Unit 4: Imperfect Competition

Market Power and the Spectrum of Imperfect Competition

In perfect competition, firms are price takers: the market price is set by supply and demand, and an individual firm can sell as much as it wants at that price. Imperfect competition begins the moment a firm becomes a price maker with market power, meaning it has some ability to influence the price it charges (usually by choosing a price or a quantity and letting the market respond). Firms can be more likely to earn long-run economic profit when competition is limited, especially when barriers to entry prevent rivals from entering and bidding profits away.

Market power matters because it changes the firm’s basic revenue logic. A perfectly competitive firm can sell one more unit without affecting price, so marginal revenue equals price. By contrast, a firm with market power faces a downward-sloping demand curve for its product: to sell more, it typically must lower its price (or at least stop raising it). As a result, marginal revenue is less than price.

Imperfect competition is usually taught as a spectrum of market structures:

• Monopoly: one seller, no close substitutes, high barriers to entry.
• Monopolistic competition: many sellers, differentiated products, relatively easy entry and exit.
• Oligopoly: a few large interdependent sellers; products may be identical or differentiated.

Even though these structures look different, the same profit logic runs through all of them:

TR = P \cdot Q

\pi = TR - TC

MR = MC

That condition is the “engine” of firm behavior across imperfectly competitive markets. What changes is how the firm’s demand curve behaves, how rivals respond, and what happens in the long run when entry is possible.

Barriers to entry you should recognize

Imperfectly competitive markets often persist because entry is difficult. Common barriers you’re expected to recognize include:

• Economies of scale / high startup costs (one large firm can have lower average cost than many small firms).
• Legal or governmental barriers (patents, copyrights, licenses).
• Control of scarce/key resources (exclusive access to an input).
• Geography (location-based advantages that keep rivals out or make entry costly).

Why marginal revenue falls below price when demand slopes downward

If the firm must lower price to sell more, selling an extra unit affects revenue in two opposing ways:

1. The firm gains revenue from the additional unit.
2. The firm loses revenue because the price drop applies to units it was already selling.

That second effect is why marginal revenue falls faster than price. For a single-price seller with a downward-sloping demand curve:

• The MR curve lies below the demand curve.
• MR can become negative at high quantities.

This explains why firms with market power restrict output relative to perfect competition: expanding output requires cutting price, which reduces revenue on earlier units.

Exam Focus

• Typical question patterns:
  • Given a graph with demand, MR, MC, and ATC, identify the profit-maximizing output and price for an imperfect competitor.
  • Compare the monopoly (or oligopoly) outcome to the perfectly competitive outcome on the same market demand and MC.
  • Identify allocative efficiency/productive efficiency and deadweight loss from a graph.
• Common mistakes:
  • Setting

P = MC

for monopolies/monopolistic competitors. For market power, the output rule is

MR = MC

then use demand to find price.

• Confusing the firm’s demand curve with the market demand curve. For monopoly they are the same, but not for monopolistic competition.

Monopoly: The Single-Price Market Power Model

A monopoly is a market with a single seller and high barriers to entry that prevent competitors from entering and eroding profits. The “single seller” part matters less than the barriers: if entry were easy, even a firm that is currently the only seller would face competitive pressure.

Why monopolies exist: barriers to entry

Common barriers to entry include:

• Economies of scale, which can create a natural monopoly.
• Legal barriers (patents, copyrights, government licenses).
• Control of key resources.
• Geographic barriers (location can make entry impractical).

A natural monopoly often has large fixed costs and economies of scale over a long range of output, so its ATC can be downward sloping over the relevant range of market demand.

The monopoly’s demand and marginal revenue

A monopolist faces the market demand curve, which is downward sloping. If it charges a single price to all buyers, marginal revenue lies below demand.

Profit maximization works in two steps:

1. Choose the quantity where

MR = MC

2. Use the demand curve at that quantity to find the price consumers will pay.

This two-step method is central to most AP Micro monopoly graph questions.

Monopoly “supply” idea and the shutdown rule

Because the monopolist chooses both price and quantity, it does not have a supply curve the way a perfectly competitive industry does. However, in AP Micro you may still be asked about the profit-maximizing quantity and whether the firm produces in the short run.

