Imperfect Competition — AP Microeconomics Micro Study Guide

For: AP Microeconomics candidates sitting AP Microeconomics.

Covers: All core Imperfect Competition subtopics per the AP Micro CED: monopoly, monopolistic competition, oligopoly with game theory, and price discrimination, plus exam tips and practice problems.

You should already know: No prior econ required.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Microeconomics style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What Is Imperfect Competition?

Imperfect competition describes any market structure where firms have some control over the price they charge (market power), unlike perfect competition where firms are price takers with zero ability to influence market price. This topic makes up 25-30% of your total AP Micro exam score per the official CED, and covers all markets that fail to meet the strict conditions of perfect competition (many small firms, identical products, zero barriers to entry, perfect information). Common synonyms include imperfect markets and non-competitive markets.

2. Monopoly — Price Maker, Deadweight Loss

A monopoly is defined as a market with one single seller of a unique good with no close substitutes, and extremely high barriers to entry that block competitors from entering the market. Common barriers to entry include patents, exclusive government licenses, natural monopoly economies of scale, and exclusive control of a key input.

Key Traits

1. Price Maker: Unlike perfectly competitive firms that face a horizontal demand curve, a monopolist faces the full downward-sloping market demand curve. To maximize profit, it first chooses the output level where marginal revenue ($MR$) equals marginal cost ($MC$), then charges the maximum price consumers are willing to pay for that output, as read from the demand curve. For any price-making firm, $MR$ always lies below the demand curve, because the firm must lower price for all units sold to sell additional output, not just the marginal unit.
2. Deadweight Loss (DWL): Because the monopolist sets $P>MC$, it produces less output than the socially optimal perfectly competitive output, creating a deadweight loss of total surplus that is captured by neither consumers nor producers.

Worked Example

A local power company (natural monopoly) faces a demand curve $P=20-2Q$, with constant $MC=\$4$ and no fixed costs.

1. For linear demand, $MR$ has the same intercept and twice the slope of the demand curve: $MR=20-4Q$
2. Set $MR=MC$ to find profit-maximizing output: $20-4Q=4⟹Q=4$
3. Plug $Q=4$ into demand to find price: $P=20-2(4)=\$12$
4. Socially optimal output (where $P=MC$): $20-2Q=4⟹Q=8$
5. DWL is the triangle between $Q=4$ and $Q=8$: $$DWL=\frac{1}{2}\ast (8-4)\ast (12-4)=\$16$$

Exam tip: Examiners almost always ask you to label DWL on a monopoly graph, so remember it is the triangle to the left of the socially optimal output, bounded by the demand curve, $MC$ curve, and monopoly output line.

3. Monopolistic Competition

Monopolistic competition is a market with many small firms, low barriers to entry and exit, and differentiated products: goods that are similar but not identical (e.g., local restaurants, clothing brands, coffee shops). Product differentiation gives each firm a small amount of market power, so they face gently downward-sloping demand curves.

Key Traits

1. Short Run: Firms act like mini-monopolists: they set $MR=MC$ to choose output, and can earn positive economic profit if $P>ATC$ (average total cost) at the profit-maximizing quantity.
2. Long Run: Low barriers to entry mean new firms enter the market when existing firms earn positive profit, stealing demand from existing firms and shifting their demand curves left. This continues until $P=ATC$ at the profit-maximizing output, so long-run economic profit is zero. Firms in this market have excess capacity: they produce less than the output that minimizes ATC, creating a small DWL that is often offset by the value consumers place on product variety.

Worked Example

A local bakery (monopolistic competition) faces short-run demand $P=10-0.5Q$, with $MC=\$2$ and $ATC=\$4$ at the profit-maximizing output.

1. $MR=10-Q$, set equal to $MC$: $10-Q=2⟹Q=8$
2. Short-run price: $P=10-0.5(8)=\$6$
3. Short-run profit: $(6-4)\ast 8=\$16$
4. In the long run, new bakeries enter, shifting the existing bakery's demand left to $P=7-0.5Q$. New profit-maximizing output: $MR=7-Q=2⟹Q=5$, price $P=7-0.5(5)=\$4.50$, which equals the new ATC at $Q=5$, so economic profit falls to zero.

