In economics, a **public good** is a good that is ''non-rivalrous''. This means: consumption of the good by one individual does not reduce the amount of the good available for consumption by others. For example, if one individual eats a cake, there is no cake left for anyone else; but breathing air or drinking water from a stream does not significantly reduce the amount of air or water available to others. 

The term ''public good'' is often used to refer to goods that are ''non-excludable'' as well as ''non-rival''. This means it is not possible to exclude individuals from the good's consumption. Fresh air may be considered a public good as it is not generally possible to prevent people from breathing it. However, technically speaking such goods should be called **pure public goods**. These are highly theoretical definitions: in the real world there may be no such thing as an absolutely non-rival or non-excludable good; but economists think that some goods in the real world approximate closely enough for these concepts to be meaningful.

Non-rivalness and non-excludability may cause problems for the production of such goods. Specifically, some economists have argued that they may lead to instances of market failure, where uncoordinated markets are unable to provide these goods in desired quantities. These issues are known as ''public goods problems'', and there is a good deal of debate and literature on how significant they are, and on what their solutions might be. These debates can become important to political arguments about the role of markets in the economy. More technically, public goods problems are related to the broader issue of externalities.

## Terminology, and types of public goods
Paul A. Samuelson is usually credited as the first economist to develop the theory of public goods. In his classic 1954 paper ''The Pure Theory of Public Expenditure'' he defined a public good, or as he called it in the paper a "collective consumption good", as follows:

This is the property that has become known as ''Non-rivalness''. In addition a ''pure public good'' exhibits a second property called ''Non-excludability'': that is, it is impossible to exclude any individuals from consuming the good.

The opposite of a public good is a ''private good'', which does not possess these properties. A loaf of bread, for example, is a private good: its owner can exclude others from using it, and once it has been consumed, it cannot be used again.

A good which is rival but ''non-excludable'' is sometimes called a ''common pool resource''. Such goods raise similar issues to public goods: the mirror to the ''public goods problem'' for this case is sometimes called the ''tragedy of the commons''. For example, it is so difficult to police deep sea fishing that the world's fish stocks can be seen as a non-excludable resource, but one which is finite and diminishing. 

It should be emphasised that these concepts are highly theoretical. For example, the definition of non-excludability states that it is impossible to exclude individuals from consumption. In reality, perhaps any good can become excludable: for example, radio or television broadcasts have in the past been used as a classic example of non-excludable goods. But as technology has developed, it is now possible to encrypt these signals so that anyone without a special decoder is excluded from the broadcast.

Many forms of creative works have characteristics of public goods. For example, a poem can be read by many people without reducing the consumption of that good by others; in this sense, it is non-rival. Similarly, the information in most patents can be used by any party without reducing consumption of that good by others. Creative works may be excludable in some circumstances, however: the individual who wrote the poem may decline to share it with others by not publishing it. Copyrights and patents function to encourage the creation of such non-rival goods by providing temporary monopolies, or, in the terminology of public goods, providing a legal mechanism to enforce excludability for a limited period of time. Note that for public goods, the "lost revenue" of the producer of the good is not part of the definition: a public good is a good whose consumption does not reduce any other's consumption of that good.

As well as public goods there can be public ''bads'' that have negative externality effects instead of positive ones. For example, pollution or political corruption may be ''bads'' that show some of the same non-excludability and non-rivalness properties.

The economic concept of public goods should not be confused with the expression "the public good", which is usually an application of a collective ethical notion of "the good" in political decision-making. Another common confusion is that public goods are goods provided by the public sector. Although it is often the case that Government is involved in producing public goods, this is not necessarily the case. Public goods may be ''naturally'' available, they may be produced by private individuals and firms, by non-state collective action, or they may not be produced at all.

The theoretical concept of public goods does not distinguish with regards to the geographical region in which a good may be produced or consumed. However some theorists use the term 
**global public good** to mean a public good which is non-rival and non-excludable throughout the whole world, as opposed to a public good which exists in just one national area. Knowledge is held to be an example of a global public good

### Collective good
**Collective goods** (or social goods) are defined as public goods that could be delivered as private goods, but are usually delivered by the government for various reasons, including social policy, and financed from public funds like taxes.

''Note: Some writers have used the term **public good** to refer only to non-excludable **pure public goods**. They may then call excludable public goods **club goods**.

## Examples of public goods
공공재의 대표적 예로서는 국가의 방어 시스템, 법률의 유지 (사유재산의 보호를 포함한), 공공시설 , 깨끗한 공기와 다른 자연, 그리고 발명이나 창작, 또는 소프트웨어 발명등의 정보재가 있다.
Some goods -such as orphan drugs- require special governmental incentives to be produced, but can't be classified as public goods since they don't fulfill the above requirements (Non-excludable and non-rivalrous.)

