7.4 Private costs and benefits, externalities and social costs and benefits

Social cost is the total cost imposed by an activity on society: private cost paid by the decision-maker plus any external cost imposed on third parties. In symbols, MSC = MPC + MEC.

When an external cost is positive, marginal social cost lies above marginal private cost. If consumers and firms consider only private cost, market output is higher than the socially efficient quantity.

A delivery vehicle pays fuel and labour costs, but its congestion and emissions impose costs on others. Those external costs make the social cost of an extra delivery higher than the firm’s private cost.

Social cost is not the government’s budget cost and should not be counted twice if the private payment already compensates the affected party.

Social benefit is the total benefit to society: private benefit received by the consumer plus any external benefit received by third parties. In symbols, MSB = MPB + MEB.

With a positive external benefit, the social marginal benefit curve lies above the private marginal benefit curve. If buyers consider only private benefit, market consumption is below the socially efficient quantity.

A vaccinated person receives private protection, while lower transmission benefits others. The external benefit makes the social value of an extra vaccination greater than the individual’s private value.

A positive externality is not simply a product that is popular; it requires a benefit spilling over to people outside the transaction.

A negative externality imposes an uncompensated cost on a third party; a positive externality gives an uncompensated benefit. They can arise in consumption or production.

The key test is whether the affected person is outside the buyer-seller decision and whether the spillover is omitted from the market price. Negative externalities usually cause overproduction or overconsumption; positive ones cause underproduction or underconsumption.

Noise from a nightclub is a negative consumption or production spillover depending on the source. A firm’s research that other firms learn from creates a positive production spillover.

Externality signs are about the direction of the spillover, not whether the activity itself is “good” or “bad” in every context.

A negative production externality makes MSC exceed MPC, while a negative consumption externality makes MSB fall below MPB. Positive externalities reverse the relevant gap.

Label the private and social curves first, then compare the market intersection with the social intersection where MSB = MSC. The diagram shows the direction of misallocation; it does not by itself choose the best policy.

If education creates benefits for classmates, MSB lies above MPB and the market consumes too little. If a chemical factory pollutes a river, MSC lies above MPC and the market produces too much.

Do not shift both curves automatically: production spillovers change cost, consumption spillovers change benefit, and some activities can contain both.

Deadweight welfare loss is the net social surplus lost when output differs from the efficient quantity. Mutually beneficial trades are missed, or resources are used where their social cost exceeds their social benefit.

On a standard diagram it is the area between the relevant marginal social benefit and marginal social cost curves over the units between market and efficient output. The shape and size depend on elasticities and the gap.

A monopoly that restricts output below the point where P=MC creates a triangle of lost surplus: consumers who value an extra unit above its cost cannot buy it.

A transfer from consumer to producer is not automatically deadweight loss; the loss is the surplus that disappears, not merely who receives it.

Asymmetric information exists when one party knows more relevant information than another. Moral hazard occurs when someone takes more risk because another party bears part of the consequences after an agreement.

Before a transaction, hidden information can cause adverse selection; after it, hidden action can create moral hazard. Monitoring, contracts, deductibles and disclosure can reduce—but not always remove—the problem.

An insured driver may take less care because the insurer covers much of the loss. A larger excess or monitoring device makes the driver face more of the marginal cost.

Moral hazard is not simply dishonesty and does not require a hidden type; it is a changed action caused by the incentive structure after protection is provided.

Cost-benefit analysis estimates the social costs and social benefits of a project, including external effects that do not have a market price. A project is worthwhile in the basic test when estimated net social benefit is positive.

Analysts must define the counterfactual, value time, uncertainty and distribution, and avoid double counting. Shadow prices or stated preferences may be used for non-market effects, but they are estimates rather than exact facts.

A new rail line has construction and operating costs, time savings, fare revenue, congestion relief and noise. Counting only ticket revenue would understate the social benefit; counting both time savings and an already included wage payment could double count.

A positive net present value is not a guarantee: assumptions, discount rate, distribution and omitted risks can change the decision.