Public good

In economics, a public good is a commodity, product or service that is both non-excludable and non-rivalrous and which is typically provided by a government and paid for through taxation. Use by one person neither prevents access by other people, nor does it reduce availability to others, so the good can be used simultaneously by more than one person. This is in contrast to a common good, such as wild fish stocks in the ocean, which is non-excludable but rivalrous to a certain degree. If too many fish were harvested, the stocks would deplete, limiting the access of fish for others. A public good must be valuable to more than one user, otherwise, its simultaneous availability to more than one person would be economically irrelevant.
Capital goods may be used to produce public goods or services that are "...typically provided on a large scale to many consumers." Similarly, using capital goods to produce public goods may result in the creation of new capital goods. In some cases, public goods or services are considered "...insufficiently profitable to be provided by the private sector...., in the absence of government provision, these goods or services would be produced in relatively small quantities or, perhaps, not at all."
Public goods include knowledge, official statistics, national security, common languages, law enforcement, broadcast radio, flood control systems, aids to navigation, and street lighting. Collective goods that are spread all over the face of the Earth may be referred to as global public goods. This includes physical book literature, but also media, pictures and videos. For instance, knowledge can be shared globally. Information about men's, women's and youth health awareness, environmental issues, and maintaining biodiversity is common knowledge that every individual in the society can get without necessarily preventing others access. Also, sharing and interpreting contemporary history with a cultural lexicon is another source of knowledge that the people can freely access.
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. Public goods may also become subject to restrictions on access and may then be considered to be club goods; exclusion mechanisms include toll roads, congestion pricing, and pay television with an encoded signal that can be decrypted only by paid subscribers.
There is debate in the literature on the definition of public goods, how to measure the significance of public goods problems in an economy, and how to identify remedies.

Academic literature

is usually credited as the economist who articulated the modern theory of public goods in a mathematical formalism, building on earlier work of Wicksell and Lindahl. 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:

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...

Many mechanisms have been proposed to achieve efficient public goods provision in various settings and under various assumptions.

Lindahl tax

A Lindahl tax is a type of taxation brought forward by Erik Lindahl, an economist from Sweden, in 1919. His idea was to tax individuals for the provision of a public good according to the marginal benefit they receive. Public goods are costly and eventually someone needs to pay the cost. It is difficult to determine how much each person should pay. So, Lindahl developed a theory of how the expense of public utilities needs to be settled. His argument was that people would pay for the public goods according to the way they benefit from the good. The more a person benefits from these goods, the higher the amount they pay. People are more willing to pay for goods that they value. Taxes are needed to fund public goods and people are willing to bear the burden of taxes. Additionally, the theory dwells on people's willingness to pay for the public good. From the fact that public goods are paid through taxation according to the Lindahl idea, the basic duty of the organization that should provide the people with this services and products is the government.

Vickrey–Clarke–Groves mechanism

s are one of the best-studied procedures for funding public goods. VCG encompasses a wide class of similar mechanisms, but most work focuses on the Clarke Pivot Rule which ensures that all individuals pay into the public good and that the mechanism is individually rational.
The main issue with the VCG mechanism is that it requires a very large amount of information from each user. Participants may not have a detailed sense of their utility function with respect to different funding levels. Compare this with other mechanisms that only require users to provide a single contribution amount. This, among other issues, has prevented the use of VCG mechanisms in practice. However, it is still possible that VCG mechanisms could be adopted among a set of sophisticated actors.

Quadratic funding

Quadratic funding is one of the newest innovations in public goods funding mechanisms. The idea of Quadratic voting was turned into a mechanism for public goods funding by Buterin, Hitzig, and Weyl and is now referred to as quadratic funding.
Quadratic funding has a close theoretical link with the VCG mechanism, and like VCG, it requires a subsidy in order to induce incentive compatibility and efficiency. Both mechanisms also fall prone to collusion between players and sybil attacks. However, in contrast to VCG, contributors only have to submit a single contribution – the total contribution to the public good is the sum of the square roots of individual contributions. It can be proved that there is always a deficit that the mechanism designer must pay.
One technique to reduce collusion is to identify groups of contributors that will likely coordinate and lower the subsidy going to their preferred causes.

Assurance contracts

First proposed by Bagnoli and Lipman, In 1989, assurance contracts have each funder agree to spend a certain amount towards a public good conditional on the total funding being sufficient to produce the good. If not everyone agrees to the terms, then no money is spent on the project. Donors can feel assured that their money will only be spent if there is sufficient support for the public good. Assurance contracts work particularly well with smaller groups of easily identifiable participants, especially when the game can be repeated.
Several crowdfunding platforms such as Kickstarter and IndieGoGo have used assurance contracts to support various projects. Assurance contracts can be used for non-monetary coordination as well, for example, Free State Project obtained mutual commitments for 20,000 individuals to move to New Hampshire in a bid to influence the politics of the state.
Alex Tabarrok suggested a modification called dominant assurance contracts where the mechanism designer gives every contributor a refund bonus if the contract fails. For example, in addition to returning their contributions, the mechanism designer might give all contributors an additional $5 if the total donations are not sufficient to support the project. If there is a chance that the contract will fail, a refund bonus incentivizes people to participate in the mechanism, making the all-pay equilibrium more likely. This comes with the drawback that the mechanism designer must pay the participants in some cases, which is a common theme.
Zubrickas proposed a simple modification of dominant assurance contracts where people are given a refund bonus proportional to the amount they offered to donate, this incentivizes larger contributions than the fixed refund from Tabarrok's original proposal. There have been many variations on the idea of conditional donations towards a public good. For example, the Conditional Contributions Mechanism allows donors to make variable sized commitments to fund the project conditional on the total amount committed. Similarly, the Binary Conditional Contributions Mechanism allows users to condition their donation on the number of unique funders. Extensions such as the Street Performer Protocol consider time-limited spending commitments.

Lotteries

have historically been used as a means to finance public goods. Morgan initiated the first formal study of lotteries as a public goods funding mechanism. Since then, lotteries have undergone extensive theoretical and experimental research. Combined with their historical success, lotteries are a promising crowdfunding mechanism.
They work by using an external source of funding to provide a lottery prize. Individual "donors" buy lottery tickets for a chance to receive the cash prize, knowing that ticket sales will be spent towards the public good. A winner is selected randomly from one of the tickets and the winner receives the entire lottery prize. All lottery proceeds from ticket sales are spent towards the public good.
Like the other mechanisms, this approach requires subsidies in the form of a lottery prize in order to function. It can be shown that altruistic donors can generate more funding for the good by donating towards the lottery prize rather than buying tickets directly.
Lotteries are approximately efficient public goods funding mechanisms and the level of funding approaches the optimal level as the prize grows. However, in the limit of large populations, contributions from the lottery mechanism converge to that of voluntary contributions and should fall to zero.

Role of non-profits

Public goods provision is in most cases part of governmental activities. In the introductory section of his book, Public Good Theories of the Nonprofit Sector, Bruce R. Kingma stated that;

In the Weisbrod model nonprofit organizations satisfy a demand for public goods, which is left unfilled by government provision. The government satisfies the demand of the median voters and therefore provides a level of the public good less than some citizens'-with a level of demand greater than the median voter's-desire. This unfilled demand for the public good is satisfied by nonprofit organizations. These nonprofit organizations are financed by the donations of citizens who want to increase the output of the public good.