Tags: perfect competition, price taker, shutdown rule, monopoly, marginal revenue, deadweight loss, price discrimination, first degree, second degree, third degree, consumer surplus, producer surplus, welfare, competitive equilibrium, ECON 323, Texas A&M, microeconomics
This topic covers the two polar market structures (perfect competition and monopoly), the welfare consequences of each, and the pricing strategies monopolists use to capture surplus. Perfect competition maximises total welfare; monopoly creates deadweight loss by restricting output. Price discrimination allows monopolists to extract more surplus, with three distinct degrees.
Perfectly competitive market
A market where firms are price takers, products are homogeneous (identical), and there is free entry and exit. No single firm can influence the market price.
Price taker
A firm that accepts the market price as given. Its demand curve is perfectly horizontal at the market price.
Shutdown rule (short run)
A firm should shut down if the market price falls below the minimum of average variable cost (P < AVC). At that point the firm cannot even cover its variable costs by producing, so it loses less by stopping production and paying only fixed costs.
Monopoly
A market with a single seller. The monopolist faces the entire market demand curve and has pricing power.
Marginal revenue (monopolist)
The additional revenue from selling one more unit. For a monopolist, MR lies below the demand curve for all positive output, because lowering the price to sell an extra unit reduces the revenue earned on all previous units.
Deadweight loss (DWL)
The loss in total surplus that occurs when the market outcome is not efficient. In monopoly, DWL arises because the monopolist produces less than the competitive quantity, leaving mutually beneficial trades unrealised.
Consumer surplus
The area above the market price and below the demand curve. It measures the benefit consumers receive from paying less than their maximum willingness to pay.
Producer surplus
The area below the market price and above the supply curve. It measures the benefit producers receive from selling at a price above their minimum acceptable price.
Competitive equilibrium
The market outcome where supply equals demand. Total welfare (consumer surplus + producer surplus) is maximised at this point.
First-degree price discrimination (perfect price discrimination)
Charging each individual consumer the maximum price they are willing to pay. Extracts all consumer surplus and transfers it to the producer.
Second-degree price discrimination
Charging different prices based on the quantity consumed or product version chosen. Examples include quantity discounts, block pricing for utilities, and "buy one get one free" offers.
Third-degree price discrimination
Dividing consumers into identifiable groups with different demand elasticities and charging each group a different price. Examples include student discounts, senior citizen pricing, and business vs economy airfares.
Information good
A product with high fixed costs of creation but negligible marginal costs of reproduction. Examples: software, digital media, online courses.
Sensitivity analysis
A technique for examining how an optimal decision changes when key economic variables (costs, demand, prices) are varied. Helps firms understand risk and robustness of a strategy.
Satisficing
A model of firm behaviour where the goal is to achieve a satisfactory level of performance against a benchmark rather than to maximise profits absolutely.
Key assumptions: price-taking behaviour, homogeneous products, free entry and exit
The firm's demand curve is horizontal at the market price
Profit-maximising rule: produce where P = MC (since P = MR for a price taker)
Short-run losses are tolerated as long as P ≥ AVC (the firm covers variable costs and contributes toward fixed costs)
If P < AVC, the firm shuts down immediately because every unit produced makes losses worse
Worked example (shutdown decision):
P = $25, ATC = $30, AVC = $20.
The firm is making a loss because P < ATC ($25 < $30)
However, P > AVC ($25 > $20), so the shutdown rule says: keep producing
By producing, the firm covers all variable costs and puts $5 per unit toward fixed costs
Shutting down would mean paying all fixed costs with zero revenue, a worse outcome
The monopolist is the sole seller and faces the downward-sloping market demand curve
MR < P for all positive output (the MR curve lies below the demand curve)
For a linear demand curve P = a - bQ, the MR function is MR = a - 2bQ (same intercept, twice the slope)
Profit-maximising rule: produce where MR = MC, then read the price off the demand curve
Worked example (monopoly profit maximisation):
Demand: P = 100 - Q. MC = 20.
