Oligopoly and Game Theory — AP Microeconomics
1. Core Definition of Oligopoly ★★☆☆☆ ⏱ 2 min
Oligopoly is a market structure defined by a small number of large, strategically interdependent firms, protected by high barriers to entry that block new competitors from entering the market. Unlike perfect competition (where firms are price takers) or monopoly (where there is only one firm), oligopolistic firms must account for the actions of their rivals when setting price, output, advertising, or other strategic choices. Game theory is the formal framework economists use to model this interdependent decision making, predicting the outcomes firms will choose and comparing them to socially optimal outcomes.
Oligopoly and game theory make up approximately 4% of the total AP Microeconomics exam score, and questions appear in both multiple-choice (MCQ) and free-response (FRQ) sections. A standard convention on the exam is that payoff matrices list the row player’s payoff first, followed by the column player’s payoff.
2. Measuring Market Concentration ★★☆☆☆ ⏱ 3 min
To classify markets as oligopolistic, economists first measure how much market share is controlled by the largest firms. Two standard measures tested on the AP exam are the 4-firm concentration ratio (CR4) and the Herfindahl-Hirschman Index (HHI).
CR_4 = s_1 + s_2 + s_3 + s_4
The main limitation of CR4 is that it does not distinguish between a market with one very large firm and three small ones, versus a market with four equal large firms, even though these two markets have very different levels of market power.
\text{HHI} = \sum_{i=1}^n s_i^2
The standard AP exam threshold is that HHI above 2500 indicates a highly concentrated (oligopolistic) market.
Exam tip: If market shares are given as decimals (e.g. 0.35 instead of 35), multiply the sum of squared decimals by 10,000 to get the standard percentage-based HHI that matches AP exam thresholds.
3. Dominant Strategy and Nash Equilibrium ★★★☆☆ ⏱ 4 min
Game theory models strategic interactions between players (oligopoly firms) who choose strategies (e.g. set high price, advertise heavily) and receive payoffs (usually profit) based on the combination of all players' choices. Simultaneous-move games, where both players choose at the same time, are most often represented by a payoff matrix.
The best response method is the easiest approach to find these outcomes: for each of the opponent's possible choices, mark the highest payoff for your player. If one strategy is marked for all opponent choices, it is dominant. Any cell where both payoffs are marked is a Nash equilibrium.
Exam tip: Always explicitly mark best responses in your FRQ working. AP graders award partial credit for correct best response calculations even if you get the final equilibrium wrong, as long as your method is correct.
4. Collusion, Cartels, and Prisoner's Dilemma ★★★☆☆ ⏱ 3 min
Collusion is an agreement between oligopoly firms to restrict output and raise price, acting like a single monopolist to maximize total industry profit, then split the profits between members. A formal collusive agreement is called a cartel; tacit collusion is an informal unwritten agreement to coordinate prices. Collusion is illegal in most developed countries, and its inherent instability is a core AP exam topic.
The prisoner's dilemma is a classic game that explains why cartels are almost always unstable. In a prisoner's dilemma, both firms have a dominant strategy to cheat on the collusive agreement (by cutting price or increasing output to capture more market share), leading to an equilibrium where both firms earn lower profit than they would if they both complied. The individual incentive to cheat overwhelms the collective benefit of cooperation, so cartels break down unless they have a way to punish cheating. In repeated games where firms interact over time, strategies like tit-for-tat can sustain collusion over time.
Exam tip: When asked why cartels are unstable, always explicitly mention the individual incentive to cheat on the collusive agreement. This is the core point AP graders look for, not just a generic statement that 'cartels break down.'
5. Sequential Games and Entry Deterrence ★★★★☆ ⏱ 3 min
Sequential games are games where one player moves first, then the second player observes the first move and chooses their own strategy. A common application in oligopoly is entry deterrence, where an incumbent firm (existing in the market) moves first, and a potential entrant decides whether to enter the market after observing the incumbent’s choice.
To solve sequential games, we use backward induction: start from the last mover’s choice, find their optimal decision for every possible first move, then work back to the first mover, who chooses their strategy to maximize their own payoff given the anticipated response of the second mover.
Exam tip: Never solve sequential games starting from the first mover. Working forward ignores the first mover's ability to anticipate the second mover's response, which almost always leads to an incorrect equilibrium. Always use backward induction.
Common Pitfalls
Why: Students forget the standard convention that the row player’s payoff comes first, and scan the matrix left to right without checking.
Why: Students confuse the special case of dominant strategy equilibrium with the general definition of Nash equilibrium.
Why: Some textbooks teach HHI as a decimal between 0 and 1, but AP always uses the percentage-based HHI between 0 and 10,000.
Why: Students confuse inefficiency for firms with inefficiency for society.
Why: Students associate equilibrium with 'optimal' and forget the prisoner’s dilemma example where both are worse off at equilibrium.