Difficulty: Intermediate to Advanced | Prerequisites: Production Theory and Costs notes, Consumer Choice notes, Demand and Elasticity notes.
This topic brings together everything from the course. Consumer choice gave you demand; production theory gave you costs; now you see how firms decide what to produce and how markets reach equilibrium. Perfect competition is the benchmark: the ideal case where no one has market power and prices reflect true costs. From there, you can measure welfare (consumer and producer surplus), see how taxes and regulations distort outcomes, and understand what happens when competition breaks down (monopoly, monopsony).
This is where the course shifts from "how do agents optimise" to "what does the market look like as a result." If you are behind, make sure your cost curves and elasticity are solid before diving in.
Under perfect competition, firms are price takers and maximise profit where MC = MR (= price). In the short run, a firm produces as long as price covers AVC; in the long run, free entry drives economic profit to zero. Consumer surplus plus producer surplus equals total welfare, and the competitive outcome maximises it. Taxes, price controls, and market power all create deadweight loss by moving the market away from the efficient quantity.
Perfect competition
A market structure defined by four assumptions: homogeneous product, firms are price takers, perfect information, and free entry/exit in the long run.
Price taker
A firm that cannot affect the market price. If it raises its price, it loses all customers. Think of it as: the firm faces a perfectly elastic (horizontal) demand curve.
Profit maximisation
The process by which a firm determines the price and output level that returns the greatest profit. In the short run: set MC = MR.
Accounting profit
Total revenue minus total explicit costs (the costs you can see on an invoice).
Economic profit
Total revenue minus all costs, both explicit and implicit (opportunity costs). In simple terms, this is the profit after accounting for what the owner could have earned elsewhere.
Producer surplus
Revenue minus variable costs. The benefit to the firm of producing rather than shutting down. In the short run, PS is greater than profit when fixed costs are positive.
Consumer surplus
The difference between willingness to pay and the price actually paid, summed across all units purchased.
Total welfare (total surplus)
Consumer surplus plus producer surplus. The efficient outcome is where total welfare is maximised.
Deadweight loss (DWL)
The reduction in total welfare caused by a market distortion (tax, price control, monopoly).
Short-run supply curve
The portion of the MC curve that lies above AVC. Below AVC, the firm shuts down.
Shutdown condition
The firm should shut down when price falls below AVC (P < AVC). At shutdown, the firm's loss equals its fixed costs.
Break-even point
Where price equals ATC. The firm earns zero economic profit.
Monopoly
A market with a single seller. The monopolist sets price above marginal cost and produces a smaller quantity than the competitive outcome.
Lerner index
A measure of monopoly power: (P - MC) / P. Under perfect competition, the Lerner index is 0. The higher the index, the greater the firm's pricing power.
Monopsony
A market with a single buyer (of an input). The monopsonist pays a price that depends on the number of units purchased, unlike a competitive buyer who takes the price as given.
Excise tax
A fixed amount per unit of a good. On buyers, it shifts demand left; on sellers, it shifts supply left.
Ad valorem tax
A tax applied as a percentage of price (e.g. 7.5% sales tax).
Price ceiling
A legal maximum price. If set below equilibrium, it creates a shortage (quantity demanded exceeds quantity supplied).
Price floor
A legal minimum price. If set above equilibrium, it creates a surplus (quantity supplied exceeds quantity demanded).
Tax incidence
How the burden of a tax is divided between buyers and sellers. Determined by relative elasticities: the more inelastic side bears more of the tax.
EITC (Earned Income Tax Credit)
A wage subsidy for low-income workers. Increases labour supply, which competes down the market wage, but the EITC payment tops the worker's income above the original level.
Four critical assumptions
Homogeneous product: Firm A's product is a perfect substitute for Firm B's. No product differentiation.
Price takers: firms cannot affect the market price. If they raise their price, they lose all customers.
Perfect information: both consumers and producers have full information. No moral hazard or adverse selection.
Free entry and exit: no barriers (no patents, no licensing). Firms can enter or leave freely in the long run.
Short-run profit maximisation
Set MC = MR. Under perfect competition, MR = P (the demand curve facing the firm is horizontal).
The firm produces the quantity where MC = P.
Produce or shut down?
Produce if total revenue covers variable costs: TR >= VC, or equivalently P >= AVC.
Shut down if P < AVC. At shutdown, the firm's loss equals its fixed costs (-Profit = -FC).
The key question in the short run: "how do we lose the least amount of money?"
