Topic 4.1 Notes – Introduction to Imperfectly Competitive Markets

1. What Imperfectly Competitive Markets Are

You already know perfect competition: many firms, identical products, price takers, and in long-run equilibrium we get P = MR = MC, which means allocative efficiency.

Imperfect competition happens when one or more of those assumptions breaks.

Core characteristics

• Fewer firms (sometimes one, sometimes a few)
• Market power → firms are price makers
• Barriers to entry
• Often product differentiation
• P > MC at the profit-maximizing output → inefficiency

Here are the four market structures you need to recognize:

• Monopoly
  • One seller, unique product
  • Examples: local utility companies, DeBeers diamonds, patented drugs, Windows operating system.
• Oligopoly
  • A few large firms (often fewer than 10)
  • Examples: airlines, cereal companies, automobile manufacturers, cell phone carriers, cable TV.
• Monopolistic competition
  • Many firms, but differentiated products
  • Examples: restaurants, clothing brands, hairdressers, makeup companies.
• Monopsony (factor market)
  • One buyer of a resource
  • Example: a single large employer in a small town.

All of them share one key trait: the firm faces a downward-sloping demand curve.

2. Price Makers and the Downward-Sloping Demand Curve

In perfect competition, the firm’s demand curve is perfectly elastic. Here, it isn’t.

If an imperfectly competitive firm wants to sell more output, it must lower its price.

That means:

• Demand is downward sloping
• Marginal revenue (MR) is below demand
• P ≠ MR

Here’s what that looks like on a monopoly-style graph.

Monopoly profit maximization: MR = MC, price from demand

Why is MR below demand?

When the firm lowers price to sell one more unit:

• It gains revenue from that extra unit
• It loses revenue on all previous units because price fell

So MR < P. On the graph, that’s why the marginal revenue curve lies below the demand curve at every positive quantity.

The profit rule stays the same:

$$\textbf{MR = MC}$$

In the graph, that happens where the MR and MC curves intersect. The firm then goes up to the demand curve to find the price it can charge for that quantity.

Students constantly write “produce where P = MC” for monopolies. That is wrong. Produce where MR = MC, charge the demand-curve price.

Fun fact you may hear about but don’t need for AP: the Lerner Index measures market power using how much P exceeds MC.

3. Inefficiency and Deadweight Loss

In imperfect competition, at the profit-maximizing quantity:

• MR = MC
• But since MR < P → P > MC

That gap is the problem.

Allocative efficiency happens when:

$$P = MC$$

If P > MC, society values the good more than it costs to produce, but the firm is restricting output to keep price high.

Result:

• Output is too low
• Price is too high
• Some mutually beneficial trades do not happen
• We get deadweight loss (DWL)

The graph below shows the monopoly outcome compared to the efficient outcome. Notice that the firm produces at $Q^M$ where MR = MC, charges $P^M$ from the demand curve, and produces less than the efficient quantity $Q^*$ where demand intersects MC. The blue triangle between the demand and MC curves from $Q^M$ to $Q^*$ is the deadweight loss.

Monopoly deadweight loss

AP-style explanation you should be ready to write in one clean sentence:
“At the monopoly quantity, price exceeds marginal cost, so the market is allocatively inefficient.”

4. Barriers to Entry and Why They Matter

If firms earn economic profit, why don’t new firms enter?

Because of barriers to entry, which protect market power.

Major barriers

• High fixed/start-up costs
  Airlines need planes, pilots, licenses. Utilities need infrastructure.
• Economies of scale
  Large firms produce at lower average cost. This creates natural monopolies, like electricity grids.
• Legal barriers
  Patents, copyrights, licenses, government franchises.
• Exclusive ownership of key resources
  Control of raw materials or geographic advantages.
• Brand loyalty / reputation (non-price competition)
  Advertising builds customer loyalty, making entry harder.

Barriers reduce entry, limit competition, and allow firms to earn long-run economic profit (except monopolistic competition, which breaks even in the long run).

Big Picture Comparison

Market structure shapes behavior. Even though every firm wants to maximize profit, the structure determines whether that leads to efficiency or inefficiency.