What Is an Externality? Positive and Negative Examples

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An externality is a cost or benefit that an economic activity imposes on a third party who did not choose to bear it, and that is not reflected in the price of the good or service involved. When a factory’s smoke harms nearby residents, the harm is a negative externality. When a neighbor’s well-kept garden raises the appeal of the whole street, the benefit is a positive externality. In both cases, the people affected are outside the transaction. The buyer and seller set a price that ignores them.

Markets respond to prices. If a price leaves out part of the true cost or benefit to society, the market produces too much of some things and too little of others.

How an Externality Works

Every economic decision has a private side and a social side. A producer compares its own costs (labor, materials, energy) with the revenue it expects. A consumer compares the price with the satisfaction a purchase brings. Neither has a financial reason to count effects that land on other people.

Economists express this with two pairs of relationships:

• Marginal social cost (MSC) = marginal private cost (MPC) + marginal external cost (MEC)
• Marginal social benefit (MSB) = marginal private benefit (MPB) + marginal external benefit (MEB)

When MEC and MEB are zero, private and social values line up and the market quantity is the efficient quantity. When MEC is positive, the supply curve, which reflects private cost, sits below the true social cost curve and the market produces more than is efficient. When MEB is positive, the demand curve, which reflects private benefit, sits below the true social benefit curve and the market produces less than is efficient.

The efficient level is where marginal social benefit equals marginal social cost. Any output beyond that point, in the negative case, adds units whose total cost to society exceeds their value. Any shortfall, in the positive case, leaves out units that society values more than they cost. The lost value in either case is called deadweight loss.

Technical vs. Pecuniary Externalities

Not every effect on a third party causes inefficiency. A technical externality changes what others can produce or consume: polluted water forces a downstream brewery to spend more on filtration. A pecuniary externality works only through prices: a new coffee chain opens and a nearby café loses customers and has to cut its prices. The café is worse off, but the price change is the market transferring value normally. Policy discussions focus on the technical kind, because those are the effects prices fail to carry.

The Four Types of Externalities

Externalities are sorted by sign (positive or negative) and source (production or consumption), which gives four types.

Negative Production Externality

The side effect comes from how a good is made. A chemical plant that stores waste improperly can contaminate groundwater; a power station emits carbon dioxide and particulates; a mine releases runoff into a river. The firm captures the revenue while part of the cost falls on people who breathe the air or use the water.

Positive Production Externality

Production can also spill benefits onto others. Corporate research and development raises the firm’s own profits but also adds to the stock of knowledge other firms and researchers build on. An orchard provides forage for a neighboring beekeeper’s bees, which in turn pollinate the orchard. The producer is not paid for the spillover, so it has less reason to supply as much as society would want.

Negative Consumption Externality

Here the harm comes from using a good. Driving a car to work adds exhaust and congestion for everyone else on the road. Secondhand smoke, loud music late at night and heavy drinking that leads to noise or unsafe streets are other examples. The consumer pays the purchase price but not the cost borne by those nearby.

Positive Consumption Externality

Some consumption benefits other people. A vaccinated person lowers the chance that others catch the disease. A person who completes more education tends to bring skills to employers, who then spend less on training, and to the wider economy through higher productivity. The individual weighs only their own benefit when deciding how much to consume, so total consumption stays below the level that would maximize social benefit.

Worked Example: A Polluting Steel Mill (Hypothetical)

Assume a local steel market where:

• Demand (marginal private benefit): P = 120 − Q
• Supply (marginal private cost): P = 20 + Q
• Each unit of output causes $20 of pollution damage downstream, so MEC = 20
• Q is measured in thousands of tons and P in dollars per ton

Market outcome. Setting demand equal to private supply: 120 − Q = 20 + Q, so Q = 50 and P = $70. The market produces 50,000 tons at $70 per ton.

Efficient outcome. Marginal social cost is MPC + MEC = 40 + Q. Setting demand equal to social cost: 120 − Q = 40 + Q, so Q = 40 and P = $80. Society is best off at 40,000 tons.

The overproduction. The market makes 10,000 tons too many. Each of those tons costs society more than buyers value it. The deadweight loss is the triangle between the social cost curve and the demand curve over that range: 0.5 × 10,000 tons × $20 = $100,000.

A corrective tax. If the government charges a tax of $20 per ton, equal to the marginal external cost (assumed constant here), the producer’s effective cost curve shifts up to 40 + Q. The market then settles at 40,000 tons. Buyers pay $80, the mill keeps $60 after tax, and the government collects $20 × 40,000 = $800,000. The deadweight loss disappears in this simplified model because the tax makes the polluter face the full cost of its output.

The buyer’s price rises from $70 to $80. Firms pass part of a corrective tax on to consumers through higher prices, and how much depends on the elasticities of supply and demand.

For a positive externality the logic reverses: if each unit gave $20 of benefit to others, a subsidy of $20 per unit would raise output to the efficient level.

