N.

Gregory Mankiw

Economics
Principles of

Sixth Edition

10
Externalities
In this chapter,
look for the answers to these questions:
• What is an externality?
• Why do externalities make market outcomes
inefficient?
• What public policies aim to solve the problem of
externalities?
• How can people sometimes solve the problem of
externalities on their own? Why do such private
solutions not always work?

1
 Introduction
▪ One of the Ten Principles from Chapter 1:
Markets are usually a good way
to organize economy activity.
In absence of market failures, the competitive
market outcome is efficient, maximizes total surplus.
▪ One type of market failure:
externality, the uncompensated impact of one
person’s actions on the well-being of a bystander.
▪ Externalities can be negative or positive,
depending on whether impact on bystander is
adverse or beneficial.
2
Introduction
▪ Self-interested buyers and sellers neglect the
external costs or benefits of their actions,
so the market outcome is not efficient.
▪ Another principle from Chapter 1:
Governments can sometimes
improve market outcomes.
In presence of externalities, public policy can
improve efficiency.

3
Examples of Negative Externalities
▪ Air pollution from a factory
▪ The neighbor’s barking dog
▪ Late-night stereo blasting from
the dorm room next to yours
▪ Noise pollution from
construction projects
▪ Health risk to others from
second-hand smoke
▪ Talking on cell phone while driving makes the
roads less safe for others
4
 Recap of Welfare Economics
P The market for gasoline
$5
The market eq’m
maximizes consumer
+ producer surplus.
4
Supply curve shows
3 private cost, the costs
$2.50 directly incurred by sellers.
2
Demand curve shows
private value, the value
1
to buyers (the prices they
are willing to pay).
0
0 10 20 25 30 Q
(gallons) 5
Analysis of a Negative Externality
P The market for gasoline
$5 Social cost
= private + external cost
4 external
cost Supply (private cost)
3 External cost
= value of the
2 negative impact
on bystanders
1 = $1 per gallon
(value of harm
0 from smog,
0 10 20 30 Q greenhouse gases)
(gallons) 6
Analysis of a Negative Externality
P The market for gasoline
The socially
$5 optimal quantity
Social
cost is 20 gallons.
4
S
3 At any Q < 20,
value of additional gas
2 exceeds
At any Qsocial
> 20, cost.
D social cost of the
1 last gallon is
greater than its value
0 to society.
0 10 20 25 30 Q
(gallons) 7
Analysis of a Negative Externality
P The market for gasoline
$5
Social Market eq’m
cost (Q = 25)
4
S is greater than
social optimum
3 (Q = 20).

2 One solution:
D tax sellers
1 $1/gallon,
would shift
0 S curve up $1.
0 10 20 25 30 Q
(gallons) 8
“Internalizing the Externality”
▪ Internalizing the externality: altering incentives
so that people take account of the external effects
of their actions
▪ In our example, the $1/gallon tax on sellers makes
sellers’ costs = social costs.
▪ When market participants must pay social costs,
market eq’m = social optimum.
(Imposing the tax on buyers would achieve the
same outcome; market Q would equal optimal Q.)

9
Examples of Positive Externalities
▪ Being vaccinated against
contagious diseases protects
not only you, but people who
visit the salad bar or produce
section after you.
▪ R&D creates knowledge
others can use.
▪ People going to college raise
the population’s education Thank you for
not contaminating
level, which reduces crime
the fruit supply!
and improves government.
10
Positive Externalities
▪ In the presence of a positive externality,
the social value of a good includes
▪ private value – the direct value to buyers
▪ external benefit – the value of the
positive impact on bystanders

▪ The socially optimal Q maximizes welfare:

▪ At any lower Q, the social value of
additional units exceeds their cost.
▪ At any higher Q, the cost of the last unit
exceeds its social value.
11
ACTIVE LEARNING 1
Analysis of a positive externality
P The market for flu shots
External benefit
$50 = $10/shot

40
▪ Draw the social
value curve.
S
30 ▪ Find the socially
optimal Q.
20 ▪ What policy would
internalize this
10 externality?
D

0 Q
0 10 20 30
ACTIVE LEARNING 1
Answers Socially optimal Q
P The market for flu shots
= 25 shots.
$50
external To internalize the
40 benefit externality, use
subsidy = $10/shot.
S
30

Social value
20 = private value
+ $10 external benefit
10
D

0 Q
0 10 20 25 30
 Effects of Externalities: Summary
If negative externality
▪ market quantity larger than socially desirable
If positive externality
▪ market quantity smaller than socially desirable
To remedy the problem,
“internalize the externality”
▪ tax goods with negative externalities
▪ subsidize goods with positive externalities

