Unit 6 Test: Market Failure and the Role of Government

When Markets Fail to Achieve Efficiency

Unit 6 addresses the conditions under which unregulated markets produce inefficient outcomes and examines the policy tools governments use to correct these failures. Externalities, public goods, and common resources are the three primary market failure categories tested in AP Microeconomics, and each has a corresponding graphical and policy framework.

Key Topics in Unit 6

Negative Externalities

A negative externality occurs when a third party bears costs that are not reflected in the market price. The classic example is pollution from production. In this case, the social cost of production exceeds the private cost, so the supply curve in the market reflects only private costs. The socially optimal quantity is lower than the market quantity. This creates a deadweight loss — overproduction relative to the efficient outcome. AP FRQs ask students to draw the negative externality graph showing the divergence between the private supply curve (MPC) and the social supply curve (MSC) and to identify the deadweight loss region.

Positive Externalities

A positive externality occurs when third parties receive benefits not captured in the market price, such as education or vaccinations. The social benefit exceeds the private benefit, meaning demand reflects only private benefits. The market underproduces relative to the socially optimal quantity. AP FRQs ask students to show the gap between the marginal private benefit (MPB) curve and the marginal social benefit (MSB) curve and identify the deadweight loss from underproduction.

Pigouvian Taxes and Subsidies

Government can correct externalities by adjusting incentives. A Pigouvian tax on a negative externality raises the private cost of production, shifting the supply curve up to align with the social cost curve and reducing output to the efficient level. A Pigouvian subsidy for a positive externality increases the private benefit, shifting the demand curve up to align with the social benefit curve and increasing output to the efficient level.

Public Goods

Public goods are non-excludable and non-rival in consumption. Because of the free-rider problem, private markets underprovide public goods — individuals cannot be excluded from enjoying them, so no one has an incentive to pay. Government provision or funding is the typical solution. AP questions ask students to distinguish public goods from private goods and to identify whether a good exhibits excludability and rivalry.

Common Resources

Common resources are rival but non-excludable. Because individuals can use them without paying, they tend to be overused — the tragedy of the commons. Examples include fisheries, groundwater, and public grazing land. Government solutions include regulation, quotas, or assigning property rights.

The Coase Theorem

The Coase theorem states that if property rights are clearly defined and transaction costs are low, private bargaining between affected parties can solve externality problems without government intervention. AP questions test conceptual understanding of when the Coase theorem applies and when it does not.

AP FRQ Skills for Unit 6

1. Draw the externality graph with private and social cost or benefit curves, labeling the market quantity, socially optimal quantity, and deadweight loss triangle.
2. Identify the correct Pigouvian tax or subsidy amount needed to achieve the socially efficient outcome.
3. Classify goods by excludability and rivalry and identify the appropriate government response.