Positive Externality vs Negative Externality
Positive Externality and Negative Externality are two Market Failure & Government concepts in AP Economics that students often mix up. In short: positive externality is a positive externality is a benefit enjoyed by a third party not involved in a transaction, such as vaccination or education. Meanwhile, negative externality is a negative externality is a cost imposed on a third party who is not part of a market transaction, such as pollution. Here is how they compare side by side.
A positive externality is a benefit enjoyed by a third party not involved in a transaction, such as vaccination or education.
Because buyers ignore these external benefits, the market underproduces relative to the socially optimal quantity. The marginal social benefit exceeds the marginal private benefit. Governments correct it with subsidies or public provision.
A negative externality is a cost imposed on a third party who is not part of a market transaction, such as pollution.
Because producers ignore these external costs, the market overproduces relative to the socially optimal quantity, creating deadweight loss. The marginal social cost exceeds the marginal private cost. Governments correct it with taxes, regulation, or tradable permits.