In economics, an externality is the cost or benefit that affects a third party who did not choose to incur that cost or benefit. Externalities often occur when the production or consumption of a product or service's private price equilibrium cannot reflect the true costs or benefits of that product or service for society as a whole. This causes the externality competitive equilibrium to not be a Pareto optimality.
Externalities can be either positive or negative. Governments and institutions often take actions to internalize externalities, thus market-priced transactions can incorporate all the benefits and costs associated with transactions between economic agents. The most common way this is done is by imposing taxes on the producers of this externality. This is usually done similar to a quote where there is no tax imposed and then once the externality reaches a certain point there is a very high tax imposed. However, since regulators do not always have all the information on the externality it can be difficult to impose the right tax. Once the externality is internalized through imposing a tax the competitive equilibrium is now Pareto optimal.
For example, manufacturing activities that cause air pollution impose health and clean-up costs on the whole society, whereas the neighbors of individuals who choose to fire-proof their homes may benefit from a reduced risk of a fire spreading to their own houses. If external costs exist, such as pollution, the producer may choose to produce more of the product than would be produced if the producer were required to pay all associated environmental costs. Because responsibility or consequence for self-directed action lies partly outside the self, an element of externalization is involved. If there are external benefits, such as in public safety, less of the good may be produced than would be the case if the producer were to receive payment for the external benefits to others. For the purpose of these statements, overall cost and benefit to society is defined as the sum of the imputed monetary value of benefits and costs to all parties involved.

History of the concept

Two British economists are credited with having initiated the formal study of externalities, or "spillover effects": Henry Sidgwick is credited with first articulating, and Arthur C. Pigou is credited with formalizing the concept of externalities.
The word externality is used because the effect produced on others, whether in the form of profits or costs, is external to the market.


A negative externality is any difference between the private cost of an action or decision to an economic agent and the social cost. In simple terms, a negative externality is anything that causes an indirect cost to individuals. An example is the toxic gases that are released from industries or mines, these gases cause harm to individuals within the surrounding area and have to bear a cost to get rid of that harm. Conversely, a positive externality is any difference between the private benefit of an action or decision to an economic agent and the social benefit. A positive externality is anything that causes an indirect benefit to individuals. For example, planting trees makes individuals' property look nicer and it also cleans the surrounding areas.

Formal definition

Suppose that there are different possible allocations and different agents, where and. Suppose that each agent has a type and that each agent gets payoff, where is the transfer paid by the -th agent. A map is a social choice function if
for all An allocation is ex-post efficient if
for all and all
Let denote an ex-post efficient allocation and let denote an ex-post efficient allocation without agent. Then the externality imposed by agent on the other agents is
where is the type vector without its -th component. Intuitively, the first term is the hypothetical total payoff for all agents given that agent does not exist, and the second term is the actual total payoff for all agents given that agent does exist.


Voluntary exchange is by definition mutually beneficial to both business parties involved because the parties would not agree to undertake it if either thought it detrimental to their interests. However, a transaction can cause effects on third parties without their knowledge or consent. From the perspective of those affected, these effects may be negative, or positive. Neoclassical welfare economics asserts that, under plausible conditions, the existence of externalities will result in outcomes that are not socially optimal. Those who suffer from external costs do so involuntarily, whereas those who enjoy external benefits do so at no cost.
A voluntary exchange may reduce societal welfare if external costs exist. The person who is affected by the negative externalities in the case of air pollution will see it as lowered utility: either subjective displeasure or potentially explicit costs, such as higher medical expenses. The externality may even be seen as a trespass on their lungs, violating their property rights. Thus, an external cost may pose an ethical or political problem. Negative externalities are Pareto inefficient, and since Pareto efficiency underpins the justification for private property, they undermine the whole idea of a market economy. For these reasons, negative externalities are more problematic than positive externalities.
Positive externalities, while Pareto efficient, are still market failures that undermine allocative efficiency because less of the good will be produced than would be optimal for society as a whole in a theoretical model with no government. If those externalities were internalized, the producer would be incentivized to produce more. Goods with positive externalities include education, public health initiatives and law enforcement.
Positive externalities are often associated with the free rider problem. For example, individuals who are vaccinated reduce the risk of contracting the relevant disease for all others around them, and at high levels of vaccination, society may receive large health and welfare benefits ; but any one individual can refuse vaccination, still avoiding the disease by "free riding" on the costs borne by others.
There are a number of theoretical means of improving overall social utility when negative externalities are involved. The market-driven approach to correcting externalities is to "internalize" third party costs and benefits, for example, by requiring a polluter to repair any damage caused. But in many cases, internalizing costs or benefits is not feasible, especially if the true monetary values cannot be determined.
Laissez-faire economists such as Friedrich Hayek and Milton Friedman sometimes refer to externalities as "neighborhood effects" or "spillovers", although externalities are not necessarily minor or localized. Similarly, Ludwig von Mises argues that externalities arise from lack of "clear personal property definition."


