Perfect competition

What you'll learn

• What perfect competition means and why its assumptions matter.
• How to draw and explain the industry and firm diagrams.
• How short-run supernormal profit or losses adjust into long-run equilibrium.
• How to evaluate perfect competition using allocative and productive efficiency.

1. The basic idea

A market structure is the set of features of a market that influence how firms behave: for example, how many firms there are, whether products are similar, and whether new firms can enter.

A firm is an individual business. An industry is all firms supplying a particular good or service.

Perfect competition is mainly a benchmark model. Real markets rarely satisfy every assumption, but the model helps you judge how competitive markets tend to work and what “efficient” outcomes might look like.

2. Revenue, costs and profit

Before the diagrams, you need the revenue and cost language.

• Total revenue (TR) is all money received from sales.
• Average revenue (AR) is revenue per unit sold: $AR=\frac{TR}{Q}$.
• Marginal revenue (MR) is the extra revenue from selling one more unit: $MR=\frac{\Delta TR}{\Delta Q}$.
• Average cost (AC) is cost per unit of output.
• Marginal cost (MC) is the extra cost of producing one more unit.
• Normal profit is the minimum reward needed to keep an entrepreneur in the market. In economics, it is treated as part of cost.
• Supernormal profit is profit above normal profit, where total revenue exceeds total economic cost.

In perfect competition, the firm sells every unit at the same market price. So:

$$AR=MR=P$$

This is why the individual firm’s demand curve is horizontal.

3. The assumptions behind perfect competition

Perfect competition depends on several strong assumptions.

Many buyers and sellers

There are so many firms that each one is tiny relative to the whole market. No single firm can raise or lower the market price.

Homogeneous products

A homogeneous product is identical regardless of which firm sells it. If one wheat farmer charges more than the market price, buyers can switch to identical wheat elsewhere.

Perfect information

Perfect information means buyers and sellers know all relevant prices, product qualities and production methods. Consumers can find the cheapest supplier, and firms know when profits are available.

Freedom of entry and exit

There are no significant barriers to entry or barriers to exit. A barrier to entry is anything that makes it difficult for new firms to enter, such as patents, high start-up costs or strong brand loyalty.

Profit maximisation

Firms are assumed to choose the output where profit is maximised. For a competitive firm, this occurs where $MR=MC$, provided marginal cost is rising.

4. Short-run equilibrium for the firm and industry

The short run is a time period in which at least one factor of production is fixed. In market-structure diagrams, it also usually means the number of firms in the industry is fixed.

In the industry diagram, market demand and market supply determine the equilibrium price. The individual firm then takes this price as given. In the firm diagram, the demand curve is horizontal at that price, labelled $AR=MR$.

The competitive firm chooses output where $MR=MC$. If price is above average cost at that output, the firm earns supernormal profit.

A firm can also make subnormal profit, often called an economic loss, if price is below average cost. In the short run, the firm might continue producing if it covers its variable costs, but losses create pressure for firms to leave in the long run.

5. Long-run adjustment

The long run is a time period in which all factors of production are variable and firms can enter or exit the market.

If firms earn supernormal profit, new firms are attracted into the industry. Industry supply shifts right. The market price falls. For each existing firm, the horizontal $AR=MR$ line shifts down.

Entry continues until only normal profit remains. At that point:

$$P=AR=MR=MC=AC$$

In the standard long-run diagram, this occurs at the minimum point of $AC$.

If firms are making subnormal profit, the reverse happens. Some firms leave, industry supply shifts left, price rises, and losses are reduced until normal profit is restored.

6. Efficiency in perfect competition

Allocative efficiency

Allocative efficiency occurs when resources are allocated according to consumer preferences. In a simple private market with no externalities, this happens where:

$$P=MC$$

Price represents the value consumers place on the last unit. Marginal cost represents the cost of producing that last unit. If $P=MC$, society is producing the quantity where marginal benefit equals marginal cost.

Perfect competition achieves allocative efficiency because firms produce where $MR=MC$, and in perfect competition $MR=P$.

Productive efficiency

Productive efficiency occurs when output is produced at the lowest possible average cost. This means production is at the minimum point of the $AC$ curve.

In the long run, perfect competition achieves productive efficiency because free entry and exit push firms to the point where price equals minimum average cost.

7. Evaluating perfect competition

Perfect competition has strong theoretical benefits. It tends to produce low prices, no supernormal profit in the long run, and efficient resource allocation. Consumers benefit from firms being unable to restrict output or charge higher prices.

However, the model depends on unrealistic assumptions. Many real markets involve branding, imperfect information, transport costs, patents, advertising or economies of scale. UK supermarkets, for example, are not perfectly competitive because large firms have brand power, pricing strategies and high barriers to entry.

Perfect competition may also be weak for dynamic efficiency, which means improving products or production methods over time through innovation. If firms only earn normal profit in the long run, they may lack funds or incentives for major research and development.

There is also a product-variety issue. Homogeneous goods keep prices low, but consumers often value differentiated products, such as different phone designs, restaurant experiences or clothing brands.

Finally, perfect competition only guarantees private efficiency if there are no externalities. If production creates pollution, then private marginal cost may be below social marginal cost, so $P=MC$ for firms would not mean the socially best output.