Perfect Competition

Perfect competition is a theoretical market structure in which many small firms sell an identical product to many buyers, anyone can enter or leave the market freely, and everyone has full information about prices and quality. Under these conditions no single firm can affect the market price; each is a "price taker" that can sell as much as it likes at the going price and nothing above it.

No real market meets all the conditions, but the model is the benchmark against which economists judge every other market structure. It sits at one end of a spectrum that runs through monopolistic competition and oligopoly to monopoly at the other end. It is central to neoclassical economics and to the case for free markets.

Imagine a large farmers' market where hundreds of stalls sell exactly the same potatoes, every price is chalked on a board everyone can see, and anyone can rent a stall tomorrow. If one seller asks a cent more than the others, nobody buys from them. If potatoes become very profitable, new sellers arrive and the extra profit is competed away.

In that market, no seller sets the price; the crowd does. Each seller decides only how many potatoes to bring. The result is that buyers pay roughly what it costs to grow the last potato, and no one earns more than a normal return for long.

• Many buyers and sellers. Each is too small for its own decisions to move the market price.
• Identical products. Buyers see one firm's output as a perfect substitute for another's, so there is no brand loyalty.
• Free entry and exit. There are no legal, financial or technical barriers to starting or closing a business.
• Perfect information. Everyone knows the prices and quality on offer, ruling out problems such as asymmetric information.
• No transaction costs or externalities. Prices reflect all the costs and benefits of production.

Because the firm takes the price as given, each extra unit sold brings in exactly the market price, so price equals marginal revenue. The firm maximizes profit by producing up to the point where:

• $P$ is the market price.
• $MR$ is marginal revenue, the extra revenue from one more unit.
• $MC$ is marginal cost, the extra cost of producing one more unit.

Worked example: a wheat farm's marginal cost per tonne is $MC = 100 + 5q$ dollars, where $q$ is tonnes produced. If the market price is $200, the farm sets $100 + 5q = 200$, so $q = 20$ tonnes. Producing a 21st tonne would cost $205, more than it sells for.

In the short run, firms can earn profits or make losses. In the long run, profits attract entrants and losses drive firms out, until price equals the minimum of average total cost. Firms then earn zero economic profit: they cover all costs, including a normal return on capital and the opportunity cost of the owner's time, but nothing more.

The long-run outcome has two efficiency properties. It is allocatively efficient, because price equals marginal cost, so goods are produced exactly up to the point where buyers value them at what they cost to make. It is productively efficient, because firms produce at the lowest possible average cost. The first fundamental theorem of welfare economics states that, under these conditions, a competitive equilibrium is Pareto efficient: no one can be made better off without making someone else worse off.

The model also clarifies what goes wrong elsewhere. Departures from its assumptions explain market failure, the losses caused by market power, and the case for antitrust law. Markets that come close include many agricultural commodities, such as wheat traded through commodities markets, and highly liquid financial markets where thousands of traders deal in standardized contracts.

• Unrealistic assumptions. Most products are differentiated, information is costly and entry often needs large investment or licenses, and economies of scale favor large firms.
• Static view of competition. Joseph Schumpeter argued that the competition that matters is the arrival of new products and methods, a process he called creative destruction, which needs the temporary profits perfect competition rules out.
• Competition as a process. Friedrich Hayek argued in "The Meaning of Competition" (1946) that the model describes the end state of competition and leaves out the rivalry that produces it.
• Alternative benchmarks. The theory of contestable markets, developed by William Baumol, John Panzar and Robert Willig in 1982, holds that even a market with few firms can deliver competitive prices if entry and exit are cheap and quick.

Supporters reply that the model is not meant to be realistic, only to identify the conditions under which markets allocate resources well, and that is why it remains the first market structure taught in economics.

• OpenStax Principles of Economics: Perfect Competition and Why It Matters