Externalities may arise between producers, between consumers or between consumers and producers. Externalities can be negative when the action of one party imposes costs on another, or positive when the action of one party benefits another.
NegativeNegative externalities in consumptionNegative externalities in production
PositivePositive externalities in consumptionPositive externalities in production


A negative externality is an economic activity that imposes a negative effect on an unrelated third party. It can arise either during the production or the consumption of a good or service. Pollution is termed an externality because it imposes costs on people who are "external" to the producer and consumer of the polluting product. Barry Commoner commented on the costs of externalities:
Clearly, we have compiled a record of serious failures in recent technological encounters with the environment. In each case, the new technology was brought into use before the ultimate hazards were known. We have been quick to reap the benefits and slow to comprehend the costs.

Many negative externalities are related to the environmental consequences of production and use. The article on environmental economics also addresses externalities and how they may be addressed in the context of environmental issues.
Examples for negative production externalities include:
Examples of negative consumption externalities include:
A positive externality is the positive effect an activity imposes on an unrelated third party. Similar to a negative externality, it can arise either on the production side, or on the consumption side.

A positive production externality occurs when a firm's production increases the well-being of others but the firm is uncompensated by those others, while a positive consumption externality occurs when an individual's consumption benefits other but the individual is uncompensated by those others.
Examples of positive production externalities LTM
Examples of positive consumption externalities include:
The existence or management of externalities may give rise to political or legal conflicts.
Collective solutions or public policies are implemented to regulate activities with positive or negative externalities.


A position externality "occurs when new purchases alter the relevant context within which an existing positional good is evaluated." Robert H. Frank gives the following example:
Frank notes that treating positional externalities like other externalities might lead to "intrusive economic and social regulation." He argues, however, that less intrusive and more efficient means of "limiting the costs of expenditure cascades"—i.e., the hypothesized increase in spending of middle-income families beyond their means "because of indirect effects associated with increased spending by top earners"—exist; one such method is the personal income tax.


Inframarginal externalities are externalities in which there is no benefit or loss to the marginal consumer. In other words, people neither gain nor lose anything at the margin, but benefits and costs do exist for those consumers within the given inframarginal range.


Technological externalities directly affect a firm's production and therefore, indirectly influence an individual's consumption; and the overall impact of society; for example Open-source software or free software development by corporations.

Supply and demand diagram

The usual economic analysis of externalities can be illustrated using a standard supply and demand diagram if the externality can be valued in terms of money. An extra supply or demand curve is added, as in the diagrams below. One of the curves is the private cost that consumers pay as individuals for additional quantities of the good, which in competitive markets, is the marginal private cost. The other curve is the true cost that society as a whole pays for production and consumption of increased production the good, or the marginal social cost. Similarly, there might be two curves for the demand or benefit of the good. The social demand curve would reflect the benefit to society as a whole, while the normal demand curve reflects the benefit to consumers as individuals and is reflected as effective demand in the market.
What curve is added depends on the type of externality that is described, but not whether it is positive or negative. Whenever an externality arises on the production side, there will be two supply curves. However, if the externality arises on the consumption side, there will be two demand curves instead. This distinction is essential when it comes to resolving inefficiencies that are caused by externalities.