The shutdown rule is based on variable costs. In the short run, a firm produces if price covers average variable cost:

• Produce if

P \ge AVC

• Shut down if

P < AVC

This also connects to the common AP statement that the firm’s short-run “supply” behavior corresponds to the portion of marginal cost above AVC.

Profit or loss for a monopolist

Once price and quantity are determined, profit depends on average total cost at that quantity:

• Positive economic profit if

P > ATC

• Economic loss if

P < ATC

A common misconception is “monopolies always earn profit.” They often can, but profit is not guaranteed if demand is low or costs are high.

Monopoly vs perfect competition: efficiency and deadweight loss

To understand why monopoly is often criticized, compare it to the perfectly competitive benchmark.

• In perfect competition, the market outcome tends toward

P = MC

which is allocatively efficient.

• In monopoly, the firm restricts output so that, at the monopoly quantity,

P > MC

That wedge signals underproduction relative to the allocatively efficient level and creates deadweight loss (DWL).

Graphically:

• The competitive (allocatively efficient) quantity is where demand intersects MC.
• The monopoly quantity is where MR intersects MC.
• DWL is the triangle between demand and MC over the quantities that would have been produced in competition but are not produced by the monopolist.

A worked graph-style example (no picture required)

Suppose a monopolist has:

• Market demand:

P = 100 - Q

• Marginal cost (constant):

MC = 20

First derive total revenue and marginal revenue.

TR = P \cdot Q

TR = (100 - Q)Q = 100Q - Q^2

MR = 100 - 2Q

Set marginal revenue equal to marginal cost.

100 - 2Q = 20

2Q = 80

Q = 40

Now use demand to find price.

P = 100 - 40 = 60

So the monopoly chooses

Q = 40

and charges

P = 60

Compare to the allocatively efficient (competitive) outcome where price equals marginal cost. Set demand equal to MC:

100 - Q = 20

Q = 80

Competition would produce 80 units at price 20. Monopoly produces 40 at price 60: higher price, lower quantity, and deadweight loss.

Exam Focus

• Typical question patterns:
  • Compute monopoly quantity/price from a demand curve (or table) by finding MR and setting

MR = MC

• Identify DWL and label consumer surplus, producer surplus, and monopoly profit on a graph.
• Determine whether a monopolist earns profit or loss given ATC and price.
• Identify the monopoly quantity (at

MR = MC

) and then find price “up to demand.”

• Common mistakes:
  • Finding the monopoly price by setting

P = MC

instead of using demand after finding

Q

.

• Labeling DWL as “monopoly profit.” Profit is a rectangle based on

P - ATC

; DWL is a triangle of lost surplus.

• Treating monopoly as allocatively efficient because it produces where

MR = MC

. Allocative efficiency requires

P = MC

.

Price Discrimination: When the Same Firm Charges Different Prices

Price discrimination occurs when a firm sells the same (or very similar) product at different prices not explained by cost differences. The core goal is to capture more consumer surplus as revenue. To price discriminate, a firm needs market power.

Conditions required for price discrimination

For price discrimination to work, three conditions are typically necessary:

1. Market power (a downward-sloping demand curve).
2. Ability to separate consumers into groups or identify purchase characteristics correlated with willingness to pay.
3. Limited arbitrage: consumers who buy at a low price cannot easily resell to consumers who face a higher price.

If arbitrage is easy, price differences collapse.

Types: imperfect vs perfect price discrimination

It helps to distinguish two common ideas:

• Imperfect price discrimination: charging different prices based on buyer willingness to pay, but not capturing all willingness to pay.
• Perfect price discrimination: charging each consumer the maximum they are willing to pay.

Under perfect price discrimination:

• The firm produces the allocatively efficient quantity where

P = MC

• There is no deadweight loss.
• Consumer surplus is driven to zero, and surplus is captured as producer surplus.

Under imperfect price discrimination, output and welfare effects are case-specific: it often raises profit and shifts surplus from consumers to producers, and it may increase output relative to a single-price monopoly (potentially reducing DWL).

