Chapter 20 — Key Takeaways
Monopolistic competition
Four features: many firms, differentiated products, free entry/exit, some market power.
Short run: looks like monopoly (MR = MC, P > MC, positive profit possible). Long run: looks like competition (entry drives profit to zero; P = ATC). But unlike perfect competition, P > MC and the firm has excess capacity.
The trade-off: variety (many differentiated products) vs. efficiency (no firm produces at minimum ATC). Most consumers prefer the variety.
Example: Millbrook's 60 restaurants — each is unique, entry is easy, no one earns above-normal returns for long.
Oligopoly
Defining feature: few firms, significant barriers to entry, and strategic interaction — each firm's best move depends on what competitors do.
Game theory is the analytical tool. The prisoner's dilemma captures the central tension: each firm's individual incentive is to defect (cut prices, cheat on a cartel), even though all firms would be better off cooperating (keeping prices high).
Nash equilibrium: a set of strategies where no player can improve by changing unilaterally. In the prisoner's dilemma, (defect, defect) is the Nash equilibrium — collectively worse than (cooperate, cooperate).
Cartels are agreements to cooperate. They face the prisoner's dilemma: each member has an incentive to cheat. OPEC is the most famous real cartel — partially successful but plagued by chronic cheating.
The four market structures compared
| Perfect competition | Monopolistic competition | Oligopoly | Monopoly | |
|---|---|---|---|---|
| Firms | Very many | Many | Few | One |
| Products | Identical | Differentiated | Either | Unique |
| Entry | Free | Free | Barriers | Very high barriers |
| Market power | None | Some | Significant | Maximum |
| Long-run profit | Zero | Zero | Possible | Persistent |
| Key tool | S&D | MR=MC + tangency | Game theory | MR=MC monopoly |
Themes this chapter touched
- Markets power+imperfect — most real markets are in the messy middle
- Tradeoffs — variety vs. efficiency; cooperation vs. defection
- Behavioral — brand loyalty, advertising
- Affects daily life — restaurants, airlines, wireless, soft drinks
One sentence summary
Most real markets live in the messy middle between perfect competition and monopoly — monopolistic competition produces variety at the cost of efficiency, while oligopoly produces strategic interaction where the prisoner's dilemma explains why cartels form and why they break down.