The Optimal Level of Public Goods

Public goods are non-rivalrous, meaning one person's consumption doesn't reduce availability for others. Therefore, total marginal benefit is calculated by adding individual marginal benefits vertically rather than horizontally. This reflects that society receives the combined benefit of all individuals consuming the same unit simultaneously, unlike private goods where consumption is divided among consumers.

The efficient quantity of a public good occurs where total marginal benefit equals marginal cost. This point balances the additional cost of providing one more unit against the benefit it delivers to society. At this intersection, the government provides the socially optimal level, ensuring the good is available at the quantity that maximizes overall social welfare.

In private markets, individuals only pay for benefits they personally receive. Each person is willing to pay only up to their individual marginal benefit equals marginal cost. Since private consumers ignore benefits others receive, the quantity supplied falls below the socially optimal level, resulting in underprovision of public goods like national defense.

Public goods exhibit non-excludability, making it economically infeasible for private firms to prevent non-paying consumers from benefiting. Governments can collect payment from all potential consumers through taxes and provide public goods at socially optimal quantities. This ensures adequate provision of essential services like national defense, public parks, and street lighting.

Marginal benefit is the additional satisfaction an individual receives from one more unit; different people value public goods differently. Marginal cost is the government's expense to produce that additional unit. For public goods, society's total marginal benefit combines all individual benefits, while marginal cost remains the provider's production expense.

Non-excludability means it is not economically feasible for private firms to exclude non-paying consumers from benefiting from a public good. Once provided, everyone can access it regardless of payment. This characteristic, combined with non-rivalry, prevents private markets from supplying public goods efficiently, necessitating government intervention.

The efficient quantity, where total marginal benefit equals marginal cost, exceeds what private markets supply. Private consumers only account for their individual marginal benefit, leading to underprovision. Government provision at the efficient quantity ensures society receives the full social benefit, making public goods available at socially optimal levels rather than insufficient private market quantities.