An externality is a cost or benefit arising from an economic activity that affects people other than the decision-maker and is not fully incorporated into prices or contractual arrangements. In economics, externalities explain why individually advantageous decisions may produce socially inefficient outcomes. Pollution imposes external costs, while research can generate external benefits. The defining feature is the divergence between the incentives facing the actor and the consequences for others, rather than whether the activity is intentional or commercially organized. (imf.org)
Types and examples
Negative externalities impose costs on third parties. A factory may release pollutants that damage neighboring property or reduce other businesses’ production opportunities without compensating those affected. Positive externalities provide benefits that the actor cannot fully capture. Research and development, for example, can create knowledge spillovers when discoveries assist other firms or researchers without equivalent payment to the original developer. Both types can arise from production, consumption, or investment. (imf.org)
Externalities also differ in geographical reach and duration. Noise may mainly affect nearby residents, whereas emissions of greenhouse gases influence the global climate and can affect people far from the source and in later generations. This makes climate change an example in which the parties generating an external cost and those bearing it are widely separated. (imf.org)
Externalities overlap with, but are not identical to, public goods. A public good is non-excludable and non-rival: people cannot readily be prevented from benefiting, and one person’s use does not reduce another’s. These characteristics can prevent providers from capturing the full benefits of provision. Externality analysis instead focuses on effects of decisions that fall outside the relevant payment or contractual relationship. (imf.org)
Private and social incentives
In microeconomics and welfare economics, the standard analysis distinguishes private costs and benefits from social costs and benefits. For an activity with an external cost, marginal social cost equals private marginal cost plus the additional cost imposed on others. For an activity with an external benefit, marginal social benefit includes both the decision-maker’s benefit and the additional benefit accruing to others. These are marginal quantities: they concern the consequences of one additional unit of activity. (books.core-econ.org)
A market equilibrium determined by private supply and demand need not maximize total social surplus. In the standard competitive model, an unpriced negative externality leads to excessive activity relative to the socially efficient level; a positive externality leads to insufficient activity. This divergence is a form of market failure. The efficiency benchmark compares marginal social benefit with marginal social cost, rather than assuming that every harmful activity should disappear. (elibrary.imf.org)
For pollution, the corresponding benchmark balances the benefit of further emission reductions against their cost. Efficient control therefore need not mean zero emissions. Different sources may face very different costs of reducing pollution, so requiring identical reductions from every source can be more expensive than allocating reductions according to marginal abatement costs. (epa.gov)
Property rights and bargaining
Property rights and private agreements can sometimes bring external effects into decision-making. Affected parties may negotiate compensation or changes in behavior when they can identify one another, establish enforceable rights, and reach an agreement at sufficiently low transaction cost. This approach is associated with Ronald Coase and his 1960 article The Problem of Social Cost. (nobelprize.org)
The Coase theorem describes the possibility of efficient bargaining under idealized conditions, including costless transactions. Coase emphasized that this hypothetical result does not establish that actual markets automatically resolve externalities. Finding parties, negotiating, and enforcing agreements consume resources. When these costs are substantial, the allocation of rights and the institutional arrangement affect outcomes. Comparing bargaining, firms, and public intervention therefore requires comparing their practical operating costs. (nobelprize.org)
Bargaining may be particularly difficult when many people are affected, necessary information is unavailable, or obligations cannot be enforced. Institutions may address these problems through contracts, tort law, or other rules requiring compensation, but each arrangement has informational and administrative limitations. (books.core-econ.org)
Taxes, subsidies, and regulation
Internalization means changing incentives so that actors take external costs or benefits into account. A Pigouvian tax, named after Arthur Cecil Pigou, charges for an activity generating external harm. In the basic model, a tax equal to marginal external damage at the efficient activity level aligns private incentives with the social benchmark. A Pigouvian subsidy performs the corresponding function for external benefits. Neither is simply a payment equal to total harm or total benefit. (books.core-econ.org)
Economic regulation can also impose emission limits, performance standards, or technology requirements. Market-based instruments instead give actors flexibility in selecting how to comply. Pollution charges encourage reductions whenever avoiding the charge is cheaper than continuing to emit, while also creating incentives to develop less costly control methods. (epa.gov)
Under emissions trading, authorities establish an emissions cap and tradable allowances. Sources can reduce emissions or acquire allowances from others. Trading allows sources with relatively low reduction costs to undertake more abatement, while higher-cost sources purchase allowances. Monitoring emissions, tracking allowances, and enforcing compliance are essential components of such systems. (epa.gov)
Measurement and policy design
Policy design requires estimates of external damages or benefits, behavioral responses, and implementation costs. Damages can vary across locations, so a uniform emissions price or unrestricted allowance trading may not reproduce the efficient geographical pattern of reductions. Existing taxes and other distortions can also change the effects of a corrective policy. Efficiency and distribution are separate considerations: tax revenue, compensation, and allowance allocation determine who receives payments and who bears costs. (19january2021snapshot.epa.gov)