AP Microeconomics Unit 4 ReviewImperfect Competition

AP Micro Unit 4, Imperfect Competition, is about what happens when firms have market power, meaning they can set price above marginal cost instead of taking the market price as given. The single biggest idea is that monopolies, oligopolies, and monopolistically competitive firms all charge a price greater than marginal cost, which creates deadweight loss and makes these markets allocatively inefficient. This unit also introduces game theory, the tool for analyzing how a handful of interdependent firms make strategic decisions. It is worth 15-22% of the AP exam, making it one of the heaviest units in the course.

What this unit covers

What "imperfect" means and where the inefficiency comes from

• Imperfectly competitive markets include monopoly, oligopoly, and monopolistic competition in product markets, plus monopsony in factor markets (you'll see monopsony in Unit 5).
• The defining feature is a downward-sloping demand curve facing the firm. To sell more units, the firm has to lower its price. That's the opposite of a perfectly competitive firm, which can sell all it wants at the market price.
• Because the firm must cut price on all units to sell one more, marginal revenue is less than price (MR < P). The MR curve sits below the demand curve, and for a linear demand curve it falls twice as fast.
• Every imperfectly competitive firm still maximizes profit the same way, by producing where MR = MC. The difference is what price it charges. The firm goes up from the MR = MC quantity to the demand curve to set price, so P > MC.
• P > MC is the source of allocative inefficiency. Units that consumers value more than they cost to produce never get made, and the lost surplus is deadweight loss.

Monopoly and price discrimination

• A monopoly is a single seller protected by barriers to entry, things like patents, control of a key resource, government licenses, or economies of scale. Without barriers, profit would attract entrants and the monopoly would erode.
• A natural monopoly is the special case where economies of scale run through the entire range of market demand, so one firm can serve everyone at lower average cost than two or more firms could (think utilities).
• On the monopoly graph, you should be able to shade and calculate consumer surplus, producer surplus, profit or loss (the rectangle between price and ATC at the chosen quantity), and deadweight loss (the triangle between demand and MC from the monopoly quantity to the efficient quantity).
• Price discrimination means charging different consumers different prices for the same product, which a firm with market power can do under certain conditions (it can identify willingness to pay and prevent resale).
• A perfectly price-discriminating monopolist is the famous twist. It charges every buyer exactly their willingness to pay, so the demand curve becomes its MR curve. It produces where P = MC, the same efficient quantity as a competitive market, but it captures all the surplus. Deadweight loss disappears and consumer surplus equals zero.

Monopolistic competition, the in-between structure

• Many firms, low barriers to entry, and differentiated products (restaurants, clothing brands). Differentiation, often through advertising, is what gives each firm its slice of market power and its downward-sloping demand curve.
• In the short run, these firms can earn positive, negative, or zero economic profit, just like any firm.
• In the long run, free entry and exit drive economic profit to zero. Entry shifts each firm's demand curve left until it is tangent to ATC at the profit-maximizing quantity.
• Even at zero profit, the long-run outcome is still inefficient in two ways. Price is greater than marginal cost (allocative inefficiency), and the firm produces less than the quantity that minimizes ATC, which is called excess capacity (productive inefficiency).

Oligopoly and game theory

• An oligopoly has a few large firms, high barriers to entry, and interdependence. Each firm's best move depends on what its rivals do, which is why you analyze oligopolies with payoff matrices instead of MR = MC graphs.
• A game is a situation where each player's payoff depends on both their own choice and others' choices. A strategy is a complete plan of action for playing the game.
• A dominant strategy is the best choice no matter what the other player does. A Nash equilibrium is the outcome where no player can improve their payoff by changing their own strategy alone. Check every cell of the matrix for this.
• Oligopolists have an incentive to collude, acting like a single monopolist to raise prices, sometimes formally through a cartel. But collusion is unstable. Each firm has an individual incentive to cheat by cutting price or raising output, which is exactly the prisoner's dilemma. The collusive outcome is best jointly, but it usually isn't a Nash equilibrium.
• You should also be able to calculate how big a payoff change (a side payment, fine, or reward) would have to be to flip a player's dominant strategy.

Unit 4, Imperfect Competition at a glance

Why Unit 4, Imperfect Competition matters in AP Micro

Unit 3 built the perfect competition model and showed why it is efficient. Unit 4 is where that ideal breaks down and the course gets realistic, since most markets you actually buy from look more like monopolistic competition or oligopoly than perfect competition. The recurring theme is that the same profit-maximizing rule (MR = MC) produces inefficient outcomes once firms have market power.

• It explains why prices can't always be trusted to coordinate buyers and sellers. When P > MC, the price signal overstates the true cost of production and output ends up too low.
• It introduces strategic interdependence through game theory, a way of thinking that applies beyond economics (negotiations, arms races, advertising wars).
• It sets up the case for antitrust policy and regulation, since deadweight loss from market power is one of the justifications for government intervention you'll evaluate in Unit 6.