14
Public Policies Toward Externalities
Two approaches:
▪ Command-and-control policies regulate
behavior directly. Examples:
▪ limits on quantity of pollution emitted
▪ requirements that firms adopt a particular
technology to reduce emissions
▪ Market-based policies provide incentives so that
private decision-makers will choose to solve the
problem on their own. Examples:
▪ corrective taxes and subsidies
▪ tradable pollution permits
15
Corrective Taxes & Subsidies
▪ Corrective tax: a tax designed to induce private
decision-makers to take account of the social
costs that arise from a negative externality
▪ Also called Pigouvian taxes after Arthur Pigou
(1877-1959).
▪ The ideal corrective tax = external cost
▪ For activities with positive externalities,
ideal corrective subsidy = external benefit

16
Corrective Taxes & Subsidies
▪ Other taxes and subsidies distort incentives and
move economy away from the social optimum.
▪ Corrective taxes & subsidies
▪ align private incentives with society’s interests
▪ make private decision-makers take into account
the external costs and benefits of their actions
▪ move economy toward a more efficient
allocation of resources

17
Corrective Taxes vs. Regulations
▪ Different firms have different costs of pollution
abatement.
▪ Efficient outcome: Firms with the lowest
abatement costs reduce pollution the most.
▪ A pollution tax is efficient:
▪ Firms with low abatement costs will reduce
pollution to reduce their tax burden.
▪ Firms with high abatement costs have greater
willingness to pay tax.
▪ In contrast, a regulation requiring all firms to
reduce pollution by a specific amount not efficient.
18
Corrective Taxes vs. Regulations
Corrective taxes are better for the environment:
▪ The corrective tax gives firms incentive to continue
reducing pollution as long as the cost of doing so
is less than the tax.
▪ If a cleaner technology becomes available,
the tax gives firms an incentive to adopt it.
▪ In contrast, firms have no incentive for further
reduction beyond the level specified in a
regulation.

19
Example of a Corrective Tax: The Gas Tax
The gas tax targets three negative externalities:
▪ Congestion
The more you drive, the more you contribute to
congestion.
▪ Accidents
Larger vehicles cause more damage in an
accident.
▪ Pollution
Burning fossil fuels produces greenhouse gases.

20
ACTIVE LEARNING 2
A. Regulating lower SO2 emissions
▪ Acme and US Electric run coal-burning power plants.
Each emits 40 tons of sulfur dioxide per month,
total emissions = 80 tons/month.
▪ Goal: Reduce SO2 emissions 25%, to 60 tons/month
▪ Cost of reducing emissions:
$100/ton for Acme, $200/ton for USE
Policy option 1: Regulation
Every firm must cut its emissions 25% (10 tons).
Your task: Compute the cost to each firm and
total cost of achieving goal using this policy.
ACTIVE LEARNING 2
A. Answers
▪ Each firm must reduce emissions by 10 tons.
▪ Cost of reducing emissions:
$100/ton for Acme, $200/ton for USE.
▪ Compute cost of achieving goal with this policy:
Cost to Acme: (10 tons) x ($100/ton) = $1000
Cost to USE: (10 tons) x ($200/ton) = $2000
Total cost of achieving goal = $3000
ACTIVE LEARNING 2
B. Tradable pollution permits
▪ Initially, Acme and USE each emit 40 tons SO2/month.
▪ Goal: reduce SO2 emissions to 60 tons/month total.
Policy option 2: Tradable pollution permits
▪ Issue 60 permits, each allows one ton SO2 emissions.
Give 30 permits to each firm.
Establish market for trading permits.
▪ Each firm may use all its permits to emit 30 tons,
may emit < 30 tons and sell leftover permits,
or may purchase extra permits to emit > 30 tons.
Your task: Compute cost of achieving goal if Acme
uses 20 permits and sells 10 to USE for $150 each.
ACTIVE LEARNING 2
B. Answers
▪ Goal: reduce emissions from 80 to 60 tons
▪ Cost of reducing emissions:
$100/ton for Acme, $200/ton for USE.
Compute cost of achieving goal:
Acme
▪ sells 10 permits to USE for $150 each, gets $1500
▪ uses 20 permits, emits 20 tons SO2
▪ spends $2000 to reduce emissions by 20 tons
▪ net cost to Acme: $2000 − $1500 = $500
continued…
ACTIVE LEARNING 2
B. Answers, continued
▪ Goal: reduce emissions from 80 to 60 tons
▪ Cost of reducing emissions:
$100/ton for Acme, $200/ton for USE.
USE
▪ buys 10 permits from Acme, spends $1500
▪ uses these 10 plus original 30 permits, emits 40 tons
▪ spends nothing on abatement
▪ net cost to USE = $1500
Total cost of achieving goal = $500 + $1500 = $2000
Using tradable permits, goal is achieved at lower total
cost and lower cost to each firm than using regulation.
Tradable Pollution Permits
▪ A tradable pollution permits system reduces
pollution at lower cost than regulation.
▪ Firms with low cost of reducing pollution
do so and sell their unused permits.
▪ Firms with high cost of reducing pollution
buy permits.
▪ Result: Pollution reduction is concentrated
among those firms with lowest costs.

