Externalities – Definition

  • Externalities occur when producing or consuming a good cause an impact on third parties not directly related to the transaction.
  • Externalities can either be positive or negative. They can also occur from production or consumption

Positive Externality in Production

A farmer grows apple trees. An external benefit is that he provides nectar for a nearby beekeeper who gains increased honey as a result of the farmers’ orchard.

Negative Externality in Production

Making furniture by cutting down rainforests in the Amazon leads to negative externalities to other people. Firstly it harms the indigenous people of the Amazon rainforest. It also leads to higher global warming as there are fewer trees to absorb carbon dioxide.

Positive Externality in Consumption.

If you take a three-year training course in IT. You gain skills but also other people in the economy can benefit from your knowledge.

Negative Externality in Consumption

If you smoke in a crowded room, other people have to breathe in your smoke. This is unpleasant for them and can leave them exposed to health problems associated with smoking.

Pigou on Externalities

In 1920, Arthur C. Pigou wrote The Economics of Welfare which is an early exposition of this concept

Pigou noted that private business pursued their own marginal private interests. However, industrialists were not concerned with any external costs to others in society. In other words, they had no incentive to internalise the full social costs of their actions and this led to deadweight welfare loss.

Pigou used an example of a contractor building a factory in the middle of a neighbourhood. The factory leads to external costs faced by those living in the locality. These external costs include

  • Pollution,
  • Congestion
  • Damage to health
  • Loss of light

Pigou also used the example of alcohol. Alcohol has a private benefit to firms but also the sale of alcohol leads to additional costs to society in terms of increased demand for healthcare and law and order.


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