The provision of a lighthouse has often been used as the standard example of a public good, since it is difficult to exclude ships from using its services and no ship's use detracts from that of others. However, since most of the benefit of a lighthouse accrues to ships using particular ports, lighthouse maintenance fees can often profitably be bundled with port fees . This has been sufficient to fund actual lighthouses. 

A public good's status may change over time. Technological progress can significantly impact excludability of traditional public goods: encryption allows broadcasters to sell individual access to their programming. The costs for electronic road pricing have fallen dramatically, paving the way for detailed billing based on actual use. On the other hand, technological progress can also create new public goods. The simplest examples are street lights: they are relatively recent inventions (by historical standards), one person's enjoyment of them does not detract from other persons' enjoyment, and it is impossible to charge individuals separately for the amount of light they presumably use.

## 무임승차자 문제
Public goods provide a very important example of market failure, in which market-like behavior of individual gain-seeking does not produce efficient results. The production of public goods results in positive externalities which are not remunerated. Because no private organisation can reap all the benefits of a public good which they have produced, there will be insufficient incentives to produce it voluntarily. Consumers can take advantage of public goods without contributing sufficiently to their creation. This is called the free rider problem, or occasionally, the "easy rider problem" (because consumer's contributions will be small but non-zero).

For example, consider national defense, a standard example of a pure public good. A purely rational person (also known as ''homo economicus'') is an individual who is extremely individualistic, considering only those benefits and costs that directly affect him or her. Public goods give such a person incentive to be a free rider. 

Suppose this purely rational person thinks about exerting some extra effort to defend the nation. The benefits to the individual of this effort would be very low, since the benefits would be distributed among all of the millions of other people in the country. Further, there is a very high possibility that he could get injured or killed during the course of his or her military service. 

On the other hand, the free rider knows that he or she cannot be excluded from the benefits of national defense, regardless of whether he or she contributes to it. There is also no way that these benefits can be split up and distributed as individual parcels to people. So the free rider would not voluntarily exert any extra effort, unless there is some inherent pleasure or material reward for doing so .

In the case of information goods, an inventor of a new product may benefit all of society. But hardly anyone is willing to pay for the invention if they can benefit from it for free.

## Possible solutions to the free rider problem

### Dominant assurance contracts
Assurance contracts are contracts in which participants make a binding pledge to contribute to a contract for building a public good, contingent on a quorum of a predetermined size being reached. Otherwise their money is refunded. A dominant assurance contract is a variation in which an entrepreneur creates the contract and refunds the initial pledge plus an additional sum of money if the quorum is not reached. In game theory terms this makes pledging to build the public good a dominant strategy: the best move is to pledge to the contract regardless of the actions of others.

### Coasian solution
The ''coasian solution'', named for the economist Ronald Coase and unrelated to the Coase theorem, proposes a mechanism by which potential beneficiaries of a public good band together and pool their resources based on their willingness to pay to create the public good. Coase (1960) argued that **if** the transaction costs between potential beneficiaries of a public good are sufficiently low, and it is therefore easy for beneficiaries to find each other and pool their money based on the public good's value to them, then an adequate level of public goods production can occur even under competitive free market conditions. However, Coase (1988) famously wrote: "The world of zero transaction costs has often been described as a Coasian world. Nothing could be further from the truth. It is the world of modern economic theory, one which I was hoping to persuade economists to leave."

A similar alternative for arranging funders of public goods production is to produce the public good but refuse to release it into the public until some form of payment to cover costs is met. Author Stephen King, for instance, authored chapters of a new novel downloadable for free on his website while threatening not to release subsequent chapters unless a certain amount of money was raised. Sometimes dubbed ''holding for ransom'', this method of public goods production is a modern application of the street performer protocol for public goods production.

In some ways, the formation of governments and government-like communities such as homeowners associations can be thought of as applied instances of practicing the coasian solution by creating institutions to reduce the transaction costs.

### Government provision
If voluntary provision of public goods will not work, then the obvious solution is making their provision involuntary.  One general solution to the problem is for governments or states to impose taxation to fund the production of public goods. The difficulty is to determine how much funding should be allocated to different public goods, and how the costs should be split . 

Sometimes the government provides public goods using "unfunded mandates". An example is the requirement that every car be fit with a catalytic converter. This may be executed in the private sector, but the end result is predetermined by the state: the individually involuntary provision of the public good clean air. Unfunded mandates have also been imposed by the U.S. federal government on the state and local governments, as with the Americans with Disabilities Act, for example.

