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A negative externality is a cost imposed on unrelated third parties who are not involved in an economic transaction. These external costs, such as pollution from a coal-fired power plant, are not reflected in the market price. As a result, consumers pay less than the true social cost, leading to overproduction and market inefficiency.

Private marginal cost represents only the expenses incurred by a producer, such as fuel and labor. Social marginal cost includes both private costs and external marginal costs, such as pollution damage. The difference between these two costs reveals the hidden burden that society bears but the market does not account for in pricing.

Overproduction occurs because the market equilibrium is determined by private marginal cost, which is lower than social marginal cost. Producers and consumers face artificially low prices that do not include external costs. The socially optimal quantity, where social marginal cost meets demand, is lower than the market quantity, creating deadweight loss.

Deadweight loss is the reduction in social welfare that occurs when too many units of a good are produced due to unpriced external costs. The triangle on a supply-demand graph shows the loss of economic efficiency. This loss demonstrates that the market fails to allocate resources optimally when negative externalities are not accounted for in pricing.

Pollution taxes and emissions permits are corrective measures that align private incentives with social welfare. These interventions increase the cost of production to reflect external damages, shifting the effective supply curve upward. By making producers bear the full social cost, these mechanisms reduce overproduction and move the market toward the socially optimal quantity.

Market prices reflect only the private costs borne by producers and consumers, not the external costs imposed on third parties. When a chemical plant discharges untreated wastewater, downstream communities suffer health and environmental damage, but neither the plant nor its customers pay for these costs. This market failure occurs because property rights to clean water are not clearly defined or enforced.

Negative externalities cause market failure because the market price does not reflect the true social cost of production. When external costs are ignored, the market produces more output than is socially optimal, resulting in inefficient resource allocation. Corrective policies are necessary to internalize external costs and restore market efficiency.