External costs

The graph shows the effects of a negative externality. For example, the steel industry is assumed to be selling in a competitive market – before pollution-control laws were imposed and enforced. The marginal private cost is less than the marginal social or public cost by the amount of the external cost, i.e., the cost of air pollution and water pollution. This is represented by the vertical distance between the two supply curves. It is assumed that there are no external benefits, so that social benefit equals individual benefit.
If the consumers only take into account their own private cost, they will end up at price Pp and quantity Qp, instead of the more efficient price Ps and quantity Qs. These latter reflect the idea that the marginal social benefit should equal the marginal social cost, that is that production should be increased only as long as the marginal social benefit exceeds the marginal social cost. The result is that a free market is inefficient since at the quantity Qp, the social benefit is less than the social cost, so society as a whole would be better off if the goods between Qp and Qs had not been produced. The problem is that people are buying and consuming too much steel.
This discussion implies that negative externalities are more than merely an ethical problem. The problem is one of the disjunctures between marginal private and social costs that are not solved by the free market. It is a problem of societal communication and coordination to balance costs and benefits. This also implies that pollution is not something solved by competitive markets. Some collective solution is needed, such as a court system to allow parties affected by the pollution to be compensated, government intervention banning or discouraging pollution, or economic incentives such as green taxes.

External benefits

The graph shows the effects of a positive or beneficial externality. For example, the industry supplying smallpox vaccinations is assumed to be selling in a competitive market. The marginal private benefit of getting the vaccination is less than the marginal social or public benefit by the amount of the external benefit. This marginal external benefit of getting a smallpox shot is represented by the vertical distance between the two demand curves. Assume there are no external costs, so that social cost equals individual cost.
If consumers only take into account their own private benefits from getting vaccinations, the market will end up at price Pp and quantity Qp as before, instead of the more efficient price Ps and quantity Qs. This latter again reflect the idea that the marginal social benefit should equal the marginal social cost, i.e., that production should be increased as long as the marginal social benefit exceeds the marginal social cost. The result in an unfettered market is inefficient since at the quantity Qp, the social benefit is greater than the societal cost, so society as a whole would be better off if more goods had been produced. The problem is that people are buying too few vaccinations.
The issue of external benefits is related to that of public goods, which are goods where it is difficult if not impossible to exclude people from benefits. The production of a public good has beneficial externalities for all, or almost all, of the public. As with external costs, there is a problem here of societal communication and coordination to balance benefits and costs. This also implies that vaccination is not something solved by competitive markets. The government may have to step in with a collective solution, such as subsidizing or legally requiring vaccine use. If the government does this, the good is called a merit good. Examples include policies to accelerate the introduction of electric vehicles or promote cycling, both of which benefit public health.


Externalities often arise from poorly defined property rights. While property rights to some things, such as objects, land, and money can be easily defined and protected, air, water, and wild animals often flow freely across personal and political borders, making it much more difficult to assign ownership. This incentivizes agents to consume them without paying the full cost, leading to negative externalities. Positive externalities similarly accrue from poorly defined property rights. For example, a person who gets a flu vaccination cannot own part of the herd immunity this confers on society, so they may choose not to be vaccinated.

