Review AP Micro Unit 4 to understand how monopolies, oligopolies, and monopolistically competitive firms set prices above marginal cost, create deadweight loss, and make strategic decisions. This unit covers the graphs, calculations, and game theory tools that explain real-world market power.
Use the topic guides, practice questions, FRQ practice, and AP score calculator available for this unit to focus your review.
In Unit 3 you studied perfectly competitive firms that are price takers. Unit 4 introduces firms with market power: they face downward-sloping demand curves, must lower price to sell more, and consistently charge above marginal cost. That gap between price and marginal cost is the source of allocative inefficiency and deadweight loss across all four market structures in this unit.
An imperfectly competitive firm must lower its price on all units to sell one more. That means the extra revenue from the new unit is less than its price, so the MR curve falls below the demand curve and declines twice as fast for a linear demand. This is the foundation of every monopoly and monopolistic competition graph in the unit.
At the monopoly output where MR = MC, the price charged exceeds marginal cost. Units between the monopoly quantity and the competitive quantity would have generated net gains for society but are not produced. That lost surplus is the deadweight loss triangle, and it appears on every imperfect competition graph you draw or read.
Oligopolists are interdependent: each firm's best choice depends on what rivals do. Game theory formalizes this with payoff matrices. A dominant strategy is best regardless of the rival's choice. A Nash equilibrium is the outcome where no player wants to deviate unilaterally. The prisoner's dilemma explains why cartels are unstable even when collusion would raise joint profits.
The AP Micro enduring understanding for this unit is that even when all firms share the goal of profit maximization, the structure of the market constrains what they can do. A monopolist faces no rivals and earns long-run profit. A monopolistic competitor earns zero economic profit in the long run but still operates with excess capacity and P > MC. An oligopolist must anticipate rivals. In every case, the deviation from P = MC is the measure of inefficiency, and the size of the deadweight loss triangle is the measure of welfare loss.
Defines monopoly, oligopoly, monopolistic competition, and monopsony. Establishes that all imperfectly competitive firms face downward-sloping demand, charge P > MC, and are protected by barriers to entry such as patents, high startup costs, or exclusive resource ownership.
Covers the monopoly graph in detail: MR = MC determines quantity, price is read off demand, and the profit rectangle and deadweight loss triangle are calculated. Also introduces natural monopoly and the role of barriers to entry in sustaining long-run profit.
Explains how a firm with market power charges different prices based on willingness to pay. Perfect price discrimination sets D = MR, produces the competitive quantity, eliminates deadweight loss, and converts all consumer surplus into producer profit.
Analyzes short-run profit or loss and the long-run adjustment to zero economic profit through free entry and exit. In long-run equilibrium, demand is tangent to ATC, but P > MC and excess capacity remain, making the market allocatively inefficient.
Introduces payoff matrices, dominant strategies, Nash equilibrium, and the prisoner's dilemma. Connects game theory to oligopoly behavior including the incentive to collude, the incentive to cheat on cartels, and how side payments can change strategic outcomes.
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Across 14k multiple-choice practice attempts for this unit.
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Review Price Discrimination with attention to how the concept appears in AP-style source and evidence questions.
Review Oligopoly and Game Theory with attention to how the concept appears in AP-style source and evidence questions.
Review Monopolistic Competition with attention to how the concept appears in AP-style source and evidence questions.
Review Introduction to Imperfectly Competitive Markets with attention to how the concept appears in AP-style source and evidence questions.
Imperfectly competitive markets break at least one assumption of perfect competition. The defining feature is that firms face a downward-sloping demand curve, so they must lower price to sell additional units. This makes marginal revenue fall below price for every unit after the first, and it means price will always exceed marginal cost at the profit-maximizing output. Barriers to entry protect these firms from competition and allow inefficiency to persist.
| Structure | Number of firms | Product type | Barriers to entry | Long-run profit |
|---|---|---|---|---|
| Monopoly | One | Unique | Very high | Positive |
| Oligopoly | Few | Similar or differentiated | High | Positive |
| Monopolistic competition | Many | Differentiated | Low | Zero |
| Monopsony | One buyer | Homogeneous input | High | Positive (buyer side) |
A monopoly is a single seller protected by barriers to entry. It maximizes profit by producing where MR = MC, then reads the price off the demand curve above that quantity. Because price exceeds both marginal cost and average total cost at the profit-maximizing output, the monopolist earns positive economic profit in both the short run and the long run. A natural monopoly occurs when long-run average costs decline throughout the entire range of market demand, making a single firm the lowest-cost producer.
