23.01.11 · economics / market-structures

Oligopoly and Monopolistic Competition

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Anchor (Master): Mas-Colell, Whinston & Green, Microeconomic Theory; Tirole, Theory of Industrial Organization

Intuition [Beginner]

Most real-world markets are neither perfectly competitive nor pure monopolies. They fall somewhere in between. Two intermediate market structures cover this terrain:

Monopolistic competition describes markets with many firms selling products that are similar but not identical. Think of restaurants, clothing brands, or coffee shops. Each firm has a small amount of market power because its product is slightly different, but there are enough competitors that no firm can dominate. In the long run, entry and entry drive economic profit to zero -- just like perfect competition -- but firms charge above marginal cost because products are differentiated.

Oligopoly describes markets dominated by a small number of large firms. Think of airlines, automobile manufacturers, or mobile carriers. Each firm must consider how its rivals will react to its decisions. If one airline cuts prices, the others will match. If one raises prices, the others may not follow. This strategic interdependence is the defining feature of oligopoly and makes it the most complex market structure to analyze.

Visual [Beginner]

  Market Structure Spectrum
  (arranged by market power, left to right)

  Perfect      Monopolistic      Oligopoly         Monopoly
  Competition  Competition
  
  Many firms   Many firms        Few firms          One firm
  Identical    Differentiated    Interdependent     Unique product
  product      products          decisions
  P = MC       P > MC            P > MC             P >> MC
  Zero eco    Zero eco profit    May earn           Earns
  profit       (long run)        eco profit         eco profit

  Examples:    Restaurants,      Airlines,          Utilities,
  Agriculture  Clothing          Soft drinks        Patented drugs

Worked Example [Beginner]

Oligopoly: The Prisoner's Dilemma in Pricing

Two gas stations sit across the street from each other. Each can set a high price ($5/gallon) or a low price ($4/gallon). Their daily profits (in dollars) depend on both firms' choices:

                    Station B
                 High Price  Low Price
 Station A  HP |   800, 800 |  400, 1000 |
           LP |  1000, 400 |   600, 600  |

If both charge high prices, each earns $800. If one cuts price while the other holds high, the discounter captures more customers and earns $1000 while the other gets only $400. If both cut prices, they earn $600 each.

The dominant strategy for each firm is to charge a low price (regardless of what the other does, low price yields more profit). Yet both would be better off if they could cooperate and charge high prices. This is the prisoner's dilemma of oligopoly pricing: rational individual decisions lead to a collectively suboptimal outcome.

Monopolistic Competition: Short Run vs. Long Run

A coffee shop in a college town faces demand $P = 10 - Q$ for its specialty lattes, with costs $T C = 4 Q + 2$ .

Short run: The shop maximizes profit where $M R = M C$ . $M R = 10 - 2 Q$ and $M C = 4$ , giving $Q = 3$ and $P = $7$ . Profit = $T R - T C = 21 - 14 = $7$ .

Long run: The positive profit attracts new coffee shops. Demand for this particular shop shifts inward (fewer customers per shop) until profit is zero. In long-run monopolistic competition, firms earn zero economic profit but charge a price above marginal cost, and operate with excess capacity (not at minimum ATC).

Check Your Understanding [Beginner]

Formal Definition [Intermediate+]

Monopolistic Competition (Chamberlin, 1933)

A monopolistically competitive market has the following properties:

Many firms: Each firm's actions have negligible effects on other firms.
Differentiated products: Each firm sells a product that is a close but imperfect substitute for competitors' products.
Free entry and exit: No barriers to entry or exit.

Each firm faces a downward-sloping demand curve. The firm maximizes:

$q_{i} max P_{i} (q_{i}) \cdot q_{i} - C (q_{i})$

Short-run equilibrium: $M R_{i} = M C_{i}$ (same as monopoly).

Long-run equilibrium: Entry occurs until profit is zero:

$P_{i} = A T C_{i} and M R_{i} = M C_{i}$

The zero-profit condition holds because the demand curve is tangent to the ATC curve. This implies the firm produces less than the output that minimizes ATC -- a feature called excess capacity.

