Oligopoly vs Duopoly
Oligopoly and Duopoly are related concepts in AP Economics that students often mix up. An oligopoly is a market structure dominated by a small number of large interdependent firms. A duopoly is a market with only two sellers, the simplest kind of oligopoly, where each firm's best price or output depends on what the other one chooses. Here is how they compare side by side.
Firms in an oligopoly are mutually aware of each other’s actions and often engage in strategic behavior, such as price leadership or collusion. High barriers to entry limit competition and can lead to sustained economic profits.
With only two firms, neither can plan in isolation, so the outcome is a strategic problem rather than a calculation, and the price that emerges depends on how the two compete. If they choose quantities, each holds output back a little and price settles between the competitive and the monopoly level. If they set prices on identical products and both have spare capacity, undercutting by a cent wins the whole market, and price can be driven down to marginal cost even with two sellers. If they coordinate, openly or quietly, they can approach the monopoly price, but each has a private incentive to shave price and take the extra sales, which is why such arrangements break down. A duopoly is simply an oligopoly with two firms, and it is not a duopsony, which has two buyers.
Oligopoly vs Duopoly: The General Category and Its Smallest Case
| Oligopoly | Duopoly | |
|---|---|---|
| Number of sellers | A few, few enough that each one's choice matters | Exactly two |
| How the two terms relate | The general category | A case that sits inside it |
| Whom each firm must predict | Several rivals at once | One rival, whose reaction is far easier to model |
| Ease of tacit coordination | Harder as more firms are added | Easiest, since any defection is obvious and traceable |
| Usual role in a course | The realistic description of concentrated industries | The teaching case used to derive the standard models |
| Price compared with competition | Above marginal cost, falling as firms are added | Above marginal cost, and usually the highest of the oligopoly cases |
| Share held by the top two | Less than all of it whenever a third firm exists | All of it, by definition |
Adding one more seller moves the price by a predictable amount
Take an illustrative market where the price is 100 dollars minus one dollar for every unit the industry sells, and any firm can make a unit for 10 dollars. The gap between the highest price anyone would pay and the cost of a unit is 90 dollars, and that gap drives everything. A single seller makes 45 units and charges 55 dollars. Two firms choosing output at the same time each settle on 30 units, so 60 units are made, the price is 40 dollars, and each firm earns 30 dollars a unit on 30 units, or 900 dollars. Add a third firm and each one settles on 22.5 units, so 67.5 units are made, the price falls to 32.5 dollars, and each earns 22.5 dollars a unit on 22.5 units, or 506.25 dollars. The pattern is that each firm produces the 90 dollar gap divided by one more than the number of firms, so output climbs and price slides toward cost as sellers are added. This is the /glossary/cournot-competition result, and it explains why a duopoly is usually the worst case for buyers among oligopolies rather than a different animal from them.
Two firms is where interdependence is easiest to see, not where it starts
What makes a market an oligopoly is not a specific count. It is that each firm has to think about what the others will do before it acts, because its own best price or output depends on their choices. That condition can hold with two firms, three, or eight. It stops holding once there are so many sellers that no single one can move the price, which is where the competitive models take over. A duopoly is simply the version where the interdependence is stripped to its simplest form. There is exactly one rival to predict, one set of reactions to work through, and any cheating on an understanding is obvious, since only one other firm could have done it. That is why textbooks derive the quantity, price and sequential move models with two firms and then generalize. Watch the direction of the argument on an exam. A statement about oligopolies applies to duopolies, because every duopoly is one. A statement about duopolies does not automatically apply to larger oligopolies, since coordination gets harder and price gets closer to cost as the number of sellers grows. Work through the diagrams at /micro/oligopoly to see how the models change shape.
Frequently asked questions
Is a duopoly an oligopoly?
Yes, a duopoly is an oligopoly with exactly two sellers, so everything true of oligopolies in general is true of a duopoly. The reverse does not hold, since most oligopolies have more than two firms. Duopoly is the case courses use first because there is only one rival to reason about.
How many firms are needed for an oligopoly?
There is no fixed number; a market is an oligopoly when each firm is large enough that it must anticipate its rivals' responses before setting price or output. That usually means somewhere between two and roughly ten meaningful sellers, though a market with more firms can qualify if a handful of them dominate. The test is interdependence, not a head count.
Do prices fall when a duopoly gains a third competitor?
Yes, in the standard quantity setting model each firm cuts back only part of what the newcomer adds, so total output rises and the price falls toward marginal cost. In the illustrative case above, two firms sold 60 units at 40 dollars and three firms sold 67.5 units at 32.5 dollars. Profit per firm falls as well, which is why incumbents in concentrated markets work so hard at keeping entrants out.
Get AP Econ exam tips in your inbox
Occasional emails with study tips, new interactive graphs, and exam-season reminders. Free, no spam.
No spam. Unsubscribe anytime. Read our privacy policy.
Keep track of what you have studied
A free EconLearn account adds progress tracking, your quiz history, and achievements. Studying here is free either way, and there is nothing to pay for as a student.
Create a free accountAlready have one? Sign in
Last updated