Difficulty: Intermediate | Prerequisites: Producer theory, cost curves (MC, AVC, ATC)
Perfect competition and monopoly are the two polar market structures. In perfect competition, many firms produce identical goods, price equals marginal cost in the long run, and economic profit is zero. A monopolist is the sole seller, sets price above marginal cost, earns positive economic profit, and creates deadweight loss. The exam tests both structures heavily, including graphing cost curves, finding equilibrium, and calculating welfare measures.
Perfect competition
A market structure where many buyers and sellers trade a homogeneous good, everyone has complete information, entry and exit are free, and there are no externalities, transaction costs, or government intervention. No single firm can influence the market price.
Short-run supply curve
For a perfectly competitive firm, the portion of the MC curve that lies above the minimum of AVC. Below that price the firm shuts down and produces zero.
Shutdown price
The minimum of AVC. If the market price falls below this, the firm loses less by producing nothing (it still pays FC, but avoids variable losses).
Breakeven price
The minimum of ATC. At this price, total revenue exactly covers total cost. Economic profit is zero.
Profit maximisation rule
Produce where marginal revenue equals marginal cost (MR = MC). In perfect competition, MR = P, so the rule becomes P = MC.
Long-run equilibrium (perfect competition)
P = MC = ATC. Firms earn zero economic profit. Entry and exit have driven price to the breakeven level.
Monopoly
A market with a single seller. The monopolist faces the entire market demand curve, so it is a price-maker rather than a price-taker.
Monopoly profit maximisation
Set MR = MC, then read the price off the demand curve at that quantity. Because demand is downward-sloping, MR < P, so the monopolist charges above marginal cost.
Deadweight loss (DWL)
The reduction in total surplus caused by a market outcome that differs from the competitive equilibrium. Under monopoly, DWL is the triangle between the demand curve and MC, from Q_M to Q_C.
Consumer surplus (CS)
The difference between what consumers are willing to pay and what they actually pay. Graphically, the area under the demand curve and above the price, up to the quantity sold.
Producer surplus (PS)
The difference between revenue received and variable cost. Graphically, the area above the supply curve (or MC curve) and below the price.
Lerner Index
Measures a firm's market power: L = (P - MC) / P. In perfect competition, L = 0. For a monopolist, L = 1 / |elasticity of demand|. A higher Lerner Index means more markup over marginal cost.
Price discrimination
Charging different prices to different consumers for the same product. Perfect (first-degree) price discrimination means charging each consumer their exact willingness to pay, capturing all consumer surplus and eliminating DWL.
Seven assumptions define the model:
Many buyers and sellers (no one can move the price)
Homogeneous good (Farm A's corn is identical to Farm B's)
Complete information
Free entry and exit
No externalities
No transaction costs
No government regulation, taxes, or subsidies
The firm takes the market price as given (MR = P) and chooses output where P = MC (on the rising portion of MC).
Profit = (P - ATC) * Q. If P > ATC, profit is positive. If AVC < P < ATC, the firm operates at a loss but covers variable costs (it would lose more by shutting down). If P < AVC, the firm shuts down.
The short-run supply curve is the MC curve above min AVC.
Three cases govern the adjustment:
P > breakeven price: positive economic profit attracts new firms, supply increases, price falls
Shutdown price < P < breakeven price: negative profit causes firms to exit, supply decreases, price rises
P < shutdown price: firms shut down immediately
In long-run equilibrium: P = MC = min ATC. Zero economic profit. The number of firms adjusts until this holds.
To find the number of firms in long-run equilibrium: find total market quantity demanded at the long-run price, then divide by the quantity each firm produces at min ATC.
The monopolist faces the market demand curve. Because demand slopes downward, MR < P. The monopolist maximises profit by setting MR = MC, then charging the price on the demand curve at that quantity.
Finding the total cost function: start from the production function and input prices. Use the cost-minimisation condition (MPL/w = MPK/r) to express K* and L* as functions of Q. Substitute into the cost equation wL + rK to get TC(Q). Differentiate to get MC.
Monopoly price and quantity: set MR = MC, solve for Q_M, plug into demand for P_M.
Monopoly profit: (P_M - ATC) * Q_M, or equivalently TR - TC.