### Subsidies
A government may subsidize production of a public good in the private sector. Unlike government provision, subsidies may result in some form of competitive market. The potential for cronyism  can be limited with secret bidding for the subsidies or application of the subsidies following clear general principles. Depending on the nature of a public good and a related subsidy, principal agent problems can arise between the citizens and the government or between the government and the subsidized producers; this effect and counter-measures taken to address it can diminish the benefits of the subsidy.

Subsidies can also be used in areas with a potential for non-individualism: For instance, a state may subsidize devices to reduce air pollution and appeal to citizens to cover the remaining costs..

### Privileged group
The study of collective action shows that public goods are still produced when one individual benefits more from the public good than it costs him to produce it; examples include benefits from individual use, intrinsic motivation to produce, and business models based on selling complement goods. A group that contains such individuals is called a privileged group. A historical example could be a downtown entrepreneur who erects a street light in front of his shop to attract customers; even though there are positive external benefits to neighboring businesses that aren't paying from the street light, the added customers to the paying shop provide enough revenue to cover the costs of the street light.

The existence of privileged groups is not a complete solution to the free rider problem, however, as underproduction of the public good can still result. The street light builder, for instance, would not consider the added benefit to neighboring businesses when determining whether to erect his street light, making it possible that the street light isn't built when the cost of building is too high for the single entrepreneur even when the total benefit to all the businesses combined exceeds the cost.

An example of the privileged group solution could be the Linux community, assuming that users derive more benefit from contributing than it costs them to do it. For more discussion on this topic see also Coase's Penguin.

### Merging of free riders
Another method of overcoming the free rider problem is to simply eliminate the profit incentive for free riding by buying out all the potential free riders. A property developer that owned an entire city street, for instance, would not need to worry about free riders when erecting street lights since he owns every business that could benefit from the street light without paying. Implicitly, then, the property developer would erect street lights until the marginal social benefit met the marginal social cost, since in this case they are equivalent to the private marginal benefits and costs.

While the purchase of all potential free riders may solve the problem of underproduction due to free riders in smaller markets, it may simultaneously introduce the problem of underproduction due to monopoly. Additionally, some markets are simply too large to make a buyout of all beneficiaries feasible - this is particularly visible with public goods that affect everyone in a country.

### Legislated exclusion
Another solution, which has evolved for information goods, is to create intellectual property laws, such as copyright or patents, covering the public goods. These laws attempt to remove the natural non-excludability by prohibiting reproduction of the good. Although they can solve the free rider problem, the downside of these laws is that they imply private monopoly power and thus are not Pareto-optimal. For example, in the United States, the patent rights given to pharmaceutical companies encourage them to charge high prices (above marginal cost), to advertise to convince patients to nag their doctors to prescribe the drugs, to sue even mild imitators in court, and to lobby for the extension of patent rights in a form of rent seeking.

This near-ubiquitous problem arises because the underlying marginal cost of giving the good to more people is low or zero, but, because of the limits of price discrimination , those who are unwilling or unable to pay a profit-maximising price, do not get access to the good.

Joseph Schumpeter claimed that the "excess profits," or profits over normal profit, generated by the copyright or patent monopoly will attract competitors that will make technological innovations and thereby end the monopoly. This is a continual process referred to as "Schumpeterian creative destruction", and its applicability to different types of public goods is a source of some controversy. The supporters of the theory point to the case of Microsoft, for example, which has been increasing its prices (or lowering its products' quality), and predict that these practices will make increased market shares for Linux and Apple largely inevitable.

### Public Spirit
If enough people do not think like free-riders, the private and voluntary provision of public goods may be successful. A free rider might litter in a public park, but a more public-spirited individual would not do so, getting an inherent pleasure from helping the community. In fact, a public-spirited person might voluntarily pick up some of the existing litter. If enough people do so, the role of the state in using taxes to hire professional maintenance crews is reduced. This might imply that even someone typically inclined to free-riding would not litter, since their action would have such an obvious cost. 

Public spirit may be encouraged by non-market solutions to the economic problem, such as tradition and social norms. For example, this kind of public spirit involving concepts such as nationalism and patriotism has been part of most successful war efforts, complementing the roles of taxation and conscription. To some extent, public spiritedness of a more limited type is the basis for voluntary contributions that support public radio and television. Contributions to online collaborative media like Wikipedia and many other projects utilising wiki technology can also be seen to represent an example of such public spiritedness, since they provide a public good (information) freely to all readers.

## Efficient production levels of public goods
Regardless of the method of providing public goods, the efficient level of such provision is still being subjected to economic analysis. For instance, the Samuelson condition calculates the efficient level of public goods production to be where the ratio of the marginal social cost of public and private goods production equals the ratio of the marginal social benefit of public and private goods production.

## External sources
For example, Gravelle and Rees: ''The defining characteristic of a public good is that consumption of it by one individual does not actually or potentially reduce the amount available to be consumed by another individual.''

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