Revenue: R = P × Q = (100 - Q)Q = 100Q - Q²
MR = dR/dQ = 100 - 2Q
Set MR = MC: 100 - 2Q = 20, so Q_m = 40
Price: P_m = 100 - 40 = $60
Profit: π = (P - MC) × Q = (60 - 20) × 40 = $1,600
Deadweight loss explanation:
In a competitive market, P = MC → 100 - Q = 20 → Q_c = 80 at P = $20
The monopolist produces 40 instead of 80 and charges $60 instead of $20
Consumers with willingness to pay between $20 and $60 would benefit from buying, and the cost of serving them is only $20, but these transactions never happen
The triangle of lost surplus between Q = 40 and Q = 80 is the deadweight loss
First degree:
The firm charges each buyer their exact willingness to pay
No consumer surplus remains; it is all captured as producer revenue
Eliminates deadweight loss (all efficient trades occur) but transfers all surplus to the producer
Real-world approximation: personalised negotiation (car dealerships, bespoke professional services)
Second degree:
Prices vary by quantity purchased or product tier, not by buyer identity
The firm cannot observe individual willingness to pay, so it designs a menu that induces self-selection
Examples: quantity discounts, block pricing for electricity or water, "buy one get one free" promotions
Captures more surplus than uniform pricing but less than first-degree
Third degree:
The firm segments the market into identifiable groups with different price elasticities
Each group is charged a different price: groups with less elastic demand pay more
Examples: student and senior discounts at cinemas or museums, airline pricing (business vs leisure travellers)
Requires the firm to identify groups and prevent resale between them
For a firm with market power, the profit-maximising price can be found using the elasticity markup formula:
P = MC / [1 - (1 / |E|)]
Where |E| is the absolute value of the price elasticity of demand.
Worked example:
|E| = 3, MC = $20.
P = 20 / [1 - (1/3)] = 20 / (2/3) = $30.
The more elastic the demand, the smaller the markup over marginal cost.
Total welfare = Consumer surplus + Producer surplus
Maximised at the competitive equilibrium (where supply = demand)
Any deviation from this equilibrium, whether from monopoly power, price floors, taxes, or other interventions, creates deadweight loss
Producer surplus is the area below price and above the supply curve
Consumer surplus is the area above price and below the demand curve
Concept | Formula |
|---|---|
MR (linear demand P = a - bQ) | MR = a - 2bQ |
Monopoly profit | π = (P - MC) × Q |
Rule-of-thumb pricing | P = MC / [1 - (1/|E|)] |
Competitive equilibrium | P = MC (for the market) |
Shutdown rule | Shut down if P < min AVC |
⚠️ The shutdown rule uses AVC, not ATC. A firm making losses (P < ATC) should still produce if P > AVC. This is one of the most commonly tested points in microeconomics.
⚠️ For a monopolist, MR is always below the demand curve (for positive output). If a question says MR equals the demand curve, that describes perfect competition, not monopoly.
⚠️ When computing monopoly profit, use (P - MC) × Q, not (P - ATC) × Q, unless they give you ATC instead of MC. With constant MC and no fixed costs, they are equivalent, but read the question carefully.
⚠️ Deadweight loss questions require you to compare monopoly output to competitive output. Competitive output is where P = MC. Show this comparison explicitly.
⚠️ Know all three degrees of price discrimination with examples. The exam asks you to define, differentiate, and illustrate each.
⚠️ The rule-of-thumb pricing formula is easy marks if you remember it. Plug and chug.
⚠️ Producer surplus is below price, above supply. Consumer surplus is above price, below demand. Do not mix these up.
Q: A monopolist has demand P = 100 - Q and MC = 20. What is the profit-maximising price and quantity?
A: MR = 100 - 2Q. Set MR = MC: 100 - 2Q = 20, so Q = 40. Price: P = 100 - 40 = $60.
Q: Why does a monopoly create deadweight loss?
A: The monopolist restricts output below the competitive level (where P = MC). Consumers whose willingness to pay exceeds MC but falls below the monopoly price are excluded, and those mutually beneficial trades are lost.
Q: A firm in a competitive market has P = $25, ATC = $30, AVC = $20. Should it produce?
A: Yes. P > AVC ($25 > $20), so the firm covers its variable costs and contributes $5 per unit toward fixed costs. Shutting down would mean losing all fixed costs with no revenue offset.
Q: What is first-degree price discrimination?
A: Charging each customer the maximum they are willing to pay. It extracts all consumer surplus and eliminates deadweight loss, but transfers all surplus to the producer.
Q: Give an example of third-degree price discrimination.
A: Student and senior discounts at a museum. The firm segments consumers into groups with different elasticities and charges each group a different price.
Q: Using the rule-of-thumb formula, what price should a firm charge if MC = $20 and |E| = 3?
A: P = 20 / [1 - (1/3)] = 20 / (2/3) = $30.
Q: Where is total welfare maximised in a competitive market?
A: At the competitive equilibrium, where supply equals demand and price equals marginal cost.
perfect competition, price taker, shutdown rule, AVC, monopoly, marginal revenue, MR curve, deadweight loss, DWL, price discrimination, first degree, second degree, third degree, consumer surplus, producer surplus, welfare, competitive equilibrium, markup pricing, rule of thumb pricing, elasticity, information good, sensitivity analysis, satisficing, ECON 323, microeconomic theory, Texas A&M