Consumer surplus under monopoly: the triangle under the demand curve, above P_M, from 0 to Q_M.
Deadweight loss: the triangle between demand and MC, from Q_M to Q_C (the competitive quantity).
Lerner Index: L = (P - MC) / P. For a monopolist with linear demand, this equals 1 / |Ed|, where Ed is the price elasticity of demand.
Occurs when a firm charges different prices to different consumers for the same product.
Perfect (first-degree) price discrimination: the firm charges each consumer their maximum willingness to pay. All consumer surplus is captured as profit. There is no DWL because the efficient quantity is produced, but the entire surplus goes to the firm.
Students sometimes think firms shut down whenever they make a loss. They do not. In the short run, a firm keeps producing as long as P > AVC, because revenue still covers variable costs and contributes toward fixed costs. Shutting down means losing all of FC.
A common mistake is setting P = ATC to find the profit-maximising output. The rule is P = MC (or MR = MC for a monopolist). P = ATC tells you the breakeven point, not the optimal output.
Students sometimes confuse accounting profit with economic profit. In long-run competitive equilibrium, economic profit is zero, but the firm may still earn a positive accounting profit (it covers the opportunity cost of capital).
Under monopoly, the demand curve is not the MR curve. MR lies below the demand curve for a linear demand function (and has twice the slope). Confusing the two leads to wrong answers for Q_M and P_M.
⚠️ The perfect competition problem is one of the seven large final exam problems. You will need to find the profit-maximising output, calculate profit, graph MC/AVC/ATC, identify the SR supply curve, find the breakeven price, and determine the number of firms in LR equilibrium.
⚠️ The monopoly problem is another large final exam problem. You must derive TC from the production function, find MC, set MR = MC, compute monopoly profit, CS, DWL, and the Lerner Index.
⚠️ Know the welfare comparison: competitive equilibrium maximises total surplus; monopoly reduces CS, increases PS (but not enough to offset), and creates DWL.
Q: A competitive firm has TC = 50 + 2Q^2. Find the shutdown price and breakeven price.
A: MC = 4Q. VC = 2Q^2, so AVC = 2Q. Min AVC occurs where MC = AVC: 4Q = 2Q, which gives Q = 0, AVC = 0. So the shutdown price is 0 (the firm always produces as long as P > 0). ATC = 50/Q + 2Q. Set MC = ATC: 4Q = 50/Q + 2Q, so 2Q = 50/Q, Q^2 = 25, Q = 5. Breakeven price = MC at Q = 5 = 20.
Q: A monopolist faces demand P = 100 - 2Q and has MC = 20. Find Q_M, P_M, profit, and DWL.
A: TR = 100Q - 2Q^2, MR = 100 - 4Q. Set MR = MC: 100 - 4Q = 20, Q_M = 20. P_M = 100 - 2(20) = 60. Profit = (60 - 20) * 20 = 800 (assuming no fixed costs). Competitive output: P = MC gives 100 - 2Q = 20, Q_C = 40. DWL = 0.5 * (60 - 20) * (40 - 20) = 400.
Q: What is the Lerner Index for the monopolist above at the profit-maximising price?
A: L = (P - MC) / P = (60 - 20) / 60 = 2/3. This indicates substantial market power.
Q: In long-run competitive equilibrium, if market demand at the breakeven price is 500 units and each firm produces 5 units at min ATC, how many firms are in the market?
A: 500 / 5 = 100 firms.
Perfect competition and monopoly are the benchmarks against which Cournot and Stackelberg oligopoly outcomes are measured. Cournot equilibrium falls between the competitive and monopoly outcomes.
The welfare analysis here (CS, PS, DWL) carries directly into the supply/demand/taxes topic, which adds government revenue and tax incidence.
The cost curves from producer theory are the foundation for everything in this document.
Related Terms / Search Tags
perfect competition, monopoly, market structure, profit maximisation, MR = MC, short-run supply curve, shutdown price, breakeven price, long-run equilibrium, zero economic profit, deadweight loss, DWL, consumer surplus, producer surplus, Lerner Index, market power, price discrimination, first-degree price discrimination, monopoly pricing, welfare analysis, allocative efficiency