Perfect Competition

Perfect competition is a theoretical market structure that serves as a benchmark against which real-world markets are assessed. No real market fully satisfies all the conditions of perfect competition, but the model is essential for understanding how competitive forces drive outcomes and for evaluating the efficiency of other market structures.

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Assumptions of Perfect Competition

Assumption	Explanation
Many buyers and many sellers	Each firm is so small relative to the market that it cannot influence the market price
Homogeneous products	All firms sell identical goods — consumers perceive no difference between them
Perfect information	All buyers and sellers have complete knowledge of prices, costs, and market conditions
Free entry and exit	There are no barriers to entry or exit — firms can join or leave the industry without cost
No transport costs	Products can be transported costlessly (so location confers no advantage)
Profit maximisation	All firms aim to maximise profit
Factor mobility	Factors of production can move freely between industries

Exam Tip: You must know these assumptions precisely. A common exam question asks you to "explain the characteristics of perfect competition" — listing the assumptions clearly and explaining why each matters is essential for full marks.

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The Perfectly Competitive Firm as a Price Taker

Because each firm is tiny relative to the market and sells a product identical to its rivals, no individual firm can influence the market price. The firm is a price taker — it must accept the price determined by the interaction of market supply and market demand.

The Demand Curve Facing the Firm

The individual firm's demand curve is perfectly elastic (horizontal) at the market price. This means:

Revenue Concept	Relationship	Value
Price (P)	Set by the market	Constant for the firm
Average Revenue (AR)	Revenue per unit = Price	AR = P
Marginal Revenue (MR)	Revenue from one more unit = Price	MR = P
Demand (D)	The firm can sell any quantity at the market price	D = AR = MR

This is the crucial feature of perfect competition: D = AR = MR = P. The firm's demand curve, average revenue curve, and marginal revenue curve are all the same horizontal line at the market price.

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Short-Run Equilibrium

Profit-Maximising Output

The firm maximises profit by producing at the output where MC = MR (and MC is rising). Since MR = P in perfect competition, this becomes MC = P.

Three Possible Short-Run Outcomes

Outcome	Condition	What Happens
Supernormal profit	P > ATC at the profit-maximising output	TR > TC — the firm earns more than normal profit
Normal profit	P = ATC at the profit-maximising output	TR = TC — the firm earns exactly the minimum required to stay in the industry
Loss (subnormal profit)	P < ATC but P > AVC	The firm makes a loss but continues producing because it covers variable costs and contributes to fixed costs
Shutdown	P < AVC	The firm should cease production immediately — it cannot even cover its variable costs

Exam Tip: When drawing diagrams, always show the profit-maximising output at MC = MR, and shade the profit/loss rectangle clearly. The vertical distance between AR and ATC gives the profit/loss per unit; the shaded area (profit/loss per unit × quantity) shows total profit or loss. Label everything — unlabelled diagrams score poorly.

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Long-Run Equilibrium

The defining feature of perfect competition is the adjustment mechanism that operates in the long run:

If firms earn supernormal profit in the short run:

1. New firms are attracted into the industry (free entry)
2. Industry supply increases (the market supply curve shifts right)
3. The market price falls
4. Individual firms' profits are competed away
5. Entry continues until only normal profit remains

If firms make losses in the short run:

1. Some firms exit the industry (free exit)
2. Industry supply decreases (the market supply curve shifts left)
3. The market price rises
4. Remaining firms return to normal profit
5. Exit continues until losses are eliminated

Long-Run Equilibrium Conditions

In long-run equilibrium, a perfectly competitive firm:

• Earns normal profit only (zero economic profit)
• Produces at the output where P = MC = ATC (at its minimum)
• Cannot earn supernormal profit because entry would erode it
• Cannot make a loss because exit would eliminate it