Imperfect Competition: Monopoly, Price Discrimination, and Monopolistic Competition

Monopoly

A monopoly is a market structure in which a single firm is the only producer of a good or service with no close substitutes. The key idea is not “a big firm” but a firm that faces the entire market demand. Because consumers cannot easily switch to a competitor, a monopolist has substantial market power—the ability to influence price.

What makes a monopoly possible (barriers to entry)

Monopolies persist because of barriers to entry, which are obstacles that prevent new firms from entering and competing away profits. In AP Micro, you typically explain monopoly power by pointing to one or more of these:

• Legal barriers: patents, copyrights, government licenses, or exclusive franchises.
• Control of a key resource: a firm owns an essential input.
• Economies of scale leading to a natural monopoly: one firm can produce the entire market output at a lower average cost than multiple firms could.
• Strategic barriers (less emphasized than the above): behaviors that deter entry (long-term contracts, limit pricing). On the AP exam, you usually don’t need game theory here; just show how entry is blocked.

Why this matters: barriers to entry explain why monopoly profits can persist in the long run—unlike perfect competition, where free entry eliminates economic profit.

Demand, marginal revenue, and why monopolies choose price and quantity differently

A monopolist is a price maker, but it still faces a constraint: the market demand curve. If it wants to sell more, it must lower price.

• The monopolist’s demand curve is the market demand curve.
• The monopolist’s average revenue (AR) equals price at each quantity.
• The monopolist’s marginal revenue (MR) is the change in total revenue from selling one more unit.

Total revenue is:

TR = P \cdot Q

Marginal revenue is conceptually:

MR = \frac{\Delta TR}{\Delta Q}

Because price must fall to sell extra units, MR is below demand for a monopolist. The intuition: selling one more unit adds revenue from that unit, but the lower price also reduces revenue on units you were already selling.

A common AP simplification (especially with linear demand) is:

• If demand is linear, the MR curve has the same intercept as demand and twice the slope.

Profit maximization: the same rule, different outcome

Like any firm, the monopolist maximizes profit by producing where marginal benefit equals marginal cost. For a monopolist, marginal benefit is MR.

Profit-maximizing rule:

MR = MC

Then the monopolist sets price from the demand curve at that quantity.

Why this matters: students often think “monopoly chooses price first.” The more accurate AP framing is:
1) Choose the profit-maximizing quantity where MR = MC.
2) Charge the highest price consumers will pay for that quantity (read off demand).

Profit (economic profit) is:

\pi = TR - TC

On graphs, profit is the rectangle:

• height: P - ATC at the chosen quantity
• width: Q

Efficiency and welfare: why monopoly is controversial

In perfect competition, the efficient condition is P = MC, which yields allocative efficiency. In monopoly:

• The monopolist produces where MR = MC.
• At that output, price exceeds marginal cost: P > MC.

This creates two standard inefficiencies:

1) Allocative inefficiency: since P > MC, the value consumers place on the last unit (price) exceeds the opportunity cost (MC). Society would benefit from producing more.

2) Deadweight loss (DWL): trades that would have occurred under a competitive output (where P = MC) do not occur under monopoly output.

Graphically (how you describe it on AP FRQs):

• Competitive outcome is where demand intersects MC.
• Monopoly outcome is where MR intersects MC, with price found on demand.
• DWL is the triangle between demand and MC over the quantities that are not produced due to monopoly restriction.

Important misconception to avoid: monopoly profit is not the same thing as deadweight loss. Profit is a transfer (from consumers to the firm); DWL is lost total surplus.

Does a monopoly have a supply curve?

In AP Micro, a key idea is: a monopoly does not have a supply curve.

A supply curve describes a unique relationship between price and quantity supplied. For a monopolist, quantity depends on the entire demand curve (because demand affects MR), not just on price. If demand changes, the monopolist may choose a different quantity at the same price or a different price at the same quantity.

Natural monopoly and regulation (common AP angle)

A natural monopoly exists when average total cost (ATC) is falling over the relevant range of market output (strong economies of scale). Having one firm can minimize costs, but it can still charge monopoly prices.

