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In economics, a public good is a good that is both non-excludable and non-rivalrous in that individuals cannot be effectively excluded from use and where use by one individual does not reduce availability to others.〔For current definitions of public goods see any mainstream microeconomics textbook, e.g.: Hal R. Varian, ''Microeconomic Analysis'' ISBN 0-393-95735-7; Andreu Mas-Colell, Whinston & Green, ''Microeconomic Theory'' ISBN 0-19-507340-1; or Gravelle & Rees, ''Microeconomics'' ISBN 0-582-40487-8.〕 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". Public goods include fresh air, knowledge, public infrastructure, national security, education, common language(s), widespread and high public literacy levels, flood control systems, lighthouses, and street lighting. Public goods that are available everywhere are sometimes referred to as global public goods.〔 There is an important conceptual difference between the sense of 'a' public good, or public 'goods' in economics, and the more generalized idea of 'the public good' (or common good, or public interest),"‘the’ public good is a shorthand signal for shared benefit at a societal level () (philosophical/political) sense should not be reduced to the established specific (economic) sense of ‘a’ public good." Many public goods may at times be subject to excessive use resulting in negative externalities affecting all users; for example air pollution and traffic congestion. Public goods problems are often closely related to the "free-rider" problem, in which people not paying for the good may continue to access it. Thus, the good may be under-produced, overused or degraded.〔Rittenberg and Tregarthen. ''Principles of Microeconomics'', (Chapter 6, Section 4. p. 2 ), Accessed June 20, 2012〕 Public goods may also become subject to restrictions on access and may then be considered to be club goods or private goods; exclusion mechanisms include copyright, patents, congestion pricing, and pay television. There is a good deal of debate and literature on how to measure the significance of public goods problems in an economy, and to identify the best remedies. ==Terminology, and types of 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'',〔 See also 〕 he defined a public good, or as he called it in the paper a "collective consumption good", as follows: () which all enjoy in common in the sense that each individual's consumption of such a good leads to no subtractions from any other individual's consumption of that good... This is the property that has become known as ''non-rivalry''. 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 that is rivalrous 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 enforce restrictions on deep sea fishing that the world's fish stocks can be seen as a non-excludable resource, but one which is finite and diminishing. The definition of non-excludability states that it is impossible to exclude individuals from consumption. Technology now allows radio or TV broadcasts to be encrypted such that persons without a special decoder are excluded from the broadcast. Many forms of information goods 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-rivalrous. 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 both 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. 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. Debate has been generated among economists whether such a category of "public goods" exists. Steven Shavell has suggested the following: when professional economists talk about public goods they do ''not'' mean that there are a general category of goods that share the same economic characteristics, manifest the same dysfunctions, and that may thus benefit from pretty similar corrective solutions...there is merely an infinite series of particular problems (some of overproduction, some of underproduction, and so on), each with a particular solution that cannot be deduced from the theory, but that instead would depend on local empirical factors. 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 regard to the geographical region in which a good may be produced or consumed. However, some theorists such as Inge Kaul use the term "global public good" for public goods 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 has been held to be an example of a global public good,〔Joseph E. Stiglitz, "Knowledge as a Global Public Good" in ''Global Public Goods'', ISBN 978-0-19-513052-2〕 but also as a commons, the knowledge commons. Graphically, non-rivalry means that if each of several individuals has a demand curve for a public good, then the individual demand curves are summed vertically to get the aggregate demand curve for the public good. This is in contrast to the procedure for deriving the aggregate demand for a private good, where individual demands are summed horizontally. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Public good」の詳細全文を読む スポンサード リンク
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