Negative Externalities

  • Negative externalities occur when the consumption or production of a good causes a harmful effect to a third party.

Examples of negative externalities

  • If you play loud music at night your neighbour may not be able to sleep.
  • If you produce chemicals and cause pollution as a side effect, then local fishermen will not be able to catch fish. This loss of income will be the negative externality.
  • If you drive a car, it creates air pollution and contributes to congestion. These are both external costs imposed on other people who live in the city.
  • If you build a new road, the external cost is the loss of a beautiful landscape which people can no longer enjoy.

Social cost

  • With a negative externality the Social Cost > Private Cost

Diagram of negative externality with deadweight welfare loss


  • In a free market people ignore the external costs to others, therefore output will be at Q1 (where Demand = Supply).
  • This is socially inefficient because at Q1 – Social Cost > Social Benefit.
  • Social efficiency occurs at Q2 where Social Cost = Social Benefit

The red triangle is the area of dead weight welfare loss. It indicates the area of overconsumption (where MSC is greater than MPC)

Implications of negative externalities

If goods or services have negative externalities, then we will get market failure. This is because individuals fail to take into account the costs to other people.

To achieve a more socially efficient outcome, the government could try tax the good with negative externalities. This means that consumers pay the full social cost.

See: Tax on negative externalities