How this unit connects across the course

• Marginal analysis and cost curves from Production, Cost, and the Perfect Competition Model (Unit 3) carry over directly. The MR = MC rule, ATC, and economies of scale all reappear; the only thing that changes is the firm's demand and MR curves. Perfect competition is also your efficiency benchmark for measuring deadweight loss.
• Elasticity from Supply and Demand (Unit 2) explains monopoly pricing. A monopolist never produces on the inelastic portion of its demand curve, and price discrimination works by charging more in inelastic segments.
• Monopsony in Factor Markets (Unit 5) is the mirror image of monopoly. A single buyer of labor faces an upward-sloping supply curve and pays a wage below marginal revenue product, the same logic flipped to the input side.
• Market Failure and the Role of Government (Unit 6) picks up where this unit leaves off. Deadweight loss from market power motivates antitrust laws and the regulation of natural monopolies (fair-return vs. socially optimal pricing).
• Scarcity and the marginal benefit vs. marginal cost framework from Basic Economic Concepts (Unit 1) is the deep reason P > MC matters. Society wants every unit where marginal benefit exceeds marginal cost, and market power leaves some of those units unproduced.

Key models and graphs to know

• Monopoly graph (D, MR, MC, ATC): find quantity where MR = MC, go up to demand for price, and shade profit, consumer surplus, producer surplus, and deadweight loss.
• Natural monopoly graph: ATC falls across the entire demand range, so MC sits below ATC at the relevant quantity. Used for regulation questions later in Unit 6.
• Perfect price discrimination graph: demand becomes the MR curve, output is where P = MC, consumer surplus is zero, and there is no deadweight loss.
• Monopolistic competition, short run vs. long run: short run looks like a mini monopoly graph; long run shows demand tangent to ATC at the profit-maximizing quantity, with zero profit and excess capacity.
• Payoff matrix (game theory): a 2x2 table for finding dominant strategies, Nash equilibria, and the gap between the collusive outcome and the equilibrium outcome.
• MR below demand: for a linear demand curve, MR has the same intercept and twice the slope. Total revenue is maximized where MR = 0, which is the unit-elastic point on demand.

Unit 4, Imperfect Competition on the AP exam

This unit is 15-22% of the exam, tied for the largest share in AP Micro, so expect it everywhere. On the multiple-choice section, you'll identify market structures from their characteristics, read monopoly and monopolistic competition graphs to pick out price, quantity, profit, and deadweight loss areas, and solve payoff matrix questions for dominant strategies and Nash equilibria. A classic move is giving you a demand and cost table instead of a graph and asking you to compute MR and find the profit-maximizing output.

The free-response section regularly features a long monopoly question. You draw a correctly labeled graph, mark the profit-maximizing quantity and price, shade or calculate profit and deadweight loss, then handle a twist like perfect price discrimination, a per-unit tax, or a lump-sum tax (lump-sum costs don't change MC, so quantity and price stay put while profit shrinks). Game theory shows up as a payoff matrix where you identify each player's dominant strategy, find the Nash equilibrium, explain why collusion breaks down, and calculate the payment that would change a player's choice. Practice labeling axes, curves, and points exactly, since graph labels earn points on their own.

Essential questions

• Why does a firm with market power produce less and charge more than a perfectly competitive market would, and who loses as a result?
• How can perfect price discrimination be efficient even though it leaves consumers with zero surplus?
• Why do monopolistically competitive firms earn zero economic profit in the long run yet still produce inefficiently?
• Why do oligopolists who would all profit from colluding so often end up undercutting each other?

Key terms to know

• Market power: a firm's ability to set price above marginal cost without losing all of its customers.
• Barriers to entry: obstacles like patents, licenses, or economies of scale that keep new firms out and let incumbents keep earning profit.
• Marginal revenue (MR): the change in total revenue from selling one more unit; in imperfect competition, MR is below price.
• Deadweight loss: total surplus lost because mutually beneficial units between the actual and efficient quantity are never produced.
• Natural monopoly: a market where one firm's economies of scale extend across all of demand, so a single producer is cheapest.
• Price discrimination: charging different prices to different buyers for the same product based on willingness to pay.
• Product differentiation: making your product distinct through features, branding, or advertising, which gives a firm its downward-sloping demand curve.
• Excess capacity: producing less than the output that minimizes ATC, the long-run signature of monopolistic competition.
• Interdependence: the oligopoly condition where each firm's payoff depends on rivals' choices, not just its own.
• Dominant strategy: a strategy that gives a player the highest payoff regardless of what the other player does.
• Nash equilibrium: an outcome where no player can do better by changing only their own strategy.
• Collusion: an agreement among oligopolists (formally, a cartel) to restrict output and raise price like a shared monopoly.
• Allocative efficiency: producing the quantity where P = MC; imperfectly competitive markets fail this test.

Common mix-ups

• A monopolist does not "charge where MR = MC." Quantity comes from MR = MC, but the price comes from the demand curve above that quantity. Reading price off the MR curve is one of the most common graphing errors.
• Zero economic profit does not mean efficient. Monopolistically competitive firms earn zero profit in the long run but still have P > MC and excess capacity.
• A Nash equilibrium is not necessarily the best joint outcome. In a prisoner's dilemma, both players would be better off colluding, but the equilibrium is where both defect.
• A lump-sum tax or fee shifts ATC but not MC, so the monopolist's price and quantity don't change. A per-unit tax shifts MC, so they do. FRQs love testing this distinction.