Common forms you’ll see in AP Micro contexts

You may see price discrimination described in practical terms:

• Student/senior discounts.
• Airline tickets priced by timing, refundability, Saturday-night stays.
• Coupons and rebates (self-selection).
• Resellers and “different buyer types” paying different effective prices.
• Bulk buying (for example, warehouse clubs such as Costco).

The unifying idea is charging higher prices to consumers with less elastic demand and lower prices to consumers with more elastic demand.

Output and welfare effects (important nuance)

With a single-price monopoly, output is restricted relative to the allocatively efficient level. Some types of price discrimination can increase output by selling to additional consumers who would not buy at the single monopoly price, potentially reducing deadweight loss.

Avoid the blanket statement “price discrimination always reduces welfare.” Its effect depends on how much output changes and how much surplus is redistributed.

A simple two-group intuition example

Imagine a theater sells tickets to two groups:

• Group A (less price sensitive): would still buy at higher prices.
• Group B (more price sensitive): stops buying if price is high.

If the theater must set one price, it may choose a relatively high price to maximize profit from Group A and lose many Group B customers. If it can charge two prices, it can keep a high price for A and offer a lower price to B, selling more total tickets.

Exam Focus

• Typical question patterns:
  • Identify whether a scenario is price discrimination and explain which condition (separation, arbitrage prevention, market power) makes it possible.
  • Explain why firms charge different prices to groups with different elasticities.
  • Predict changes in output and consumer surplus under price discrimination.
  • Distinguish perfect from imperfect price discrimination and connect perfect discrimination to

P = MC

and zero DWL.

• Common mistakes:
  • Calling any price difference “price discrimination.” If cost differences justify different prices (like shipping costs), it may not be price discrimination.
  • Forgetting the arbitrage condition (resale). Without it, discriminatory pricing can’t persist.
  • Claiming “no consumer surplus and no deadweight loss” for all price discrimination. Those are results associated with perfect price discrimination.

Monopolistic Competition: Differentiation with Many Firms

Monopolistic competition is a market structure with many firms, differentiated products, and relatively easy entry and exit. It is one form of imperfect competition (not a synonym for all imperfect competition). It combines features of monopoly (some market power) and perfect competition (entry drives long-run profit down).

How product differentiation creates market power

Because products are differentiated by brand, features, location, or perceived quality, each firm faces a downward-sloping demand curve for its own product. When it raises price, it does not lose all customers, because some prefer its version.

Firms choose output where

MR = MC

and then charge the price on their demand curve.

Advertising and non-price competition

Because products are differentiated, firms often compete through:

• Advertising and marketing
• Packaging, design, service quality
• Store location and customer experience

Advertising is typically treated as a cost that can shift demand outward (if successful) and can make demand more inelastic by increasing brand loyalty.

Short-run equilibrium: profits or losses are possible

In the short run, a monopolistically competitive firm can earn:

• Economic profit if

P > ATC

• Economic loss if

P < ATC

As always, the quantity choice comes from

MR = MC

and price comes from the demand curve at that quantity.

Long-run equilibrium: entry eliminates economic profit

If firms are earning economic profit, new firms enter (entry is relatively easy). Entry increases variety and competition, shifting each incumbent firm’s demand curve left until economic profit becomes zero. In long-run equilibrium:

P = ATC

However, because each firm still has a downward-sloping demand curve, it typically remains true that:

P > MC

So long-run monopolistic competition is not allocatively efficient.

Excess capacity and productive inefficiency

A classic AP result is excess capacity: in long-run monopolistic competition, the firm produces at an output where ATC is not minimized. The demand curve is tangent to ATC on the downward-sloping portion of ATC.

• Productive efficiency occurs at the minimum of ATC.
• In monopolistic competition (even in long run), the firm produces less than the minimum-ATC output, so it has excess capacity.

Equivalently, you can explain that in long-run equilibrium the firm produces in a region where economies of scale still exist, because it is operating on the declining portion of ATC.

Is monopolistic competition “bad”?

It depends on what you value:

• Inefficiencies: typically

P > MC

and excess capacity.

