[1.1.6a] External costs
External Costs
When producers or consumers make decisions, they typically consider only the costs and benefits that directly affect themselves. However, some economic activity imposes costs on third parties — people not directly involved in the transaction. These are called external costs (also known as negative externalities).
An external cost is a cost that falls on a third party as a result of economic activity, for which no compensation is paid. Because producers ignore these costs when deciding how much to produce, they tend to overproduce relative to the socially optimal level.
Examples of External Costs
A factory that releases chemicals into a river imposes costs on nearby residents (contaminated drinking water), downstream farmers (polluted irrigation) and the wider environment. The factory's private costs do not include these damages — they are borne by third parties. Other examples: air pollution from vehicles causing respiratory illness in nearby communities; noise pollution from airport flights affecting local residents.
When a driver uses a busy road, they slow down every other road user — a cost imposed on third parties. This is road congestion. Each individual driver considers only their own time cost, not the delay they cause to thousands of other drivers. The cumulative external cost of congestion — lost productivity, fuel waste and pollution — is substantial in most urban economies.
Activities such as deforestation, mining or intensive farming can cause lasting damage to ecosystems, biodiversity and soil quality — costs borne by society at large and future generations. Carbon emissions from fossil fuel combustion contribute to climate change, imposing enormous external costs on communities worldwide through rising sea levels, extreme weather and disrupted food systems.
The diagram shows that when producers only consider their private marginal cost (MPC), they produce Qₘ — more than the socially optimal output Q*. The marginal social cost (MSC) lies above MPC by the amount of the external cost. Overproduction generates a welfare loss to society.
Key Takeaways
- An external cost is a cost imposed on third parties by economic activity, for which no compensation is paid.
- External costs cause overproduction — firms produce more than the socially optimal level because they ignore costs borne by others.
- Examples include pollution, congestion and environmental damage.
- External costs are a form of market failure — the free market outcome is inefficient from society's perspective.