Externalities: private, external and social costs and benefits

Costs and benefits

• Private costs: costs paid by the producer or consumer directly involved (a firm's wages and materials; the price a consumer pays).
• External costs: costs imposed on third parties who are not part of the transaction and are not compensated (pollution, noise, congestion).
• Social costs = private costs + external costs.
• Private benefits: benefits to those directly involved (the satisfaction of the consumer, the revenue of the firm).
• External benefits: benefits to third parties who do not pay for them.
• Social benefits = private benefits + external benefits.

An externality exists when social and private costs or benefits differ.

The four types

• External costs of production (negative production externality): a cement works emits dust that harms local residents' health.
• External costs of consumption (negative consumption externality): smoking in public harms others; driving adds congestion and emissions; loud music disturbs neighbours.
• External benefits of production (positive production externality): a firm trains workers who later work for other firms; a beekeeper's bees pollinate nearby orchards.
• External benefits of consumption (positive consumption externality): education makes a more productive workforce and better-informed citizens; vaccination protects others from disease.

Because decision-makers ignore external effects, goods with external costs are over-produced or over-consumed and goods with external benefits are under-produced or under-consumed.

Where externalities arise

The specification names five sectors:

• Transport: congestion, air pollution, noise and accidents from road traffic (external costs); rail and public transport can reduce them.
• Health: vaccination and treatment of infectious disease (external benefits); alcohol and tobacco (external costs to health services and others).
• Education: higher productivity, lower crime, innovation (external benefits).
• Environment: carbon emissions, water pollution, deforestation (external costs); conservation (external benefits).
• Finance: when banks take excessive risks and fail, households and firms who were not party to the loans suffer job losses and lower incomes (external costs).

Worked example

A chemical plant dumps waste into a river, reducing the catch of a fishing business downstream. Identify the private cost, external cost and social cost.

1. Private cost: the plant's own costs of production (labour, materials, energy).
2. External cost: the lost income of the fishing business and harm to river users, third parties not compensated.
3. Social cost: the private cost plus the external cost.

Answer: The external cost is borne by the fishing business; the social cost is private plus external cost.

Check yourself

1. Traffic congestion caused by drivers on a city road is an example of
   • an external benefit of production
   • an external cost of consumption
   • an external benefit of consumption
   • a private benefit
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   an external cost of consumption. Each driver's use of the road imposes delays and pollution on others.

2. A firm trains its workers, who later move to other firms in the same industry. This is
   • an external cost of production
   • an external cost of consumption
   • a private cost
   • an external benefit of production
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   an external benefit of production. Other firms gain skilled workers without paying for the training.

3. Social cost is equal to
   • private cost minus external cost
   • private cost plus external cost
   • external cost only
   • private benefit plus external benefit
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   private cost plus external cost. Society's cost includes the firm's own cost and the cost to third parties.

4. If a good creates external costs in its production, the free market will
   • under-produce the good
   • produce the social optimum
   • over-produce the good compared with the social optimum
   • not produce the good at all
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   over-produce the good compared with the social optimum. Producers ignore the external cost, so private cost is too low and output too high.

5. A factory's private cost of producing a tonne of steel is $400, and its pollution causes $90 of damage per tonne to local residents. What is the social cost per tonne?
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   $490. Social cost = private + external = 400 + 90 = $490.

6. A student's private benefit from a degree is valued at $50,000 and the external benefit to society is $15,000. What is the social benefit?
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   $65000. Social benefit = private + external = $65,000.

7. Explain how an externality can arise in the financial sector.
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   When a bank takes excessive risks and fails, the costs fall on third parties such as households and firms who lose jobs, savings or access to credit, and taxpayers if the government rescues the bank. These external costs are not paid by the bank.. Identify the action, the third parties and why they bear the cost.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Economics specification (2018)

Last updated 8 November 2026. Spotted a mistake? Tell us.