26
Tradable Pollution Permits
in the Real World
▪ SO2 permits traded in the U.S. since 1995.
▪ Nitrogen oxide permits traded in the northeastern
U.S. since 1999.
▪ Carbon emissions permits traded in Europe since
January 1, 2005.

27
Corrective Taxes vs.
Tradable Pollution Permits
▪ Like most demand curves, firms’ demand for the
ability to pollute is a downward-sloping function of
the “price” of polluting.
▪ A corrective tax raises this price and thus
reduces the quantity of pollution firms demand.
▪ A tradable permits system restricts the supply of
pollution rights, has the same effect as the tax.
▪ When policymakers do not know the position of
this demand curve, the permits system achieves
pollution reduction targets more precisely.
28
Objections to the
Economic Analysis of Pollution
▪ Some politicians, many environmentalists argue
that no one should be able to “buy” the right to
pollute, cannot put a price on the environment.
▪ However, people face tradeoffs. The value of
clean air and water must be compared to their
cost.
▪ The market-based approach reduces the cost of
environmental protection, so it should increase the
public’s demand for a clean environment.

29
Private Solutions to Externalities
Types of private solutions:
▪ Moral codes and social sanctions,
e.g., the “Golden Rule”
▪ Charities, e.g., the Sierra Club
▪ Contracts between market participants and the
affected bystanders

30
Private Solutions to Externalities
▪ The Coase theorem:
If private parties can costlessly bargain over the
allocation of resources, they can solve the
externalities problem on their own.

31
The Coase Theorem: An Example
Dick owns a dog named Spot.
Negative externality:
Spot’s barking disturbs Jane,
Dick’s neighbor.
The socially efficient outcome
maximizes Dick’s + Jane’s well-being.
▪ If Dick values having Spot more See Spot bark.
than Jane values peace and quiet,
the dog should stay.
Coase theorem: The private market will reach the
efficient outcome on its own…
32
The Coase Theorem: An Example
▪ CASE 1:
Dick has the right to keep Spot.
Benefit to Dick of having Spot = $500
Cost to Jane of Spot’s barking = $800
▪ Socially efficient outcome:
Spot goes bye-bye.
▪ Private outcome:
Jane pays Dick $600 to get rid of Spot,
both Jane and Dick are better off.
▪ Private outcome = efficient outcome.

33
The Coase Theorem: An Example
▪ CASE 2:
Dick has the right to keep Spot.
Benefit to Dick of having Spot = $1000
Cost to Jane of Spot’s barking = $800
▪ Socially efficient outcome:
See Spot stay.
▪ Private outcome:
Jane not willing to pay more than $800,
Dick not willing to accept less than $1000,
so Spot stays.
▪ Private outcome = efficient outcome.
34
The Coase Theorem: An Example
▪ CASE 3:
Jane has the legal right to peace and quiet.
Benefit to Dick of having Spot = $800
Cost to Jane of Spot’s barking = $500
▪ Socially efficient outcome: Dick keeps Spot.
▪ Private outcome: Dick pays Jane $600 to put up
with Spot’s barking.
▪ Private outcome = efficient outcome.

The private market achieves the efficient outcome

regardless of the initial distribution of rights.
35
ACTIVE LEARNING 3
Applying Coase
Collectively, the 1000 residents of Green Valley
value swimming in Blue Lake at $100,000.
A nearby factory pollutes the lake water, and would
have to pay $50,000 for non-polluting equipment.
A. Describe a Coase-like private solution.

B. Can you think of any reasons why this

solution might not work in the real world?
Why Private Solutions Do Not Always Work
1. Transaction costs:
The costs parties incur in the process of
agreeing to and following through on a bargain.
These costs may make it impossible to reach a
mutually beneficial agreement.
2. Stubbornness:
Even if a beneficial agreement is possible,
each party may hold out for a better deal.
3. Coordination problems:
If # of parties is very large, coordinating them
may be costly, difficult, or impossible.
37
S U MMA RY

• An externality occurs when a market transaction

affects a third party. If the transaction yields
negative externalities (e.g., pollution), the market
quantity exceeds the socially optimal quantity.
If the externality is positive (e.g., technology
spillovers), the market quantity falls short of the
social optimum.
S U MMA RY

• Sometimes, people can solve externalities on

their own. The Coase theorem states that the
private market can reach the socially optimal
allocation of resources as long as people can
bargain without cost. In practice, bargaining is
often costly or difficult, and the Coase theorem
does not apply.
S U MMA RY

• The government can attempt to remedy the

problem. It can internalize the externality using
corrective taxes. It can issue permits to polluters
and establish a market where permits can be
traded. Such policies often protect the
environment at a lower cost to society than
direct regulation.