Perfect competition: its assumptions, short-run and long-run equilibrium, the role of entry and exit, and why it achieves both allocative and productive efficiency.

What this key area is asking

Perfect competition is the benchmark market structure in the Advanced Higher course. You must know its assumptions, derive its short-run and long-run equilibrium on diagrams, explain how entry and exit drive the market to long-run normal profit, and show why the long-run outcome is efficient on two counts: allocative and productive. Every other structure is then judged against this ideal, so the efficiency results here are reused constantly in evaluation.

The assumptions of perfect competition

The assumptions are strong and rarely met in full, but markets such as some agricultural commodities and foreign exchange come close. The value of the model is as a benchmark, not a literal description.

The firm as a price taker

Because the firm is one of many selling an identical product, it cannot raise its price (buyers would switch to identical rivals) and has no reason to lower it (it can sell all it wants at the market price). Its demand curve is therefore horizontal at the market price, so:

The market price itself is set by total industry demand and supply; the individual firm takes that price as given and decides only its output.

Short-run equilibrium

In the short run the firm maximises profit where $MC = MR$. Three outcomes are possible, found by comparing $AR$ with $AC$ at that output:

• $AR > AC$: abnormal (supernormal) profit.
• $AR = AC$: normal profit only.
• $AR < AC$: a loss (continue while $P \ge AVC$, otherwise shut down).

Long-run equilibrium: the role of entry and exit

The defining feature is what happens next, because entry and exit are free:

• If firms earn abnormal profit, new firms enter. Industry supply shifts right, the market price falls, and each firm's horizontal demand line drops until abnormal profit is competed away.
• If firms make losses, some exit. Industry supply shifts left, the price rises, until the remaining firms earn normal profit.

Long-run equilibrium is reached when only normal profit is earned, so no firm wants to enter or leave. At this point the firm produces where:

Why perfect competition is efficient

This long-run position is the source of the model's importance.

Both forms of efficiency hold simultaneously in long-run perfect competition, which is why it is the yardstick for judging monopoly, oligopoly and the case for intervention.

The limitations of the benchmark

Perfect competition is efficient in a narrow, static sense, but the assumptions cut out things that matter:

• Earning only normal profit leaves no surplus to fund research and development, so dynamic efficiency (innovation over time) may be weak.
• Identical products mean no variety or branding, which consumers may value.
• There are no economies of scale to exploit with many tiny firms, so unit costs may be higher than a larger firm could achieve.

These limitations explain why the real benefits of competition (innovation, choice, scale) are often debated in the structures and intervention area, rather than taken for granted.

Worked example: tracing the adjustment

Why this matters

Perfect competition sets the standard for what an "ideal" market achieves: price equal to marginal cost and production at lowest average cost, with no lasting abnormal profit. Holding this benchmark in your head lets you explain, by contrast, exactly how monopoly and oligopoly fall short (higher price, restricted output, lasting abnormal profit) and exactly why governments might intervene. It is the reference point for the entire structures and intervention area.

Try this

Q1. State the condition for allocative efficiency and explain why a perfectly competitive firm meets it. [3 marks]

• Cue. Allocative efficiency requires $P = MC$. The firm sets $MC = MR$, and as a price taker $P = MR$, so $P = MC$; the value of the last unit equals its production cost.

Q2. Explain why a perfectly competitive firm can earn abnormal profit in the short run but not the long run. [3 marks]

• Cue. In the short run the price may exceed $AC$. With no barriers, abnormal profit attracts entry, raising supply and cutting price until $AR = AC$ and only normal profit remains, so abnormal profit cannot persist.