Chapter 4: The Market Forces of Supply and Demand

How price and quantity get determined in every competitive market

Section 1

Markets and Competition

A market is a group of buyers and sellers of a particular good or service. Markets take many forms — some are highly organized (the stock market), others are informal (the ice cream vendors at a summer festival).

In this chapter we focus on the competitive market: many buyers, many sellers, and a single going price that nobody can move on their own. Think "ice cream cones on a hot beach" or "wheat at the Chicago grain exchange." Each individual trader is a price taker.

Perfectly Competitive

Textbook ideal

Identical product, so many buyers and sellers that no one has market power. The model of this chapter.

Other market forms

Monopoly & between

Later chapters cover monopoly (1 seller), oligopoly (a few), and monopolistic competition (many with differentiated products).

Why start here? Supply and demand is the single most important tool in economics. It explains prices, shortages, surpluses, and how markets respond to shocks — from gas prices to Taylor Swift tickets.

Section 2

Demand

The quantity demanded is how much of a good buyers are willing and able to purchase. The law of demand is a nearly universal empirical fact:

Law of demand   ⇒   when P ↑, Qd ↓ (all else equal)

Higher price, lower quantity demanded. "All else equal" (ceteris paribus) is doing a lot of work — it's what lets us plot a demand curve in the first place.

Demand Schedule & Demand Curve

Catherine's weekly demand for ice cream cones:

Price ($/cone)Quantity demanded
$0.0012
$0.5010
$1.008
$1.506
$2.004
$2.502
$3.000

Plot P on the vertical axis, Q on the horizontal axis, connect the dots — that's the demand curve.

Market Demand vs. Individual Demand

The market demand curve is the horizontal sum of every individual buyer's demand. At each price, add up how much each person would buy. Lots of individual downward-sloping curves → one big downward-sloping market curve.

Section 3

Shifts in the Demand Curve

A change in the good's own price causes a movement along the demand curve — that's a change in quantity demanded. But anything else that affects buyers shifts the entire curve — that's a change in demand.

The single most-tested distinction in Chapter 4: a change in own price is a movement along the curve. Everything else is a shift of the curve.

Five Things That Shift Demand

Income Normal good: Y ↑ → D shifts right inferior good: opposite
Related prices Substitute price ↑ → D shifts right complement price ↑ → D shifts left
Tastes Prefer it more → D shifts right fads, ads, health scares
Expectations Expect P ↑ tomorrow → D ↑ today buy now to beat the hike
Number of buyers More buyers → D shifts right market demand = sum
Normal good

Income ↑ → Demand ↑

Most goods: steak, concerts, new cars. When you earn more, you buy more.

Inferior good

Income ↑ → Demand ↓

Ramen, bus rides, store-brand cereal. Once you have more money, you switch away.

Substitutes

Pepsi & Coke

Pepsi price ↑ → people switch to Coke → DCoke shifts right.

Complements

Peanut butter & jelly

PB price ↑ → PB&J sandwiches are pricier overall → Djelly shifts left.

A rightward shift (D₁ → D₂) means "more demanded at every price." A leftward shift (D₁ → D₀) means the opposite.

Change in Demand vs. Change in Quantity Demanded

CauseWhat happens on the graph?Name
The good's own price changes Slide along the existing demand curve Change in quantity demanded
Anything else (income, tastes, expectations, related prices, # of buyers) Whole curve shifts left or right Change in demand

Section 4

Supply

The quantity supplied is how much of a good sellers are willing and able to sell. The law of supply:

Law of supply   ⇒   when P ↑, Qs ↑ (all else equal)

Higher price → higher profit margin → producers expand output (and new producers jump in). The supply curve slopes upward.

Five Things That Shift Supply

Input prices Costs ↑ → S shifts left wages, materials, energy
Technology Better tech → S shifts right produces more per $
Expectations Expect P ↑ tomorrow → S ↓ today hold inventory, sell later
Number of sellers More sellers → S shifts right entry expands market supply
Natural events Hurricane, pest → S shifts left Mankiw groups these with inputs/tech

Same logic as demand: own-price change → movement along; everything else → shift.

Section 5

Equilibrium: Where Supply Meets Demand

Put the two curves on the same graph. The single point where they cross is called the market equilibrium. At that intersection:

Equilibrium price P*   where Qs = Qd "market-clearing price"
Equilibrium quantity Q*   the amount traded at P*

Surplus and Shortage

What happens at any price other than P*?

Surplus (P > P*)

Qs > Qd

Too many unsold units. Sellers cut price. Price falls back toward P*.

Shortage (P < P*)

Qd > Qs

Empty shelves, long lines. Sellers raise price. Price rises back toward P*.

Law of supply and demand: price adjusts to bring quantity supplied and quantity demanded into balance. Surpluses push prices down, shortages push prices up — until Qs = Qd.

Section 6

The Three-Step Method

Mankiw's three-step recipe for analyzing any change in a market. Memorize this — it's the framework for every supply-and-demand question in the course:

Step 1 Does the event shift D, S, or both?
Step 2 Which direction — left or right?
Step 3 Compare new equilibrium to old: what happens to P*, Q*?

Worked Example: A Hot Summer

Hot summer hits. What happens in the ice cream market?

Step 1. Hot weather makes people want ice cream more — it affects tastes, a demand shifter.  → Shifts demand, not supply.
Step 2. People want more at every price → demand shifts right.
Step 3. Along the unchanged supply curve, a rightward demand shift raises both P* and Q*. Ice cream gets more expensive and more of it gets sold.

Demand shifts right (D₁ → D₂). Equilibrium moves along the fixed supply curve: P* rises, Q* rises.

All Four Pure Cases

When only one curve shifts, the effect on P and Q is unambiguous:

Demand ↑ (shifts right)

P ↑,   Q ↑

Demand ↓ (shifts left)

P ↓,   Q ↓

Supply ↑ (shifts right)

P ↓,   Q ↑

Supply ↓ (shifts left)

P ↑,   Q ↓

When Both Curves Shift

If D and S shift at the same time, one of the two equilibrium changes is ambiguous until you know the relative sizes:

ShiftP*Q*
D ↑ & S ↑ambiguousQ ↑
D ↓ & S ↓ambiguousQ ↓
D ↑ & S ↓P ↑ambiguous
D ↓ & S ↑P ↓ambiguous
Rule of thumb: if both curves move in the same direction (both up or both down), quantity is definite but price depends on which shift is bigger. If they move in opposite directions, price is definite and quantity is the unknown.

Key Takeaways

The Big Ideas in Chapter 4

  • Law of demand: P ↑ → Qd ↓. Demand curve slopes down.
  • Law of supply: P ↑ → Qs ↑. Supply curve slopes up.
  • Own-price change = movement along. Anything else = shift of the curve. This is the most-tested distinction in the chapter.
  • Demand shifts come from income (normal vs. inferior), prices of substitutes/complements, tastes, expectations, and number of buyers.
  • Supply shifts come from input prices, technology, expectations, number of sellers, and natural shocks.
  • Equilibrium: P* and Q* where S = D. Above P* → surplus (P falls). Below P* → shortage (P rises).
  • Three steps: (1) Which curve shifts? (2) Which direction? (3) Read new P*, Q* off the graph.