Possible solutions

Solutions in non-market economies

There are several general types of solutions to the problem of externalities, including both public- and private-sector resolutions:
A Pigovian tax is a tax imposed that is equal in value to the negative externality. The result is that the market outcome would be reduced to the efficient amount. A side effect is that revenue is raised for the government, reducing the amount of distortionary taxes that the government must impose elsewhere. Governments justify the use of Pigovian taxes saying that these taxes help the market reach an efficient outcome because this tax bridges the gap between marginal social costs and marginal private costs.
Some arguments against Pigovian taxes say that the tax does not account for all the transfers and regulations involved with an externality. In other words, the tax only considers the amount of externality produced. Another argument against the tax is that it does not take private property into consideration. Under the Pigovian system, one firm, for example, can be taxed more than another firm, even though the other firm is actually producing greater amounts of the negative externality.
However, the most common type of solution is a tacit agreement through the political process. Governments are elected to represent citizens and to strike political compromises between various interests. Normally governments pass laws and regulations to address pollution and other types of environmental harm. These laws and regulations can take the form of "command and control" regulation, or environmental pricing reform. The second type of resolution is a purely private agreement between the parties involved.
Government intervention might not always be needed. Traditional ways of life may have evolved as ways to deal with external costs and benefits. Alternatively, democratically run communities can agree to deal with these costs and benefits in an amicable way. Externalities can sometimes be resolved by agreement between the parties involved. This resolution may even come about because of the threat of government action.
The use of taxes and subsidies in solving the problem of externalities
Correction tax, respectively subsidy, means essentially any mechanism that increases, respectively decreases, the costs associated with the activities of an individual or company.
The private-sector may sometimes be able to drive society to the socially optimal resolution. Ronald Coase argued that an efficient outcome can sometimes be reached without government intervention. Some take this argument further, and make the political claim that government should restrict its role to facilitating bargaining among the affected groups or individuals and to enforcing any contracts that result.
This result, often known as the Coase theorem, requires that
If all of these conditions apply, the private parties can bargain to solve the problem of externalities. The second part of the Coase theorem asserts that, when these conditions hold, whoever holds the property rights, a Pareto efficient outcome will be reached through bargaining.
This theorem would not apply to the steel industry case discussed above. For example, with a steel factory that trespasses on the lungs of a large number of individuals with pollution, it is difficult if not impossible for any one person to negotiate with the producer, and there are large transaction costs. Hence the most common approach may be to regulate the firm while paying for the regulation and enforcement with taxes. The case of the vaccinations would also not satisfy the requirements of the Coase theorem. Since the potential external beneficiaries of vaccination are the people themselves, the people would have to self-organize to pay each other to be vaccinated. But such an organization that involves the entire populace would be indistinguishable from government action.
In some cases, the Coase theorem is relevant. For example, if a logger is planning to clear-cut a forest in a way that has a negative impact on a nearby resort, the resort-owner and the logger could, in theory, get together to agree to a deal. For example, the resort-owner could pay the logger not to clear-cut – or could buy the forest. The most problematic situation, from Coase's perspective, occurs when the forest literally does not belong to anyone, or in any example in which there are not well-defined and enforceable property rights; the question of "who" owns the forest is not important, as any specific owner will have an interest in coming to an agreement with the resort owner.
However, the Coase theorem is difficult to implement because Coase does not offer a negotiation method. Moreover, Coasian solutions are unlikely to be reached due to the possibility of running into the assignment problem, the holdout problem, the free-rider problem, or transaction costs. Additionally, firms could potentially bribe each other since there is little to no government interaction under the Coase theorem. For example, if one oil firm has a high pollution rate and its neighboring firm is bothered by the pollution, then the latter firm may move depending on incentives. Thus, if the oil firm were to bribe the second firm, the first oil firm would suffer no negative consequences because the government would not know about the bribing.
In a dynamic setup, Rosenkranz and Schmitz have shown that the impossibility to rule out Coasean bargaining tomorrow may actually justify Pigouvian intervention today. To see this, note that unrestrained bargaining in the future may lead to an underinvestment problem. Specifically, when investments are relationship-specific and non-contractible, then insufficient investments will be made when it is anticipated that parts of the investments’ returns will go to the trading partner in future negotiations. Hence, Pigouvian taxation can be welfare-improving precisely because Coasean bargaining will take place in the future. Antràs and Staiger make a related point in the context of international trade.