Oligopoly: Game-Theoretic Models

Cournot Model (1838): Firms choose quantities simultaneously. With $n$ identical firms facing inverse demand $P (Q)$ and cost $C (q_{i})$ :

$q_{i} max P q_{i} + j \neq = i \sum q_{j} \cdot q_{i} - C (q_{i})$

The first-order condition for each firm gives a best-response function. The Cournot-Nash equilibrium is where all best responses intersect. As $n \to \infty$ , the Cournot outcome converges to the competitive outcome ( $P = M C$ ).

Bertrand Model (1883): Firms choose prices simultaneously. With homogeneous products and constant marginal cost $c$ , the unique Nash equilibrium is $P_{1} = P_{2} = c$ -- the competitive outcome, even with only two firms. This is the Bertrand paradox: two firms are enough for perfect competition.

Stackelberg Model (1934): A leader firm moves first, the follower observes and then moves. The leader produces more than the Cournot quantity and earns more profit. Solved by backward induction: first solve the follower's best response, then the leader optimizes given that response.

Kinked Demand Curve Model (Sweezy, 1939): Firms expect rivals to match price cuts but not price increases. This creates a kink in the demand curve at the current price, and a discontinuity in the MR curve. Marginal cost can change within this gap without changing the price -- explaining price rigidity in oligopolistic markets.

Contestable Markets (Baumol, 1982)

Even a market with only one or a few firms may behave competitively if entry and exit are costless ("hit-and-run" entry). The threat of entry disciplines incumbents. The number of firms is less important than the barriers to entry.

Key Concepts [Intermediate+]

Strategic interdependence: In oligopoly, each firm's optimal strategy depends on rivals' actions, requiring game-theoretic analysis.
Nash equilibrium: A set of strategies where no player can benefit by unilaterally changing their strategy, given the other players' strategies.
Cartel: A group of firms that explicitly coordinate output and pricing to act as a collective monopolist (e.g., OPEC). Illegal in most jurisdictions.
Tacit collusion: Firms achieve collusive outcomes without explicit communication, often through price leadership or repeated interaction.
Excess capacity: In monopolistic competition, firms produce below minimum ATC in the long run, meaning they could produce more at lower average cost.
Product differentiation: Horizontal (different varieties for different tastes) vs. vertical (clear quality rankings). Both create market power.

Exercise 1. Two firms (duopoly) face demand $P = 100 - Q$ where $Q = q_{1} + q_{2}$ . Each has $M C = 10$ and no fixed costs. Find the Cournot-Nash equilibrium quantities, price, and profit per firm. Compare total output with the monopoly and competitive outcomes.

Exercise 2. In a monopolistically competitive market, explain why the tangency solution (demand tangent to ATC) implies that the firm produces on the downward-sloping portion of the ATC curve. What does this mean for productive efficiency?

Academic Perspectives [Master]

Neoclassical / Industrial Organization

The formal theory of oligopoly is grounded in game theory, following Nash (1950) and the extensive development of industrial organization by Tirole (1988). The Structure-Conduct-Performance (SCP) paradigm (Bain, 1956) linked market concentration to firm conduct (pricing, advertising, R&D) to economic performance (profits, efficiency). The "new empirical industrial organization" (Bresnahan, 1989) uses structural econometric models to estimate market power directly from firm-level data.

The Cournot model predicts that price-cost margins decline as the number of firms increases:

$\frac{P - M C}{P} = \frac{H}{ϵ _{d}}$

where $H$ is the Herfindahl-Hirschman Index (sum of squared market shares) and $ϵ_{d}$ is market demand elasticity. This provides a direct link between market concentration and market power.

Critique: The Bertrand model shows that even duopoly can produce competitive outcomes with homogeneous products. The predicted outcome is highly sensitive to modeling assumptions (simultaneous vs. sequential, quantity vs. price, one-shot vs. repeated).

Austrian School

Austrian economists reject the equilibrium focus of oligopoly theory. Kirzner (1973) argued that the real question is not what equilibrium results from strategic interaction but how the competitive process discovers opportunities. From this perspective, the distinction between market structures is less important than whether government barriers prevent entry.

Critique: The Austrian dismissal of formal modeling provides few testable predictions about when and how market power will be exercised.