Regulation attempts to address the tradeoff:

• Marginal cost pricing: regulator sets P = MC. This is allocatively efficient, but if ATC is above MC at that output, the firm may earn losses and exit unless subsidized.
• Average cost pricing: regulator sets P = ATC (often at the quantity where the demand curve intersects ATC). This allows the firm to break even (normal profit) but is not allocatively efficient because typically P > MC.

Worked example: finding monopoly price, quantity, and profit

Suppose market demand is:

P = 100 - 2Q

Then marginal revenue is:

MR = 100 - 4Q

Assume marginal cost is constant:

MC = 20

And fixed cost is 200, so total cost is:

TC = 20Q + 200

Step 1: Set MR = MC to find quantity

100 - 4Q = 20

80 = 4Q

Q = 20

Step 2: Use demand to find price

P = 100 - 2(20) = 60

Step 3: Compute profit

TR = 60 \cdot 20 = 1200

TC = 20(20) + 200 = 600

\pi = 1200 - 600 = 600

Interpretation: the monopolist restricts output relative to the efficient level (which would be where P = MC). If you set 100 - 2Q = 20, you get Q = 40 under the efficient condition, so monopoly output is lower.

Exam Focus

• Typical question patterns:
  • Given a graph (or equations) with demand, MR, MC, and ATC, identify monopoly Q where MR = MC, then find P on demand and compute profit or loss.
  • Compare monopoly outcome to socially efficient outcome (where demand intersects MC) and shade deadweight loss.
  • Natural monopoly regulation: compare outcomes under P = MC versus P = ATC (profit, DWL, and whether a subsidy is needed).
• Common mistakes:
  • Setting P = MC for a monopolist. The monopolist sets MR = MC, then uses demand to find P.
  • Reading price from the MR curve. MR gives the profit-maximizing quantity, not the price consumers pay.
  • Claiming the monopoly “supply curve is MC.” Monopolies do not have supply curves because output depends on demand.

Price Discrimination

Price discrimination occurs when a firm charges different prices to different consumers for the same product (or for units of the product) and the price differences are not explained by differences in cost. The motivation is straightforward: if consumers have different willingness to pay, a firm with market power can increase profit by capturing more consumer surplus.

Conditions required for price discrimination

For price discrimination to work, three core conditions must hold:

1) Market power: the firm must face a downward-sloping demand curve (monopoly or a firm in monopolistic competition with some pricing power).

2) Ability to segment the market: the firm must identify groups with different demand elasticities (students vs. non-students, peak vs. off-peak travelers).

3) Limited resale (no arbitrage): if low-price buyers can easily resell to high-price buyers, the price differences collapse.

Why this matters: on AP questions, if any one condition fails, you should explain why price discrimination is not sustainable.

Why elasticity is the key idea

In the common AP model of third-degree price discrimination (different prices for different groups), the firm charges:

• a higher price to the group with more inelastic demand
• a lower price to the group with more elastic demand

Reason: raising price causes a smaller percentage drop in quantity when demand is inelastic, so revenue and profit are higher.

Types of price discrimination (AP-relevant overview)

You may see these categories. AP most often emphasizes the logic of third-degree.

• First-degree (perfect) price discrimination: charge each unit at the consumer’s exact willingness to pay. Conceptually captures all consumer surplus as producer surplus (hard to do perfectly in real life).
• Second-degree price discrimination: prices vary by quantity purchased or product version (bulk discounts, two-part tariffs like membership plus per-unit pricing). The consumer self-selects.
• Third-degree price discrimination: different identifiable groups pay different prices (student discounts, senior discounts, geographic pricing).

How a third-degree price discriminator chooses output and price

A third-degree price discriminator separates the market into groups (say Market 1 and Market 2) and chooses quantities in each.

Core logic:

• The firm produces total output up to the point where overall marginal revenue equals marginal cost.
• It allocates units across markets so that marginal revenue is equalized across markets (otherwise shifting a unit from one market to the other would raise total revenue).

