant-cost industry, price would eventually return to its original level; in an increasing-cost industry, it would remain below the original price.

Key Formulas

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

• Profit Maximization: $MR = MC$ or $P = MC$ in perfect competition

• Shut-Down Rule: $P < \text{minimum AVC}$ (average variable cost)