• Benefits: greater variety and innovation. Consumers may willingly trade some efficiency for choice.

Exam Focus

• Typical question patterns:
  • Use a firm graph to identify short-run profit/loss and the long-run outcome after entry/exit.
  • Explain why long-run monopolistic competition has zero economic profit but is not allocatively or productively efficient.
  • Interpret advertising/branding as shifting demand and affecting elasticity.
  • Explain long-run adjustment: short-run profits attract entry until demand is tangent to ATC and

P = ATC

.

• Common mistakes:
  • Claiming firms earn long-run economic profit in monopolistic competition. Entry drives profit to zero.
  • Confusing “excess capacity” with “unused machines.” It’s about producing less than the minimum-ATC output.

Oligopoly: Strategic Interdependence and Game Theory

An oligopoly is a market dominated by a small number of interdependent firms. Interdependence means each firm’s best decision depends on what it expects rivals to do. If an exam asks why a market is an oligopoly, a strong answer emphasizes interdependence.

Oligopolies can sell homogeneous or differentiated products. Because there are only a few major firms, actions like price cuts, output expansion, and advertising can noticeably affect the market and provoke responses.

The tension between competition and cooperation

Oligopoly behavior often reflects a conflict:

• If firms compete aggressively (cut prices, raise output), prices can fall toward competitive levels and profits shrink.
• If firms cooperate (explicitly or implicitly), they can restrict output and raise prices like a monopoly, increasing joint profits.

Cooperation is hard to sustain because each firm has an incentive to cheat by undercutting price or expanding output.

Collusion and cartels

• Collusion is when firms coordinate to reduce competition to maximize profit.
• A cartel is a formal collusive agreement to control price and output.

Collusion can push the oligopoly outcome toward the monopoly-like outcome (higher price, lower quantity), but cheating incentives make it unstable.

Graph note (how AP usually treats it)

When an oligopoly behaves like a collusive cartel, the outcome can resemble monopoly: same general logic (restrict output, raise price). In many AP settings you are not asked to draw an oligopoly firm graph; instead you are asked to reason with game theory and strategic incentives.

Game theory: modeling strategic decisions

Game theory analyzes decisions when payoffs depend on multiple players’ choices.

Key terms:

• Payoff matrix: shows payoffs (profits) under different strategy combinations.
• Dominant strategy: yields a higher payoff regardless of what the other player does.
• Nash equilibrium: a set of strategies where no player can improve payoff by unilaterally changing their own strategy.

Prisoner’s dilemma (core oligopoly story)

In the prisoner’s dilemma, both firms would be better off cooperating (high price), but each has an incentive to cheat (low price). If both cheat, both can end up worse off than if they had cooperated.

Even though (High, High) is jointly best, (Low, Low) can be the Nash equilibrium if “Low Price” is a dominant strategy.

Repeated interaction and retaliation

In real oligopolies, firms interact repeatedly. Repetition can make cooperation more stable because cheating today can trigger retaliation tomorrow (price wars). AP questions may signal this with phrases like “firms interact repeatedly,” “long-term relationships,” or “credible punishment.”

Price rigidity and kinked demand (conceptual)

Some oligopoly models predict price rigidity: prices don’t change often even when costs or demand shift modestly. The kinked demand idea explains this:

• If a firm raises price, rivals may not follow, so it loses many customers (demand is very elastic above the current price).
• If a firm lowers price, rivals match the cut, so it gains few customers (demand is inelastic below the current price).

Firms may therefore compete using advertising, product features, or service rather than frequent price changes.

Exam Focus

• Typical question patterns:
  • Identify dominant strategies and Nash equilibrium from a payoff matrix.
  • Explain why collusion increases joint profit but is unstable.
  • Use prisoner’s dilemma logic to predict likely firm behavior.
  • Define and apply: payoff matrix, dominant strategy, Nash equilibrium.
• Common mistakes:
  • Thinking “Nash equilibrium” means the best collective outcome. It means no one wants to change unilaterally.
  • Assuming oligopolies always collude. Laws, monitoring difficulty, and the number of firms make sustained collusion hard.

