Economics HL · Microeconomics Unit 2: Market Power · 7 min read · Updated 2026-05-11
Perfect competition — IB Economics HL
IB Economics HL · Microeconomics Unit 2: Market Power · 7 min read
1. Core Characteristics of Perfect Competition★★☆☆☆⏱ 15 min
Many small firms: Each firm produces an insignificant share of total market output
Homogeneous products: Products are identical across firms, so consumers have no preference
No barriers to entry or exit: Firms can join/leave the market with no extra costs in the long run
Perfect information: All firms and consumers have full information about prices and costs
Exam tip: IB examiners frequently ask why perfect competition is a theoretical model: always mention that no real market satisfies all core assumptions.
2. Short-run Equilibrium for the Firm★★★☆☆⏱ 20 min
AR = \frac{TR}{Q} = \frac{P \times Q}{Q} = P = MR
All profit-maximizing firms produce at the output where $MR = MC$. For perfectly competitive firms, this becomes $P = MC$, since $P=MR$.
Exam tip: Always label the firm's horizontal demand curve $AR = MR = P$ on your diagram: missing labels cost you marks in the IB.
3. Long-run Equilibrium for Firms and Industry★★★☆☆⏱ 20 min
The key difference between the short run and long run in perfect competition is free entry and exit. If existing firms earn abnormal profit, new firms enter the industry, increasing market supply and pushing price down until abnormal profit is eliminated. If existing firms make losses, firms exit, reducing supply and pushing price up until losses are gone.
Exam tip: Always draw two separate diagrams for long-run adjustment: one for the market (industry) and one for the individual firm.
4. Efficiency Evaluation of Perfect Competition★★★★☆⏱ 15 min
Perfect competition is considered the most efficient market structure in neoclassical economics, because it achieves both productive and allocative efficiency in the long run.
Allocative efficiency: $P = MC$, meaning marginal benefit to consumers equals marginal cost of production, so social surplus is maximized
Productive efficiency: Production occurs at the minimum point of the ATC curve, so output is produced at the lowest possible cost
Exam tip: For 15 mark essays, always evaluate both the efficiency benefits and the dynamic costs of perfect competition to reach a balanced conclusion.
Common Pitfalls
Why: You confused the industry demand curve with the individual firm's demand curve. The firm is a price taker, so demand is perfectly elastic.
Why: Free entry and entry means any abnormal profit is quickly competed away by new firms entering the market.
Why: If price is above average variable cost, the firm can cover variable costs and contribute to fixed costs, so losing less money than shutting down.
Why: Productive efficiency at minimum ATC only occurs in long-run equilibrium, not in the short run.
Why: Normal profit includes the opportunity cost of the entrepreneur's capital and time, so it is zero economic profit, not zero accounting profit.