16.18: The Optimal Level of Public Goods

Public goods are services or commodities that are non-rival, meaning all members of society can consume the good without diminishing the quality or availability of the good to anyone. Public goods also have the characteristic of non-excludability, where it is not economically feasible for private firms to exclude non-paying consumers of the goods.

This combination of nonrivalry and non-excludability prevents the private sector from providing the socially optimal level of public goods. As a result, the government can use its ability to collect payment from all potential consumers (via taxes) to provide public goods at socially optimal quantities. Examples include national defense, public parks, and street lighting. However, the challenge is to determine the socially optimal quantity of a public good to provide.

Marginal Benefit and Marginal Cost

1. Marginal benefit refers to the additional satisfaction or utility an individual receives from consuming one more unit of a good or service. For public goods, different individuals may place different values on the good. For example, some people may value a public park highly, while others may value clean air relatively more.
2. Marginal cost is the expense incurred by the government or provider to produce one additional unit of the public good. This could include costs of construction, maintenance, or other related expenses.

Total Marginal Benefit

The total marginal benefit is the sum of the individual marginal benefits received by all members of society. Since public goods are non-rivalrous, meaning one person's use does not reduce availability for others, the combined benefit represents the value society places on the good as a whole.

Finding the Efficient Quantity

The efficient quantity of a public good is found where the total marginal benefit equals the marginal cost. This is the point at which the additional cost of providing one more unit is balanced by the benefit it provides to society.

Private Market and Underprovision

In a private market, individuals would only be willing to pay up to the point where their personal marginal benefit equals the marginal cost. As a result, the quantity provided would be less than the efficient level, leading to underprovision of the public good. This is why governments step in to supply public goods, ensuring they are available at socially optimal levels.

In finding the optimal level of public goods, consider national defense as an example.

Suppose a nation has two individuals, John and Jane. The graph shows their marginal benefit curves for national defense. Their combined benefit is shown by the total marginal benefit curve, which is simply John's and Jane's marginal benefits added together vertically rather than horizontally, as it would be in a private market. This is because the public good is non-rivalrous, and additional units of the public good need not be produced to combine their benefits.

The marginal cost curve shows the cost of providing each additional unit of national defense.

The most efficient quantity of national defense is found where the total marginal benefit equals the marginal cost.

In a private market, however, each person would only be willing to pay up to the point where their individual marginal benefit equals the marginal cost. This would be at quantities Q1 for John and Q2 for Jane, which are less than the efficient quantity.

This discrepancy occurs because individuals in a market account only for the benefit they receive from their consumption.

As a result, in a private market, national defense would be underprovided. This is one reason why governments often provide such public goods.

• Public Good - A service or commodity that is non-rival and non-excludable, and is often provided by the government.
• Non-rival - A characteristic where one's use doesn't reduce availability to others.
• Non-excludable - A feature where it's impossible to prevent non-payers from accessing the good.
• Optimal Quantity of a Public Good - The level at which marginal benefit equals marginal cost.
• Marginal Benefit and Marginal Cost - The additional satisfaction vs. the cost of providing one more unit of a public good.

• Define Public Good – Understand its characteristics and importance (e.g., non-rival and non-excludable).
• Contrast Public Good and Private Goods – Identify key differences (e.g., availability, excludability).
• Explore Public Good Examples – Understand real-life instances (e.g., national defence, street lighting).
• Explain Optimal Quantity – Describe how it is achieved when marginal benefit equals marginal cost.
• Explore Underprovision in the Private Market – Discuss the underprovision of public goods in private markets and the role of government.

• What is a Public Good and how can its optimal quantity be determined?
• What are some examples of Public Goods?
• What happens when Public Goods are provided in a private market?

• Students – Provides understanding of Public Goods, its characteristics, and their importance in society.
• Educators – Aids in teaching complex economic concepts like public goods and market failure.
• Researchers – Facilitates in studying economic phenomena and understanding policy implications.
• Economy Enthusiasts – Offers insights into public finance and the role of government in providing public goods.