Revise Private costs and benefits, externalities and social costs and benefits for Cambridge 9708 (7.4)

In a free market, economic agents (consumers and producers) make decisions based on their own self-interest. Producers decide how much to supply based on their private costs of production, represented by the Marginal Private Cost (MPC) curve. This curve is effectively the firm's supply curve. Consumers decide how much to demand based on the utility or satisfaction they receive, represented by the Marginal Private Benefit (MPB) curve, which is the demand curve. The market equilibrium occurs where MPC equals MPB. At this point, the price (P_mkt) and quantity (Q_mkt) are determined. This equilibrium is privately optimal, meaning no individual consumer or producer can be made better off by changing their behaviour, assuming they only consider their own costs and benefits.

A negative externality is a cost imposed on a third party who is not directly involved in the production or consumption of a good. A classic example is pollution from a factory. These external costs are not factored into the producer's decision-making. The true cost to society is the Social Cost, which is the sum of the private cost and the external cost. This is shown by the Marginal Social Cost (MSC) curve, where MSC = MPC + External Cost. Because the MSC is higher than the MPC, the socially optimal level of output (Q_soc), where MSC = MSB, is lower than the market equilibrium output (Q_mkt). The free market therefore results in overproduction and a deadweight welfare loss, representing a misallocation of resources and market failure.

A positive externality is a benefit conferred on a third party from the production or consumption of a good. For instance, an individual receiving a vaccination not only protects themselves (private benefit) but also reduces the likelihood of others in the community contracting the disease (external benefit). The full benefit to society is the Social Benefit, which is the sum of the private benefit and the external benefit. This is represented by the Marginal Social Benefit (MSB) curve, where MSB = MPB + External Benefit. As the MSB is greater than the MPB, the socially optimal level of consumption (Q_soc) is higher than the market equilibrium (Q_mkt). The free market leads to under-consumption and a potential welfare gain that is not realised, creating a deadweight welfare loss.

The presence of externalities signifies market failure because the free market fails to achieve allocative efficiency. Allocative efficiency occurs at the output level where society's welfare is maximised, which is where Marginal Social Benefit (MSB) equals Marginal Social Cost (MSC). In a market with externalities, the private equilibrium (MPC = MPB) does not coincide with this social optimum. The divergence creates a deadweight welfare loss, shown on a diagram as the triangle pointing towards the socially optimal quantity. This triangle represents the total loss of consumer and producer surplus to society resulting from the overproduction (negative externality) or under-consumption (positive externality). The goal of government intervention, such as taxes or subsidies, is to correct this market failure and move the market towards the allocatively efficient output.

Markets equate MPC = MPB (private equilibrium). Society's optimum is where MSC = MSB.

MSC = MPC + marginal external cost (MEC) MSB = MPB + marginal external benefit (MEB)

When external costs exist, MSC lies above MPC. When external benefits exist, MSB lies above MPB.

Social optimum: $MSB = MSC$

Negative externality: $Q_{market} > Q_{social}$

Positive externality: $Q_{market} < Q_{social}$

Example: factory pollution. Producers consider only MPC, not harm to neighbours. Market produces too much — Q_market > Q_social.

Corrective policies:

• Pigouvian tax per unit = MEC at Q_social
• Regulation (emission limits, permits)
• Tradable pollution permits (cap and trade)

Example: education or vaccinations. Individual considers only MPB, not benefits to society (educated workforce, herd immunity). Market produces too little — Q_market < Q_social.

Corrective policies:

• Subsidies per unit = MEB at Q_social
• Direct provision (state schools, free vaccinations)
• Information campaigns to raise perceived benefits