Short-run supply curve
The portion of the MC curve above AVC.
Below AVC, quantity supplied is zero (the firm shuts down).
Break-even point
Where P = ATC. The firm earns exactly zero economic profit.
Below this price the firm operates at a loss; above it, the firm earns positive economic profit.
Short-run outcomes
Firms may earn positive or negative economic profit in the short run (they cannot exit the market).
Long-run equilibrium
All costs are variable (firms can adjust all inputs, purchase or sell capital).
Free entry and exit means firms enter when profits are positive (driving price down) and leave when profits are negative (driving price up).
In long-run equilibrium, all firms earn zero economic profit. Price equals the minimum of ATC.
Producer surplus
Revenue minus variable costs. Graphically, the area between the price line and the supply curve (MC above AVC).
The benefit of producing rather than shutting down.
In the short run, when fixed costs are positive, producer surplus is greater than profit.
Important distinctions
Producer surplus = Revenue - Variable Costs.
Accounting profit = Revenue - Explicit Costs (both variable and fixed).
Economic profit = Revenue - Explicit Costs - Implicit Costs (opportunity costs).
Consumer surplus
The area between the demand curve and the price line.
Represents the total "bonus" consumers receive from paying less than their maximum willingness to pay.
Total welfare
Consumer surplus + Producer surplus = Total welfare (total surplus).
The efficient outcome is where total welfare is maximised. Under perfect competition, this occurs at the market equilibrium quantity.
Any deviation from the competitive equilibrium (taxes, price controls, monopoly) creates deadweight loss.
Excise tax on buyers
Shifts the demand curve to the left.
The buyer pays the original price plus the tax; the effective price to the seller falls.
Excise tax on sellers
Shifts the supply curve to the left.
The seller receives the market price minus the tax.
Tax incidence
Tax incidence on sellers: (P - P2) * Q2.
Tax incidence on buyers: (P2 + tax - P) * Q2.
Pass-through (buyer's burden) = Es / (Es - Ed).
The more inelastic side bears more of the burden. Whether the tax is levied on buyers or sellers does not change who pays; it is elasticities that determine incidence.
Price ceilings
A maximum price set below equilibrium. Quantity demanded exceeds quantity supplied, creating a shortage.
Example: rent controls.
Price floors
A minimum price set above equilibrium. Quantity supplied exceeds quantity demanded, creating a surplus.
Example: minimum wage (in a simple model).
Subsidies and the EITC
Subsidies are the mirror image of taxes (negative taxes).
The EITC works as a wage subsidy for low-income working mothers. It increases labour supply, which pushes down the market wage, but the credit tops workers' total income above the original level.
Both PS and CS increase, but there is still deadweight loss.
The government bears the cost of the subsidy.
Monopoly pricing
A monopolist is the sole seller. It faces the entire market demand curve.
It maximises profit by setting MC = MR, but MR < P (the MR curve lies below the demand curve).
Compared to perfect competition, a monopolist charges a higher price and sells a smaller quantity.
The monopolist has no supply curve because the quantity it supplies depends on the shape of the demand curve, not just on the price.
Monopoly power
The ability to set price above marginal cost.
Measured by the Lerner index: L = (P - MC) / P.
Under perfect competition, L = 0 (price equals MC).
A higher Lerner index means greater market power.
Common misconception about monopoly
A monopolist cannot charge as high a price as it likes. It is still constrained by the demand curve. Setting a very high price means selling very few (or zero) units.
Monopsony
A single buyer of an input (e.g. the only employer in a remote area).
Unlike a competitive buyer who takes the price as given, a monopsonist's price depends on the number of units purchased.
The monopsonist faces an upward-sloping supply curve and a marginal expenditure curve above it.
Large firms that can negotiate lower input prices (e.g. large computer manufacturers buying components) exhibit a degree of monopsony power.
Average revenue and monopoly
For a monopolist, at every output level, average revenue equals price. This is true because AR = TR/Q = P*Q/Q = P.
Profit: π = P * Q - C(Q) (revenue minus costs)
SR profit max: MC = MR = P
Produce if: P >= AVC
Shutdown if: P < AVC (loss = FC)
Break-even: P = ATC
Producer surplus: TR - VC (area between price and MC above AVC)
Lerner index: L = (P - MC) / P
Tax incidence on sellers: (P - P2) * Q2
Tax incidence on buyers: (P2 + tax - P) * Q2
Pass-through: Es / (Es - Ed)
Key diagrams to know
Competitive firm's short-run diagram: horizontal demand/MR line, U-shaped MC, ATC, AVC curves, with profit/loss area shaded.
