A public goodpublic goodA good that has the attributes of nonexclusivity and nonrivalry. is a good that has two attributes: NonexcludabilitynonexcludabilityCondition in which the producer can’t prevent the use of the good by others., which means the producer can’t prevent the use of the good by others, and nonrivalrynonrivalryCondition in which many people can use a good simultaneously., which means that many people can use the good simultaneously. The classic example of a public good is national defense. National defense is clearly nonexcludable because, if we spend the resources necessary to defend our national borders, it isn’t going to be possible to defend everything except one apartment on the second floor of a three-story apartment building on East Maple Street. Once we have kept our enemies out of our borders, we’ve protected everyone within the borders. Similarly, the defense of the national borders exhibits a fair degree of nonrivalry, especially insofar as the strategy of defense is to deter an attack in the first place. That is, the same expenditure of resources protects all. It is theoretically possible to exclude some people from the use of a poem or a mathematical theorem, but exclusion is generally quite difficult. Both poems and theorems are nonrivalrous. Similarly, technological and software inventions are nonrivalrous, even though a patent grants the right to exclude the use by others. Another good that permits exclusion at a cost is a highway. A toll highway shows that exclusion is possible on the highways. Exclusion is quite expensive, partly because the tollbooths require staffing, but mainly because of the delays imposed on drivers associated with paying the tolls—the time costs of toll roads are high. Highways are an intermediate case where exclusion is possible only at a significant cost, and thus should be avoided if possible. Highways are also rivalrous at high-congestion levels, but nonrivalrous at low-congestion levels. That is, the marginal cost of an additional user is essentially zero for a sizeable number of users, but then marginal cost grows rapidly in the number of users. With fewer than 700 cars per lane per hour on a four-lane highway, generally the flow of traffic is unimpeded.^[23] As congestion grows beyond this level, traffic slows down and congestion sets in. Thus, west Texas interstate highways are usually nonrivalrous, while Los Angeles’s freeways are usually very rivalrous.

Like highways, recreational parks are nonrivalrous at low-use levels, becoming rivalrous as they become sufficiently crowded. Also like highways, it is possible, but expensive, to exclude potential users, since exclusion requires fences and a means for admitting some but not others. (Some exclusive parks provide keys to legitimate users, while others use gatekeepers to charge admission.)

Take the example of a neighborhood association that is considering buying land and building a park in the neighborhood. The value of the park is going to depend on the size of the park, and we suppose for simplicity that the value in dollars of the park to each household in the neighborhood is where n is the number of park users, S is the size of the park, and a and b are parameters satisfying 0 < a ≤ b < 1. This functional form builds in the property that larger parks provide more value at a diminishing rate, but there is an effect from congestion. The functional form gives a reason for parks to be public—it is more efficient for a group of people to share a large park than for each individual to possess a small park, at least if b > a, because the gains from a large park exceed the congestion effects. That is, there is a scale advantage—doubling the number of people and the size of the park increases each individual’s enjoyment.

How much will selfish individuals voluntarily contribute to the building of the park? That of course depends on what they think others will contribute. Consider a single household, and suppose that each household, i, thinks the others will contribute S_-1 to the building of the park. Given this expectation, how much should each household, i, contribute? If the household contributes s, the park will have size S = S _{− 1} + s, which the household values at Thus, the net gain to a household that contributes s when the others contribute S_{− 1} is

Exercise 1 shows that individual residents gain from their marginal contribution if and only if the park is smaller than Consequently, under voluntary contributions, the only equilibrium park size is S₀. That is, for any park size smaller than S₀, citizens will voluntarily contribute to make the park larger. For any larger size, no one is willing to contribute.

Under voluntary contributions, as the neighborhood grows in number, the size of the park shrinks. This makes sense—the benefits of individual contributions to the park mostly accrue to others, which reduces the payoff to any one contributor.

How large should the park be? The total value of the park of size S to the residents together is n times the individual value, which gives a collective value of and the park costs S, so from a social perspective the park should be sized to maximize which yields an optimal park of size Thus, as the neighborhood grows, the park should grow, but as we saw, the park would shrink if the neighborhood has to rely on voluntary contributions. This is because people contribute individually, as if they were building the park for themselves, and don’t account for the value they provide to their neighbors when they contribute. Under individual contributions, the hope that others contribute leads individuals not to contribute. Moreover, use of the park by others reduces the value of the park to each individual, so that the size of the park shrinks as the population grows under individual contributions. In contrast, the park ought to grow faster than the number of residents grows, as the per capita park size is which is an increasing function of n.^[24]

The lack of incentive for individuals to contribute to a social good is known as a free-rider problemfree-rider problemThe lack of incentive for individuals to contribute to a social good.. The term refers to the individuals who don’t contribute to the provision of a public good, who are said to be free ridersfree ridersIndividuals who don’t contribute to the provision of a public good., that is, they ride freely on the contributions of others. There are two aspects of the free-rider problem apparent in this simple mathematical model. First, the individual incentive to contribute to a public good is reduced by the contributions of others, and thus individual contributions tend to be smaller when the group is larger. Put another way, the size of the free-rider problem grows as the community grows larger. Second, as the community grows larger, the optimal size of the public good grows. The market failure under voluntary contributions is greater as the community is larger. In the theory presented, the optimal size of the public good is and the actual size under voluntary contributions is a gap that gets very large as the number of people grows.

The upshot is that people will voluntarily contribute too little from a social perspective, by free riding on the contributions of others. A good example of the provision of public goods is a coauthored term paper. This is a public good because the grade given to the paper is the same for each author, and the quality of the paper depends on the sum of the efforts of the individual authors. Generally, with two authors, both work pretty hard on the manuscript in order to get a good grade. Add a third author, and it is a virtual certainty that two of the authors will think the third didn’t work as hard and is a free rider on the project.

The term paper example also points to the limitations of the theory. Many people are not as selfish as the theory assumes and will contribute more than would be privately optimal. Moreover, with small numbers, bargaining between the contributors and the division of labor (each works on a section) may help to reduce the free-rider problem. Nevertheless, even with these limitations, the free-rider problem is very real; and it gets worse the more people are involved. The theory shows that if some individuals contribute more than their share in an altruistic way, the more selfish individuals contribute even less, undoing some of the good done by the altruists.