In a two-market setup:

MR_1 = MR_2 = MC

Then the firm charges prices using each market’s demand curve at its chosen quantity.

Important misconception to avoid: the firm does not set the same MR curves equal to each other because “it feels right”; it’s because profit rises by shifting output toward the market where an extra unit generates higher MR.

Effects on output, consumer surplus, and deadweight loss

Compared to single-price monopoly pricing:

• Total output often increases under price discrimination (especially when serving a more price-sensitive group becomes profitable at a lower price).
• Consumer surplus usually falls for the group charged a higher price; it may rise or fall for the lower-price group.
• Deadweight loss may shrink if output increases toward the efficient level. Price discrimination can reduce DWL relative to single-price monopoly, though it typically transfers surplus from consumers to the producer.

Compared to perfect competition, price discrimination still typically yields P > MC in at least part of the market and does not generally reach the competitive quantity.

Worked example: third-degree price discrimination with two markets

Suppose a firm has constant marginal cost:

MC = 10

Market 1 demand:

P_1 = 50 - Q_1

Market 2 demand:

P_2 = 30 - 0.5Q_2

Step 1: Find MR in each market

For a linear demand P = a - bQ, the MR is MR = a - 2bQ.

So:

MR_1 = 50 - 2Q_1

For Market 2, b = 0.5, so:

MR_2 = 30 - Q_2

Step 2: Set each MR equal to MC (profit-max condition with discrimination)

50 - 2Q_1 = 10

Q_1 = 20

And:

30 - Q_2 = 10

Q_2 = 20

Step 3: Find each market’s price from its demand

P_1 = 50 - 20 = 30

P_2 = 30 - 0.5(20) = 20

Interpretation:

• Market 1 pays a higher price (30) than Market 2 (20). That aligns with the idea that Market 1 is “less price sensitive” in this setup.
• Total output is Q = Q_1 + Q_2 = 40.

If the firm were forced to charge a single price, it would need the combined demand curve and combined MR, which is usually more algebra. On the AP exam, they often test the discrimination logic more than heavy combined-demand calculations.

Exam Focus

• Typical question patterns:
  • Identify the conditions necessary for price discrimination and explain whether a scenario (coupons, student discounts, peak pricing) satisfies them.
  • With two markets (two demand curves) and one MC curve, find Q_1 and Q_2 by setting MR_1 = MC and MR_2 = MC, then find prices from each demand.
  • Compare single-price monopoly to price discrimination in terms of output and deadweight loss (usually qualitative with a graph).
• Common mistakes:
  • Saying price discrimination means “different prices because costs differ.” If costs differ, it is not price discrimination in the standard sense.
  • Claiming discrimination requires being a monopoly specifically. It requires market power; monopolistic competitors can sometimes discriminate too.
  • Forgetting the no-resale condition: if resale is easy, arbitrage undermines different prices.

Monopolistic Competition

Monopolistic competition is a market structure with many firms that sell differentiated products and face free entry and exit in the long run. Each firm has some market power because its product is not identical to others, but competition is still intense because there are many close substitutes.

The core features (and why they matter)

1) Product differentiation

• Firms sell products that are similar but not identical (brand, location, quality, style).
• Because of differentiation, each firm faces a downward-sloping demand curve.

2) Many sellers and low barriers to entry

• Entry is relatively easy compared to monopoly.
• This is the key reason long-run economic profit tends to be driven to zero.

3) Nonprice competition

• Firms compete through advertising, branding, product features, service, and location.
• This is not “waste” automatically; it can provide information or create value, but it also raises costs.

Real-world examples often used for intuition: restaurants, clothing brands, salons, coffee shops. The point is not that these are perfect matches, but that they show differentiation plus many competing firms.

Short-run equilibrium: like a monopoly (for the individual firm)

In the short run, a monopolistically competitive firm chooses output like a monopolist:

• Profit-max output where:

MR = MC

• Price comes from the firm’s demand curve at that quantity.

Because firms can face strong demand for their specific variety (good reviews, convenient location), they can earn economic profit in the short run.