Solutions to GHG Emission Externalities

The negative effect of carbon-emissions and other GreenHouse Gases produced in production exacerbate the numerous environmental and human impacts of anthropogenic climate change. These negative effects are not reflected in the cost of producing, nor in the market price of the final goods. There are many public and private solutions proposed to combat this externality
An emissions fee, or Carbon Tax, is a tax levied on each unit of pollution produced in the production of a good or service. The tax incentivised producers to either lower their production levels or to undertake abatement activities that reduce emissions by switching to cleaner technology or inputs.
The cap-and-trade system enables the efficient level of pollution to be achieved by setting a total quantity of emissions and issuing tradable permits to polluting firms, allowing them to pollute a certain share of the permissible level. Permits will be traded from firms that have low abatement costs to firms with higher abatement costs and therefore the system is both cost-effective and cost-efficient. The cap and trade system has some practical advantages over an emissions fee such as the fact that:
1. it reduces uncertainty about the ultimate pollution level.
2. If firms are profit maximizing, they will utilize cost-minimizing technology to attain the standard which is efficient for individual firms and provides incentives to the research and development market to innovate.
3. The market price of pollution rights would keep pace with the price level while the economy experiences inflation.
The emissions fee and cap and trade systems are both incentive-based approaches to solving a negative externality problem. They provide polluters with market incentives by increasing the opportunity cost of polluting, thus forcing them to internalize the externality by making them take the marginal external damages of their production into account.
Command-and-Control regulations act as an alternative to the incentive-based approach. They require a set quantity of pollution reduction and can take the form of either a technology standard or a performance standard. A technology standard requires pollution producing firms to use specified technology. While it may reduce the pollution, it is not cost-effective and stifles innovation by incentivising research and development for technology that would work better than the mandated one. Performance standards set emissions goals for each polluting firm. The free choice of the firm to determine how to reach the desired emissions level makes this option slightly more efficient than the technology standard, however, it is not as cost-effective as the cap-and-trade system since the burden of emissions reduction cannot be shifted to firms with lower abatement.


criticizes the concept of externality because there is not enough system thinking and integration of different sciences in the concept.
Ecological economics is founded upon the view that the neoclassical economics assumption that environmental and community costs and benefits are mutually cancelling "externalities" is not warranted. Joan Martinez Alier, for instance shows that the bulk of consumers are automatically excluded from having an impact upon the prices of commodities, as these consumers are future generations who have not been born yet. The assumptions behind future discounting, which assume that future goods will be cheaper than present goods, has been criticized by Fred Pearce and by the Stern Report.
Concerning these externalities, some, like the eco-businessman Paul Hawken, argue an orthodox economic line that the only reason why goods produced unsustainably are usually cheaper than goods produced sustainably is due to a hidden subsidy, paid by the non-monetized human environment, community or future generations. These arguments are developed further by Hawken, Amory and Hunter Lovins to promote their vision of an environmental capitalist utopia in .
In contrast, ecological economists, like Joan Martinez-Alier, appeal to a different line of reasoning. Rather than assuming some form of capitalism is the best way forward, an older ecological economic critique questions the very idea of internalizing externalities as providing some corrective to the current system. The work by Karl William Kapp explains why the concept of "externality" is a misnomer. In fact the modern business enterprise operates on the basis of shifting costs onto others as normal practice to make profits. Charles Eisenstein has argued that this method of privatising profits while socialising the costs through externalities, passing the costs to the community, to the natural environment or to future generations is inherently destructive Social ecological economist Clive Spash argues that externality theory fallaciously assumes environmental and social problems are minor aberrations in an otherwise perfectly functioning efficient economic system. Internalizing the odd externality does nothing to address the structural systemic problem and fails to recognize the all pervasive nature of these supposed 'externalities'. This is precisely why heterodox economists argue for a heterodox theory of social costs to effectively prevent the problem through the precautionary principle.