2. Negative Externalities & Efficiency

Based on Dr. Said's Figure 5 - Paper Industry Example

The Setup

Suppose paper production generates pollution (waste into rivers/lakes). The paper industry operates under perfect competition. There are no positive externalities in production or consumption of paper.

Supply curve (S1) = MPC (Marginal Private Cost) - what producers actually pay
Demand curve (D) = MPB = MSB (since no external benefit, private = social benefit)
MSC = MPC + MEC - the TRUE cost to society (private cost + pollution damage)

Interactive Chart: The Problem

DWL Triangle MEC Gap

MEC Value: $10

Step-by-Step Explanation

Step 1: Why is E1 the Market Equilibrium?

In a free market, producers make decisions based on their private costs (MPC) and the market price. They don't consider the pollution damage to fishermen, swimmers, or downstream communities.

The market equilibrium is where Supply = Demand, i.e., MPC = MPB at point E1 (quantity q1, price P1).

$\text{At E1: } MPC = MPB \text{ (producers' cost = consumers' benefit)}$

Step 2: Why is E1 NOT Efficient?

E1 is NOT efficient because it ignores the external cost (MEC). At E1:

$MSC = MPC + MEC > MPC = MPB = MSB$

This means: the true cost to society of each unit between q2 and q1 is greater than the benefit. Society is worse off for each of these extra units produced.

Intuition: Imagine each ton of paper costs $50 to produce (MPC) but also causes $10 of river damage (MEC). The true cost is $60 (MSC). If consumers only value it at $55 (MSB), then producing it destroys $5 of value - but the market produces it anyway because the producer only sees the $50 cost!

Step 3: Why is E2 the Efficient Point?

The efficient point E2 is where MSC = MSB (at quantity q2, price P2).

At this point, the marginal benefit to ALL of society exactly equals the marginal cost to ALL of society (including pollution damage). No reallocation of resources could make anyone better off without making someone worse off.

The efficient output q2 is less than q1 - the market overproduces when negative externalities exist.

Step 4: What Does the DWL Triangle Mean?

The triangle E1-E2-E3 represents the Deadweight Loss (DWL) - the total efficiency loss to society from overproduction.

For each unit between q2 and q1: $MSC > MSB$, meaning each unit costs society more than it benefits.

$DWL = \frac{1}{2} \times (q1 - q2) \times (MSC_{q1} - MSB_{q1})$

This DWL can become efficiency gains if output decreases from q1 to q2.

Step 5: Why Does the Market Overproduce?

The market overproduces because producers only see MPC (their private cost), not MSC (the true social cost including pollution). They keep producing as long as MPC < market price, ignoring the damage to third parties.

The gap between q1 and q2 represents the overproduction: units that should NOT be produced because their social cost exceeds their social benefit.

Khan Academy: Negative Externalities Economics Help: Negative Externalities