Perfect Competition vs Oligopoly

The two extremes of how much one firm's choice matters

In perfect competition a single firm is so small that its output decision cannot move the market price. It takes the price as given, which is why the demand curve it faces is horizontal and why price equals marginal revenue. In an oligopoly a firm is large enough that its decisions move the market AND provoke responses, so it must think about rivals before acting. Everything else follows from that: efficiency, profit, the shape of the demand curve, and even which analytical tool applies.

Efficiency, and where the deadweight loss comes from

A perfectly competitive market in long-run equilibrium is both allocatively efficient, since price equals marginal cost, and productively efficient, since firms produce at minimum average total cost. An oligopoly generally achieves neither. Because it restricts output to hold price above marginal cost, some units that buyers value more than they cost to make never get produced, and the value of those lost trades is deadweight loss. This is the standard efficiency case against concentrated markets. The standard counter is that large firms may fund research a fragmented industry could not, which is worth a sentence in an evaluation answer rather than a whole essay.

Why entry decides the long run

Perfect competition has free entry and exit, so any economic profit draws in new firms until it disappears, and any loss drives firms out until it disappears. Long-run profit is therefore zero by construction. An oligopoly's high barriers, whether from economies of scale, patents, control of an input, or regulation, stop that process, so profit can persist indefinitely. When a question asks why one industry stays profitable and another does not, the answer is almost always the barrier rather than anything about the firms themselves. Compare the diagrams at /sandbox/perfect-competition.

Frequently asked questions