Marxian Perspective

Marxian economists view oligopoly as the natural endpoint of capitalist competition. The tendency toward concentration was a central theme in Marx's analysis of capital accumulation. Baran and Sweezy (1966) argued that monopoly capital tends to generate a surplus that cannot be profitably absorbed, leading to stagnation -- a dynamic not captured by the neoclassical focus on static deadweight loss.

Critique: The stagnation thesis has not been consistently borne out. Innovation, globalization, and new industries have created investment opportunities that Baran and Sweezy did not anticipate.

Institutional Economics

Coase (1937) and Williamson (1985) emphasize that oligopolistic firms may use vertical integration, exclusive contracts, and other organizational forms as substitutes for market transactions. The boundaries of the firm are endogenous and depend on transaction costs, not just the number of competitors. Network industries (platforms, two-sided markets) pose particular challenges: the winner-take-all dynamics of network effects may naturally produce oligopoly or monopoly, requiring new regulatory frameworks.

Behavioral and Experimental Economics

Experimental evidence (Holt, 1985; Abbink and Brandts, 2009) shows that real oligopolistic behavior deviates from Nash equilibrium predictions. Firms often collude more than theory predicts in repeated settings but fail to coordinate in complex environments. Behavioral IO (Ellison, 2006) incorporates bounded rationality, fairness concerns, and other behavioral factors into oligopoly models.

Historical Context [Master]

Cournot (1838): Antoine Augustin Cournot developed the first formal model of oligopoly, showing how firms choosing quantities would reach an equilibrium. The work was largely ignored for decades.
Bertrand (1883): Joseph Bertrand critiqued Cournot by showing that if firms choose prices instead of quantities, the outcome is competitive even with only two firms. The Cournot-Bertrand debate remains central to industrial organization.
Chamberlin and Robinson (1933): Edward Chamberlin's Monopolistic Competition and Joan Robinson's Economics of Imperfect Competition independently developed the theory of markets between monopoly and competition, introducing product differentiation and the concept of excess capacity.
von Neumann and Morgenstern (1944): Theory of Games and Economic Behavior provided the mathematical framework that would revolutionize oligopoly theory.
Nash (1950): John Nash's equilibrium concept gave game theory its central solution concept, which became the standard tool for analyzing strategic interaction in oligopolies.
Bain (1956): Joe Bain's Barriers to New Competition launched the SCP paradigm, linking market structure to performance and providing the intellectual basis for antitrust enforcement.
Baumol (1982): William Baumol's contestable markets theory challenged the focus on market structure, arguing that the threat of entry is more important than the number of incumbents.
Tirole (1988): Jean Tirole's The Theory of Industrial Organization synthesized game theory and industrial organization, earning him the Nobel Prize in 2014.

Bibliography [Master]

Abbink, K., & Brandts, J. (2009). Collusion through communication: The role of the information structure. European Economic Review, 53(1), 33-51.
Bain, J. S. (1956). Barriers to New Competition. Harvard University Press.
Baran, P. A., & Sweezy, P. M. (1966). Monopoly Capital. Monthly Review Press.
Baumol, W. J. (1982). Contestable markets: An uprising in the theory of industry structure. American Economic Review, 72(1), 1-15.
Bresnahan, T. F. (1989). Empirical studies of industries with market power. In R. Schmalensee & R. D. Willig (Eds.), Handbook of Industrial Organization, Vol. 2. Elsevier.
Chamberlin, E. H. (1933). The Theory of Monopolistic Competition. Harvard University Press.
Ellison, G. (2006). Bounded rationality in industrial organization. In R. Blundell, W. K. Newey, & T. Persson (Eds.), Advances in Economics and Econometrics. Cambridge University Press.
Holt, C. A. (1985). An experimental test of the consistent-conjectures hypothesis. American Economic Review, 75(3), 314-325.
Mas-Colell, A., Whinston, M. D., & Green, J. R. (1995). Microeconomic Theory. Oxford University Press. Chapter 12.
Nash, J. F. (1950). Equilibrium points in n-person games. Proceedings of the National Academy of Sciences, 36(1), 48-49.
Robinson, J. (1933). The Economics of Imperfect Competition. Macmillan.
Sweezy, P. M. (1939). Demand under conditions of oligopoly. Journal of Political Economy, 47(4), 568-573.
Tirole, J. (1988). The Theory of Industrial Organization. MIT Press.