4. Oligopoly and Game Theory

Oligopoly is a market with 2-10 large firms that control 70%+ of total market share, high barriers to entry, and interdependence: each firm's profit depends directly on the actions of its competitors, so firms must strategize to maximize profit. Game theory is the standard tool used to analyze oligopoly behavior.

Key Terms

1. Payoff Matrix: A table showing the profit each firm earns for every combination of actions taken by all players in the market.
2. Dominant Strategy: An action that gives a firm higher profit no matter what action its competitor takes.
3. Nash Equilibrium: A stable outcome where no firm can increase its profit by changing its action, given the actions of all other firms.
4. Collusion: An illegal agreement between firms to set high prices and restrict output to act like a monopoly. Collusive agreements are inherently unstable, because each firm has a strong incentive to cheat by lowering price to steal market share.

Worked Example

A duopoly (2-firm oligopoly) of flight operators serving a small regional route can set either a High price or Low price. The payoff matrix below shows annual profit in millions of dollars:

1. Find dominant strategies: If Firm B sets High, Firm A earns 100 for High, 140 for Low → choose Low. If Firm B sets Low, Firm A earns 30 for High, 60 for Low → choose Low. Both firms have a dominant strategy to set Low Price.
2. Nash Equilibrium: Both set Low Price, earning 60M each. Even though both would be better off colluding to set High Price, neither can improve their profit by switching to High if the other keeps their price Low. This is the classic prisoner's dilemma outcome.

5. Price Discrimination

Price discrimination is when a firm sells the identical good to different customers at different prices, with no difference in production cost for the two groups. Three conditions are required for price discrimination to work:

1. The firm has market power (is a price maker)
2. The firm can separate customers into groups with different price elasticities of demand
3. The firm can prevent resale of the good between customer groups

Common Types

1. First-Degree (Perfect) Price Discrimination: The firm charges each individual customer the maximum price they are willing to pay. This eliminates all consumer surplus, converts it to producer surplus, and produces the socially optimal output where $P=MC$, so there is zero DWL.
2. Third-Degree Price Discrimination: The firm charges different prices to distinct groups of customers (e.g., student discounts, senior discounts, business vs economy airline tickets). Firms follow the inverse elasticity rule: they charge higher prices to groups with less elastic demand, and lower prices to groups with more elastic demand.

Worked Example

A museum (local monopolist) has two customer groups: adults with demand $P=15-Q$ (elasticity = -1.8) and seniors with demand $P=10-Q$ (elasticity = -3.5). $MC$ per visitor is $\$3$.

1. Adult group: $MR=15-2Q$, set to $MC=3⟹Q=6$, $P=15-6=\$9$
2. Senior group: $MR=10-2Q$, set to $MC=3⟹Q=3.5$, $P=10-3.5=\$6.50$ The museum charges adults a higher price because their demand is less elastic, so they are less likely to stop visiting if prices rise.

6. Common Pitfalls (and how to avoid them)

• Pitfall 1: Drawing the MR curve above the demand curve for a monopolist or monopolistic competitor. Why? Students mix up price-making firms with perfect competition where $P=MR$. Correct move: Always draw MR below the downward-sloping demand curve for any price-making firm, since lowering price to sell more units reduces revenue from all existing units.
• Pitfall 2: Assuming monopolistic competition earns positive economic profit in the long run. Why? Students confuse it with monopoly, which has high barriers to entry. Correct move: Low barriers to entry mean new firms enter until demand shifts left to where $P=ATC$, so long-run economic profit is zero for monopolistic competition.
• Pitfall 3: Identifying the highest total payoff cell as the Nash equilibrium. Why? Students assume firms will collude to maximize total profit, but collusion is not enforceable in most cases. Correct move: Test each cell: if no player can increase their payoff by switching their action while others stay the same, that is the Nash equilibrium.
• Pitfall 4: Assuming all price discrimination increases deadweight loss. Why? Students associate monopoly power with higher DWL. Correct move: Perfect (first-degree) price discrimination produces the socially optimal output, so it eliminates DWL entirely, even though it captures all consumer surplus as producer surplus.
• Pitfall 5: Calculating monopoly output by setting $P=MC$ instead of $MR=MC$. Why? Students apply the perfect competition profit-maximizing rule to monopolies. Correct move: For all price-making firms, the profit-maximizing output is always where $MR=MC$, then you find price from the demand curve at that output.