| Outcome | Perfect competition | Monopoly |
|---|---|---|
| Price | P = MC | P > MC |
| Output | Allocatively efficient Q | Below efficient Q |
| Economic profit (long run) | Zero | Positive |
| Deadweight loss | None | Positive triangle |
| Consumer surplus | Maximized | Reduced |
A firm with market power can charge different prices to different buyers based on willingness to pay. The most tested case is perfect (first-degree) price discrimination, where the monopolist charges each consumer exactly their maximum willingness to pay. This changes the graph significantly: the demand curve becomes the marginal revenue curve, the firm produces where D = MR = MC, all consumer surplus is transferred to the producer as profit, and deadweight loss is eliminated entirely.
| Outcome | Single-price monopoly | Perfect price discrimination |
|---|---|---|
| Quantity produced | Below competitive Q | Equal to competitive Q |
| Consumer surplus | Positive (reduced) | Zero (fully extracted) |
| Producer surplus / profit | Positive rectangle | Entire area under demand above MC |
| Deadweight loss | Positive triangle | Zero |
Monopolistic competition combines features of monopoly and perfect competition. Firms sell differentiated products and face downward-sloping demand, giving them some price-making power. In the short run, a firm can earn positive, negative, or zero economic profit. Because barriers to entry are low, positive profit attracts new entrants, shifting each firm's demand curve left until economic profit reaches zero in the long run. The long-run equilibrium has the demand curve tangent to ATC, so P = ATC but P > MC, meaning the firm still operates with excess capacity.
An oligopoly has a few large firms with high barriers to entry. Because each firm's profit depends on rivals' choices, firms are mutually interdependent. Game theory provides the tools to analyze these decisions. A payoff matrix shows each player's outcomes for every combination of strategies. A dominant strategy is best regardless of what the rival does. A Nash equilibrium is any outcome where no player can improve their payoff by changing strategy alone. The prisoner's dilemma shows why firms have an incentive to cheat on collusive agreements even when cooperation would raise joint profits.
| Concept | Definition | How to find it |
|---|---|---|
| Dominant strategy | Best response regardless of rival's choice | Compare payoffs row by row (or column by column) for each player |
| Nash equilibrium | No player wants to deviate unilaterally | Check that each player is playing a best response to the other's strategy |
| Prisoner's dilemma outcome | Both players choose dominant strategy, both worse off than cooperation | Identify dominant strategies; note joint payoff is lower than cooperative payoff |
Try AP-style multiple-choice questions and written prompts after you review the notes.
| Term | Definition |
|---|---|
| Market Power | The ability of a firm to set price above marginal cost because it faces a downward-sloping demand curve rather than a perfectly elastic one. |
| Marginal Revenue (MR) | The additional revenue from selling one more unit. For an imperfect competitor, MR is less than price because the firm must lower price on all units to sell one more. |
| Barriers to Entry | Obstacles that prevent new firms from entering a market, including high startup costs, patents, legal restrictions, and exclusive resource ownership. They allow imperfect competitors to earn long-run profit. |
| Deadweight Loss | The loss of total surplus that occurs when a market produces less than the allocatively efficient quantity. In imperfect competition, it appears as a triangle between the demand curve and MC from the monopoly quantity to the competitive quantity. |
| allocative inefficiency | A market outcome where price exceeds marginal cost, causing underproduction relative to the socially optimal level and generating deadweight loss. |
| Natural Monopoly | A market where long-run average costs decline over the full range of demand, making a single firm the lowest-cost producer and creating a barrier to entry for rivals. |
| perfect price discrimination | A pricing strategy where a monopolist charges each consumer their maximum willingness to pay, producing the competitive quantity, extracting all consumer surplus as profit, and eliminating deadweight loss. |
| Differentiated products | Goods that are distinct in quality, features, or branding, allowing monopolistically competitive firms to have some pricing power even with many competitors. |
| dominant strategy | A strategy that yields a higher payoff for a player regardless of what the other player chooses. When both players have dominant strategies, the result is a dominant strategy equilibrium. |
| collusion | An agreement among oligopolistic firms to coordinate prices or output to increase joint profits, effectively acting like a monopolist. Collusion is unstable because each firm has an incentive to cheat. |
| Prisoner's Dilemma | A game theory scenario where individual incentives lead both players to a dominant strategy that produces a worse joint outcome than cooperation would, explaining why cartels tend to break down. |
MR = MC gives you the quantity, not the price. After finding Q, you must go up to the demand curve to read the price. Drawing the price at the MR = MC intersection is one of the most common graph errors in this unit.