Shutdown point at AVC minimum; break-even point at ATC minimum.
Long-run equilibrium with P = min ATC and zero economic profit.
Monopolist's diagram: downward-sloping demand, MR below demand, profit area between P and ATC at the MC = MR quantity.
Tax incidence diagram: original equilibrium, shifted curve, new equilibrium, buyer and seller tax burdens, DWL triangle.
Price ceiling and floor diagrams showing shortage and surplus respectively.
Students often think a monopolist can charge whatever it wants. A monopolist is still constrained by the demand curve. An extremely high price means near-zero sales.
Students confuse producer surplus with profit. PS = Revenue - VC; profit = Revenue - TC (including fixed costs). In the short run with positive fixed costs, PS > profit.
Students assume tax incidence depends on who the tax is levied on (buyers or sellers). It does not. Tax incidence depends on relative elasticities. The legal incidence is irrelevant to the economic incidence.
Students think a firm should shut down whenever it is making a loss. A firm should only shut down if P < AVC. If P > AVC but P < ATC, the firm should keep producing because it covers variable costs and offsets some fixed costs.
The produce-or-shutdown decision (P vs. AVC) is tested heavily. Know all four forms of the condition: TR - VC - FC >= -FC, TR >= VC, PQ >= VC, P >= AVC.
Be ready to calculate profit, producer surplus, and consumer surplus from given cost and demand functions.
Know the four assumptions of perfect competition and when the S&D model breaks down (monopoly, differentiated goods, asymmetric information, barriers to entry).
Expect Lerner index calculations and questions about why the pure monopolist has no supply curve.
Understand long-run equilibrium: zero economic profit, P = min ATC, free entry eliminates positive profits.
Tax incidence questions often test whether you know that elasticities (not legal assignment) determine burden.
True or false: in perfect competition, a firm should shut down whenever price is below ATC. False (shut down only when P < AVC).
Fill in the blank: in long-run competitive equilibrium, economic profit is ____. Zero.
True or false: the Lerner index under perfect competition is 0. True.
Fill in the blank: the short-run individual supply curve is the MC curve above ____. AVC.
True or false: a monopolist has a supply curve. False (quantity supplied depends on the demand curve, not just on price).
Q: Which of the following is not a correct characterisation of a pure monopoly: "A monopolist can charge as high a price as it likes"?
A: Correct, this statement is not accurate. A monopolist is constrained by the demand curve. It chooses the profit-maximising price-quantity combination, but cannot simply set any price it wants without losing sales.
Q: Compared to perfect competition, a pure monopolist will charge a ____ price and sell a ____ quantity.
A: Higher; smaller.
Q: If the market for widgets is suddenly monopolised, what should we expect?
A: A decrease in consumer surplus in the widget market (price rises, quantity falls, some surplus transfers to the monopolist and some is lost as deadweight loss).
Q: Why does a price ceiling prevent the market from reaching equilibrium?
A: When the market price is not allowed to rise to the equilibrium level, quantity demanded exceeds quantity supplied, creating a shortage.
Q: A monopsonist pays a price for a factor of production that depends on what?
A: The number of units purchased. Unlike a competitive buyer who takes the price as given, a monopsonist faces an upward-sloping supply curve, so buying more units drives up the price it pays.
Q: For a monopolist, at every output level, average revenue equals price. True or false?
A: True. AR = TR/Q = (P * Q)/Q = P.
Perfect competition is the benchmark against which every other market structure is measured. The welfare analysis here (CS + PS, deadweight loss) is the framework used to evaluate any policy intervention throughout the rest of the course. Monopoly and monopsony extend naturally into oligopoly and game theory. The cost curves from production theory are essential inputs here: MC, ATC, and AVC are the building blocks of the firm's supply decision.
Perfect competition, price taker, homogeneous good, free entry, free exit, profit maximisation, MC = MR, accounting profit, economic profit, producer surplus, consumer surplus, total welfare, total surplus, deadweight loss, DWL, short-run supply curve, shutdown condition, shutdown point, break-even point, monopoly, monopolist, Lerner index, market power, monopsony, monopsony power, marginal expenditure, excise tax, ad valorem tax, tax incidence, pass-through, price ceiling, price floor, shortage, surplus, subsidy, EITC, earned income tax credit, wage subsidy, ECO 3101, intermediate microeconomics, FIU