How Governments and Markets Address Externalities

The goal of every approach is to internalize the externality, meaning to make the decision-maker account for the cost or benefit that would otherwise fall on someone else.

Pigovian Taxes

Named after the British economist Arthur C. Pigou, who developed the idea in The Economics of Welfare (1920), a Pigovian tax is set equal to the marginal external cost of an activity, as in the worked example above. Carbon, fuel and tobacco taxes are often discussed in these terms. The tax does not ban the activity; it raises its price so that only the units worth their full social cost get produced.

Subsidies

A subsidy is the mirror image, paying producers or consumers roughly the value of the external benefit. Public funding for schooling, research grants, vaccine programs and payments to farmers for conservation fit this pattern. Subsidies can also target substitutes: incentives for energy-efficient home renovations make lower-externality options cheaper relative to higher-externality ones, nudging choices without taxing anyone.

Direct Regulation

Governments can also set rules: emission limits, required technology, zoning, bans on certain substances, or health rules such as indoor smoking restrictions. Regulation gives a predictable result for the regulated activity, but it requires monitoring and enforcement, which carry their own administrative cost. A uniform rule also ignores the fact that some firms can reduce pollution far more cheaply than others.

Tradable Permits (Cap and Trade)

Cap and trade combines a regulatory limit with a market. The government sets a total cap on emissions and issues or auctions permits that add up to that cap. Firms that cut emissions cheaply can sell unused permits, and firms that find cuts expensive can buy them. The cap fixes total quantity, while the permit price is set by supply and demand and can move over time. The Regional Greenhouse Gas Initiative (RGGI) in the United States is a mandatory cap-and-trade program for carbon dioxide emissions from power plants in a group of Northeastern and Mid-Atlantic states.

Private Bargaining and the Coase Theorem

Ronald Coase argued in 1960 that if property rights are clearly defined and bargaining costs are low, the affected parties can negotiate their way to an efficient outcome without government taxes. A rancher whose cattle trample a farmer’s crops and the farmer can strike a deal, whichever of them holds the legal right. In practice, bargaining breaks down when many people are affected (millions of residents breathing the same air) or when harm is hard to trace to a source, which is why most large-scale externalities end up handled through policy. The courts play a role too: nuisance and tort law let people harmed by a neighbor’s activity seek compensation or an injunction, which can push the cost back onto the party causing it.

How Economists Measure Externalities

Setting a tax or subsidy requires a dollar figure for the externality, and economists generally use two approaches.

• Cost of damages. Estimate the value of the harm: medical costs, lost crops, reduced property values, lower productivity, and the cost of cleaning up. Some harms are difficult or impossible to reverse, as with long-lived greenhouse gases, which makes the estimate uncertain.
• Cost of control. Estimate what it would cost to prevent the externality in the first place, for example installing scrubbers or switching fuels. This approach looks at preventive steps rather than repair.

Analysts often produce several estimates under different assumptions and report a range rather than a single number.

Why Externalities Matter for the Wider Economy

Externalities are one of the main causes of market failure, situations in which free markets do not allocate resources efficiently. That is why even strong supporters of market-based capitalism accept some government role in pollution control and public health.

They also affect how economic performance is read. Output measured by GDP counts the value of goods sold but not the uncompensated damage their production causes, and spending on cleanup after an oil spill can register as economic activity. Resources used to repair external harm carry an opportunity cost. For businesses, externalities that become regulated or taxed turn into real costs, which is why investors and companies now track potential environmental liabilities when assessing a firm.

Limitations of the Externality Framework

• Measurement is uncertain. Pigovian taxes and subsidies need an estimate of marginal external cost or benefit, which varies by location, time and the number of people affected. A mis-set tax can leave overproduction in place or cut output below the efficient level.
• Valuation involves judgment. Placing a price on health, ecosystems or future generations requires choices about discount rates and values that economists disagree on.
• Distribution is separate from efficiency. A tax that corrects an externality can still fall more heavily on lower-income households, and the model says nothing on its own about who should bear the cost.
• Not every third-party effect qualifies. Pecuniary effects and harms that the affected party agreed to and priced into a contract are not externalities in the inefficiency sense.

Externality vs. Related Concepts

Externality vs. public good. A public good is something that is non-excludable and non-rival, like national defense. Public goods are an extreme case of positive externality, since the benefit spreads to everyone, but most externalities attach to ordinary private goods such as cars or electricity.

Externality vs. market failure. Market failure is the broader category. Externalities are one cause; others include monopoly power, information asymmetry and public goods.

Externality vs. opportunity cost. Opportunity cost is the value of the next-best alternative the decision-maker gives up, so it falls on you. An externality spills onto others.

FAQs

The Bottom Line

Externalities matter to policymakers, businesses and investors because unpriced costs and benefits push markets to produce too much or too little. Pigovian taxes, subsidies, regulation, tradable permits and private bargaining all try to make decision-makers count the full effect of their choices. Each tool depends on uncertain estimates of external harm or benefit, and correcting inefficiency leaves separate questions about who bears the cost.