It is actually less inefficient. Perfect price discrimination eliminates deadweight loss because the firm produces the competitive quantity. The inefficiency shifts from quantity distortion to full surplus extraction, but total surplus equals the competitive outcome.
Zero economic profit means P = ATC, not P = MC. The firm still charges above marginal cost and produces below the ATC-minimizing output, so allocative inefficiency and excess capacity both persist in long-run equilibrium.
A dominant strategy equilibrium is a Nash equilibrium, but not every Nash equilibrium involves dominant strategies. In some games, a Nash equilibrium exists only because each player is best-responding to the other, not because one strategy dominates all others.
For a linear demand curve, MR starts at the same vertical intercept and falls at twice the rate. This means MR hits the horizontal axis at the midpoint of the demand curve's horizontal intercept, which matters for correctly drawing graphs and finding quantities.
AP Micro free-response questions frequently ask you to draw a correctly labeled imperfect competition graph and then identify or calculate specific areas. You should be ready to shade and calculate the profit rectangle as (P - ATC) x Q, the consumer surplus triangle, and the deadweight loss triangle using coordinates read from a graph or provided in a table. Both monopoly and monopolistic competition graphs appear in this format.
Multiple-choice and free-response questions often ask you to compare outcomes across market structures, such as explaining why a monopoly produces less and charges more than a perfectly competitive market, or why a monopolistically competitive firm has excess capacity in long-run equilibrium even though economic profit is zero. Be precise about which inefficiency (allocative, productive, or both) applies to each structure.
Oligopoly questions typically present a two-by-two payoff matrix and ask you to identify dominant strategies, find the Nash equilibrium, explain whether the outcome is a prisoner's dilemma, or calculate the minimum payment needed to change a player's strategy. Unlike other Unit 4 topics, this task uses a table rather than a graph, and the calculation skill (finding the incentive sufficient to alter a dominant strategy) is explicitly tested.
Open the individual guides for Unit 4 when you want a closer review of one topic.
browse guidesPractice free-response reasoning and compare your answer with scoring guidance.
practice FRQsWatch past review streams filtered to Unit 4 when you want a video walkthrough.
open videosUse unit cheatsheets for a quick visual review after you work through the notes.
open cheatsheetsEstimate your broader AP score goal after you review the course and exam format.
open calculatorAP Micro Unit 4 covers 5 topics across imperfect competition: Introduction to Imperfectly Competitive Markets (4.1), Monopoly (4.2), Price Discrimination (4.3), Monopolistic Competition (4.4), and Oligopoly and Game Theory (4.5). Together they explain how real-world firms set prices when they have market power, unlike the perfectly competitive model. See the full topic breakdown at /ap-micro/unit-4.
AP Micro Unit 4 makes up 15-22% of the AP exam, making it one of the heavier-weighted units. It covers imperfect competition, including monopoly, price discrimination, monopolistic competition, and oligopoly with game theory. Expect several multiple-choice questions and possible FRQ components drawn directly from these topics.
The AP Micro Unit 4 progress check in AP Classroom includes both MCQ and FRQ parts drawn from all five unit topics: imperfectly competitive markets, monopoly, price discrimination, monopolistic competition, and oligopoly and game theory. The MCQ section tests graph reading and concept identification, while the FRQ section asks you to analyze firm behavior and market outcomes. For matched practice on every topic the progress check covers, visit /ap-micro/unit-4.
AP Micro Unit 4 FRQs most often focus on monopoly profit maximization, price discrimination scenarios, and oligopoly game theory payoff matrices. A typical question gives you a graph or table and asks you to identify output, price, profit, and deadweight loss, or to find a dominant strategy in a game theory problem. Practice by drawing the monopoly and monopolistic competition graphs from scratch, labeling every curve, and writing out your reasoning step by step. Find Unit 4 FRQ practice at /ap-micro/unit-4.
The best place to find AP Micro Unit 4 practice questions, including multiple-choice and practice test sets, is /ap-micro/unit-4. That page has resources covering all five topics: monopoly, price discrimination, monopolistic competition, and oligopoly with game theory. Mixing MCQ drills with full practice tests helps you get comfortable with both graph-based questions and written analysis.
Start with monopoly, since it anchors the whole unit. Make sure you can draw the monopoly graph, find the profit-maximizing output where MR equals MC, mark the price on the demand curve, and shade in profit or loss. Then layer in price discrimination, monopolistic competition, and oligopoly. For game theory, practice reading payoff matrices and finding dominant strategies until it feels automatic. A solid study plan: one topic per session, draw every graph by hand, then test yourself with mixed MCQ before moving on. Full topic guides and practice are at /ap-micro/unit-4.