6.4 Positive Externalities

What is a positive externality?

A positive externality occurs when a third party not involved in a transaction receives a benefit from it, without paying for it. Examples include education (raises productivity of others), vaccinations (reduces disease spread), and public research. Because producers/consumers don't capture the full benefit, the social benefit of the good exceeds the private benefit.

Market outcome vs. socially optimal outcome

When positive externalities exist, marginal social benefit (MSB) > marginal private benefit (MPB). The free market equilibrium (where MPB = MSC) produces less than the socially optimal quantity, leading to underproduction and underconsumption. This creates a deadweight loss because the MSB curve lies above the demand curve, and too little of the good is made relative to what's efficient.

Government correction: subsidies

Governments can correct positive externalities using a per-unit subsidy to producers or consumers, shifting the supply (or demand) curve to increase output toward the socially optimal quantity (Qoptimal), where MSB = MSC. This eliminates the deadweight loss. Other tools include public provision (like public education) or mandates (like required vaccinations).

Key terms

Positive externality

A benefit received by a third party from a transaction they weren't involved in, without paying for it.

Marginal social benefit (MSB)

The total benefit to society from an additional unit of a good, including external benefits to third parties.

Underproduction

When a market produces less than the socially optimal quantity, common in markets with positive externalities.

Subsidy

A government payment that lowers production or consumption costs, used to correct underproduction from positive externalities.

Socially optimal quantity

The output level where marginal social benefit equals marginal social cost (MSB = MSC), maximizing societal welfare.

Try an AP-style question

What is the MSB?

1. $3,000
2. $5,000
3. $1,000
4. $4,000