Comparing Market Structures: Prices, Output, and Efficiency

A major goal of this unit is comparing perfect competition (as a benchmark) with monopoly, monopolistic competition, and oligopoly. The comparisons usually boil down to:

1. How much market power does the typical firm have?
2. What happens to price and output relative to the competitive outcome?
3. What are the efficiency consequences?

Quick structure comparison table

Efficiency relationships to know (often tested)

• Allocative efficiency occurs when

P = MC

• Monopoly typically produces where

P > MC

creating deadweight loss.

• Monopolistic competition in long run has

P = ATC

but typically

P > MC

and excess capacity (not productively efficient).

• Oligopoly is not pinned to one universal outcome: some are near-competitive (intense rivalry), others are near-monopoly (collusion/tacit coordination).

Surplus and deadweight loss comparisons

In general, the more market power firms have, the more likely they restrict output below the allocatively efficient level:

• Monopoly: clear restriction of output; DWL is typical.
• Monopolistic competition: some restriction; DWL exists, but product variety complicates welfare evaluation.
• Oligopoly: can range from near-competitive to near-monopoly depending on rivalry/collusion.

Ranking questions (lowest price / highest quantity)

On typical AP comparisons (assuming similar costs and demand):

• Lowest price and highest quantity: perfect competition.
• Highest price and lowest quantity: monopoly.
• Monopolistic competition usually falls between.
• Oligopoly can fall between or closer to one extreme.

When asked for a ranking, it is usually about market power: more market power tends to mean higher price and lower quantity.

Exam Focus

• Typical question patterns:
  • Rank market structures by price, quantity, efficiency, or long-run profit.
  • Use a graph to identify whether the firm is producing where

P > MC

and interpret that as allocative inefficiency.

• Explain why entry drives long-run monopolistic competition profit to zero.
  • Common mistakes:
• Treating oligopoly as always identical to monopoly. Oligopoly has multiple possible outcomes.
• Claiming “many firms” automatically implies allocative efficiency. Monopolistic competition can still have

P > MC

.

Public Policy Toward Imperfect Competition: Regulation and Antitrust

Because imperfect competition can lead to higher prices and reduced output, governments sometimes intervene. Two major policy categories are:

1. Antitrust policy (mainly for oligopoly/monopoly behavior): prevents or breaks up anti-competitive practices.
2. Regulation of natural monopolies: attempts to achieve lower prices and more efficient outcomes when competition is impractical.

Antitrust: promoting competition

Antitrust aims to reduce harmful market power. In AP terms, the focus is on outcomes:

• Less collusion
• More competition
• Lower prices and higher quantities

A strong written explanation links policy to the market power model: if a policy makes a market more competitive, it tends to push outcomes toward higher output and lower price, reducing DWL.

Natural monopoly and why regulation is different

A natural monopoly occurs when economies of scale are so strong that one firm can supply the market at a lower average cost than multiple competing firms, often with a downward-sloping ATC over the relevant quantity range. Unregulated, a natural monopolist still chooses output where

MR = MC

and then sets price from demand, which can yield high prices and restricted output.

The policy challenge is a tradeoff:

• If left unregulated, the natural monopolist may set the monopoly price.
• If forced to price at marginal cost, it may not cover total costs and could exit.

Common regulatory pricing rules

Marginal cost pricing

Regulator sets:

P = MC

This achieves allocative efficiency, but if ATC exceeds MC at that output (common for natural monopolies), then:

P < ATC

The firm earns a loss and may require a subsidy to stay in business.

Average cost pricing (fair-return pricing)

Regulator sets:

P = ATC

On a graph, this is often described as setting the regulated price at the intersection of the demand curve and ATC (sometimes written informally as “ATC = demand” at the regulated price). This yields zero economic profit and a lower price than the unregulated monopoly price, but typically:

P > MC

so some DWL may remain.

Real-world realism: regulation has costs and imperfect information

A strong AP explanation notes that regulators may not know the firm’s true costs or demand. Firms may have incentives to exaggerate costs, and regulation can reduce incentives to innovate or cut costs. Regulation is an attempt to balance efficiency with the firm’s need to cover costs.