Long-run equilibrium: entry eliminates economic profit

Here is the signature AP result.

If firms earn economic profit in the short run, new firms enter because barriers are low. Entry means:

• consumers have more choices
• each existing firm loses some customers
• each firm’s demand curve shifts left (and becomes more elastic)

Entry continues until firms earn zero economic profit (normal profit). The long-run condition is:

P = ATC

But because the firm still has a downward-sloping demand curve, it generally does not produce where P = MC. So even in long-run equilibrium:

• Allocative inefficiency: typically P > MC
• Excess capacity: the firm produces at an output lower than the output that minimizes ATC

Excess capacity means the firm is not producing at the lowest possible average total cost for its plant. This happens because the demand curve is tangent to ATC at a quantity left of minimum ATC.

Important misconception to avoid: “zero profit” does not mean firms are failing. It means economic profit is zero after including opportunity costs; firms still earn a normal return.

The efficiency tradeoff: why monopolistic competition is not simply “bad”

AP Micro often frames monopolistic competition as a tradeoff:

• Costs:

  • Some deadweight loss (since P > MC)
  • Excess capacity (higher ATC than the minimum)
  • Advertising can raise costs

• Benefits:

  • Product variety that consumers value
  • Innovation and responsiveness to consumer tastes

So the question is not only “Is it efficient?” but “Is the extra variety worth the extra cost?” You can’t answer that purely from the model; the model helps you identify what is gained and what is lost.

Advertising: shifting demand versus increasing costs

Advertising and branding can affect the firm in two main ways:

1) Increase demand: successful advertising can shift the firm’s demand curve right (more customers at each price).

2) Increase costs: advertising is an expense, often shifting ATC upward.

On an exam, you may be asked to determine whether advertising increases profits. The correct reasoning is comparative:

• If the demand increase raises revenue more than the cost increase raises total costs, profit rises.
• Otherwise, profit can fall even if the firm attracts more customers.

Worked example: short run profit, then long-run adjustment

Assume a monopolistically competitive firm has:

P = 40 - Q

So:

MR = 40 - 2Q

Costs:

MC = 10

TC = 10Q + 60

Then:

ATC = 10 + \frac{60}{Q}

Short run: find the profit-max outcome

Set MR = MC:

40 - 2Q = 10

30 = 2Q

Q = 15

Price from demand:

P = 40 - 15 = 25

Compute ATC at Q = 15:

ATC = 10 + \frac{60}{15} = 14

Profit per unit is P - ATC = 11, so economic profit is:

\pi = (25 - 14) \cdot 15 = 165

Long run: what changes?

Because there is economic profit, new firms enter. Entry reduces the firm’s demand (shifts it left) until the firm earns zero economic profit, which occurs where the firm’s demand curve is tangent to ATC:

P = ATC

You are often not asked to compute the exact new demand equation; instead, you show on a graph that:

• the demand curve shifts left
• the new tangency point occurs at a lower quantity
• profit becomes zero

The key long-run conclusions you should state clearly:

• Long-run economic profit is zero.
• The firm still sets MR = MC at its chosen quantity.
• Price equals ATC in the long run, but price remains above MC.

Comparing monopoly and monopolistic competition (conceptual table)

Exam Focus

• Typical question patterns:
  • Use a graph to show short-run profit (or loss) for a monopolistically competitive firm and explain long-run entry (or exit) shifting demand until P = ATC.
  • Compare long-run monopolistic competition to perfect competition: emphasize product differentiation, excess capacity, and P > MC.
  • Analyze advertising: does it shift demand, shift costs, or both, and what happens to profit?
• Common mistakes:
  • Saying monopolistic competition is efficient in the long run because profit is zero. Zero profit does not imply allocative efficiency; typically P > MC.
  • Confusing the firm’s demand curve with the market demand curve. Each firm has its own demand based on differentiation.
  • Claiming firms produce at minimum ATC in the long run. In monopolistic competition, the tangency occurs left of minimum ATC (excess capacity).