Part IV — Firm Behavior and Market Structure
Markets are populated by firms — and how a firm behaves depends on what kind of market it operates in.
Part II's supply curve was a black box. The supply curve told you that firms collectively were willing to provide more of a good as the price rose, but it didn't tell you why, or how a particular firm decided how much to produce, or what difference it made whether a market had one firm or ten thousand. Part IV opens that black box. Five chapters take you from the most basic question (what does it cost a firm to produce things?) to the most realistic question (what does competition look like in a real market with a handful of firms watching each other?).
By the end of Part IV, you should be able to analyze any firm's decisions — Riverside Foods, the campus coffee shop, the Walden County hospital, OPEC, Google — using the same toolkit. You should understand why some markets settle into a stable equilibrium with many firms and zero economic profit while others tip into one-firm dominance with persistent monopoly power. And you should understand the labor market — the most consequential market in the typical reader's life — well enough to evaluate the contemporary debates about minimum wage, automation, and the gig economy.
Chapter 17 — The Costs of Production opens the firm's books. Total cost, fixed cost, variable cost, average cost, marginal cost — and the reasons each cost curve has the shape it does. The chapter introduces diminishing returns and economies of scale, the two ideas that make sense of why firm cost structures look the way they do. The Riverside Foods plant in Millbrook is the running example. Sunk costs are sunk; the chapter is firm about that.
Chapter 18 — Perfect Competition introduces the benchmark model. Many firms, identical products, free entry and exit. The competitive firm as a price taker. Profit maximization where marginal revenue equals marginal cost. Why long-run economic profit is zero — and why that isn't as bad as it sounds. The chapter is honest that no real market is perfectly competitive, and explains why the model is still useful as a benchmark to compare real markets against.
Chapter 19 — Monopoly walks through the four sources of monopoly power, the monopolist's profit-maximizing decision, the deadweight loss from monopoly pricing, and the practice of price discrimination (with concrete examples from airlines, software, and student discounts). The chapter ends with the contemporary tech monopoly debate: are Google, Amazon, Apple, and Meta actually monopolies in the legal sense? The answer is contested; the chapter walks through arguments on both sides.
Chapter 20 — Monopolistic Competition and Oligopoly covers the messy middle of market structure where most real markets actually live. Monopolistic competition (many firms, differentiated products): the Millbrook restaurant scene. Oligopoly (few firms, strategic interaction): airlines, wireless carriers, soft drinks. The chapter introduces game theory just enough to show why cartels are unstable even when they would benefit all members — the prisoner's dilemma at scale, with OPEC as a real-world example of partial success against the dilemma.
Chapter 21 — Labor Markets is the longest chapter in Part IV and the deepest treatment of the labor market in the book. The chapter builds labor demand from marginal revenue product, labor supply from the work-leisure tradeoff, and brings together everything you have learned in Chapters 5–10 to give the minimum wage debate its full treatment. Then automation and AI: the empirical evidence is more nuanced than the headlines, and the gig economy is a regulatory puzzle still being worked out.
When you finish Part IV, you have completed the microeconomics half of the book. If you are taking Econ 101, you are done with the textbook content. If you are continuing to macroeconomics, Part V begins with the question: what is "the economy" and how do we measure it?