価格メカニズム: 税金

A Pigovian tax is a government levy on each unit of output that generates negative externalities, set equal to the external marginal cost. Named after economist Arthur Pigou, this tax forces producers to internalize societal costs by increasing production costs. The higher price reduces output to a socially optimal level, correcting market inefficiencies caused by pollution, health issues, or environmental damage.

A Pigovian tax shifts the supply curve leftward by increasing the cost of production to reflect the true social cost. This shift raises the market equilibrium price and reduces the quantity produced. The new equilibrium reflects the external marginal cost of pollution, moving the market toward a more efficient outcome that accounts for societal harm.

Coal-fired power plants overproduce because the market price reflects only private marginal costs—fuel, labor, and equipment—while ignoring the external marginal cost of pollution. This gap between private and social costs creates market failure, leading to excessive output and pollution beyond socially optimal levels. A Pigovian tax bridges this gap by making the price reflect true societal impact.

Pigovian taxes extend to cigarettes, which reduce smoking-related health issues; alcohol, which mitigates public health and safety concerns; and carbon emissions, which combat climate change. These taxes aim to correct market failures by increasing the price of goods that generate harmful side effects, encouraging more sustainable economic behavior across multiple sectors.

A Pigovian tax is a price-based intervention that adjusts the cost of production to internalize externalities. Unlike price vs quantity based interventions, which compare tax approaches to quantity controls like quotas, Pigovian taxes specifically target the external marginal cost. Both mechanisms aim to correct market failure, but Pigovian taxes work through price signals rather than output limits.

Private cost represents only the direct expenses of production, excluding external harms. A Pigovian tax adds the external marginal cost to private cost, creating a total social cost reflected in the new market price. This adjustment ensures producers and consumers bear the full societal burden of their economic decisions, eliminating the gap between private and social costs.

A Pigovian tax reduces pollution by increasing production costs and raising prices, which decreases output to a socially optimal quantity. By setting the tax equal to the external marginal cost, the government ensures that the market equilibrium reflects the true harm of pollution. This mechanism moves the economy toward the efficient level of pollution where marginal social benefit equals marginal social cost.