7. Practice Questions (AP Microeconomics Style)

Question 1 (1 point, Multiple Choice)

A monopolist faces a demand curve of $P=16-Q$, with constant marginal cost $MC=4$ and no fixed costs. What is the deadweight loss created by the monopoly? A) $\$18$ B) $\$32$ C) $\$9$ D) $\$24$

Worked Solution:

1. Monopoly MR: $MR=16-2Q$
2. Profit-maximizing output: $16-2Q=4⟹Q=6$, $P=16-6=\$10$
3. Socially optimal output: $16-Q=4⟹Q=12$
4. DWL: $\frac{1}{2}\ast (12-6)\ast (10-4)=\$18$ Correct answer: A

Question 2 (2 points, Short Answer)

Two laptop manufacturers, Firm X and Firm Y, are choosing whether to launch a new product line or not. The payoff matrix below shows annual profits in millions of dollars:

Worked Solution: a) Test Firm Y's payoffs: If Firm X launches, Firm Y earns 35 for Launch, 20 for No Launch → choose Launch. If Firm X does not launch, Firm Y earns 60 for Launch, 45 for No Launch → choose Launch. Yes, Firm Y's dominant strategy is to Launch. b) Test each cell: If both launch, Firm X cannot improve by switching (40 > 25) and Firm Y cannot improve by switching (35 > 20). All other cells have at least one firm that can increase profit by switching. Nash equilibrium is (Launch, Launch), with payoffs 40, 35.

Question 3 (3 points, FRQ Part)

A local coffee shop operates in a monopolistically competitive market, currently earning positive short-run economic profit. a) Draw a correctly labeled graph for the coffee shop, showing the short-run profit-maximizing price, quantity, and area of profit. (2 points) b) Explain what will happen to the coffee shop's demand curve in the long run, and why. (1 point)

Worked Solution: a) Correct graph requirements: 1) Downward sloping Demand (D) and MR curve below D, 2) Upward sloping MC curve intersecting U-shaped ATC at its minimum, 3) Profit-maximizing quantity $Q^{\ast }$ at the intersection of MR and MC, 4) Price $P^{\ast }$ on the demand curve above $Q^{\ast }$, with $P^{\ast }>ATC$ at $Q^{\ast }$, 5) Profit is the rectangle with height $(P^{\ast }-AT{C}_{Q^{\ast }})$ and width $Q^{\ast }$. b) In the long run, new coffee shops will enter the market because of low barriers to entry and positive short-run profit. This will reduce the number of customers for the existing shop, shifting its demand curve left until $P=ATC$ at the profit-maximizing quantity, so economic profit falls to zero.

8. Quick Reference Cheatsheet

Market Structure Summary

Key Rules & Formulas

1. Monopoly MR for linear demand $P=a-bQ$: $MR=a-2bQ$
2. Monopoly DWL: $\frac{1}{2}\ast (Q_{social}-Q_{monopoly})\ast (P_{monopoly}-MC)$
3. Nash Equilibrium: No player can improve payoff by switching action if others stay constant
4. Price Discrimination Conditions: Market power, separable groups, no resale; Perfect price discrimination = zero DWL; Third degree = higher price for less elastic demand groups

9. What's Next

Imperfect competition is the foundation for several high-weight AP Micro topics that appear later in the syllabus, including regulation of natural monopolies, anti-trust policy targeting collusive oligopoly behavior, and welfare analysis of government interventions like price ceilings on monopolies. You will also frequently see imperfect market contexts in FRQ questions about market failure, income distribution, and real-world industry analysis, so mastering these concepts will give you a significant edge on 40%+ of the exam content.

If you struggle with any of the concepts covered in this guide, from drawing monopoly graphs to identifying Nash equilibria in payoff matrices, you can ask Ollie, our AI tutor, for personalized explanations, extra practice questions, or step-by-step walkthroughs of past exam problems. You can also head to the homepage to access more AP Microeconomics study guides, timed practice tests, and exam strategy tips tailored to the College Board curriculum.