Exam Focus

• Typical question patterns:
  • Given a natural monopoly graph with demand, MC, and ATC, identify the regulated price/quantity under

P = MC

versus

P = ATC

.

• Explain why

P = MC

may require a subsidy for a natural monopoly.

• Predict how antitrust enforcement could affect price, output, and consumer surplus.
  • Common mistakes:
• Claiming

P = MC

always guarantees the firm stays in business. It may imply losses if

ATC > MC

.

• Confusing “zero economic profit” with “zero accounting profit.” Zero economic profit means normal profit is included in cost.

How to Read Imperfect Competition Graphs Like an AP Reader

Many Unit 4 questions are really graph-reading and graph-explaining tasks. The main skill is knowing what each curve means and the correct sequence of steps.

The four-curve firm graph: Demand, MR, MC, ATC

For monopoly and monopolistic competition (and sometimes for an oligopoly firm treated as having market power), you’ll often see:

• Demand (D): downward sloping
• MR: below demand
• MC: upward sloping (typical)
• ATC: U-shaped (typical)

The correct procedure:

1. Find the profit-maximizing quantity where

MR = MC

2. Go up to the demand curve to find the price consumers pay.
3. Compare price to ATC at that quantity to determine profit or loss.

Profit area (if price exceeds ATC) is a rectangle:

• Height:

P - ATC

• Width:

Q

Loss area (if ATC exceeds price) is also a rectangle:

• Height:

ATC - P

• Width:

Q

Deadweight loss identification

To show deadweight loss from market power:

• Identify the allocatively efficient quantity where demand intersects MC (where)

P = MC

• Identify the market-power quantity where

MR = MC

• DWL is the triangle between demand and MC between those quantities.

A frequent error is shading DWL using ATC. DWL is based on marginal benefit (demand) and marginal cost (MC).

Connecting elasticity to marginal revenue (useful intuition)

A common conceptual link:

• When demand is elastic, lowering price increases total revenue, so MR is positive.
• When demand is inelastic, lowering price decreases total revenue, so MR is negative.

A monopolist will not choose to produce where MR is negative, because increasing output there reduces total revenue while increasing total cost.

Exam Focus

• Typical question patterns:
  • Given a graph, label monopoly

Q

and

P

and compute profit using ATC.

• Shade consumer surplus, producer surplus, DWL, and monopoly profit.
• Identify the socially efficient output and compare it to the firm’s chosen output.
  • Common mistakes:
• Picking the quantity at the intersection of demand and MC (that is the efficient quantity, not the monopoly quantity).
• Using the intersection of MR and demand to set price (price comes from demand at the chosen quantity, not from MR).

Extended Example: From Market Structure to a Full Written Explanation

AP free-response questions often reward you for chaining ideas together: firm choice → price → profit → efficiency.

Consider a monopolistically competitive firm in the short run:

• It has market power (downward-sloping demand), so it chooses

Q

where

MR = MC

.

• Suppose at that

Q

the price on the demand curve lies above ATC. Then it earns positive economic profit.

Now connect to the long run:

• Because entry is relatively easy, those profits attract entrants.
• Entry shifts each existing firm’s demand curve left (fewer customers at each price), reducing profit.
• The process continues until the firm’s demand curve is tangent to ATC at the profit-maximizing quantity, so

P = ATC

and economic profit is zero.

Finally, connect to efficiency:

• Even at long-run zero profit, the firm still typically has

P > MC

so it is not allocatively efficient.

• It also produces at less than minimum ATC, so it has excess capacity and is not productively efficient.

This step-by-step causal explanation is exactly what AP graders look for.

Exam Focus

• Typical question patterns:
  • Write a coherent paragraph explaining the adjustment from short-run profit to long-run equilibrium in monopolistic competition.
  • Compare long-run outcomes across market structures using both words and a graph.
  • Explain how a change in costs or demand affects price and output under monopoly.
• Common mistakes:
  • Saying “firms enter until profits are maximized.” Entry continues until economic profit is zero (normal profit).
  • Mixing up firm-level and industry-level curves (especially when explaining entry effects).