1.3 Externalities: positive and negative externalities of production and consumption, the divergence of private and social costs and benefits, the welfare loss, and the social optimum.

What this dot point is asking

OCR wants you to define externalities, distinguish private from social costs and benefits, draw and analyse the four externality diagrams, identify the welfare loss, and explain why each type makes the free market over-produce or under-produce relative to the social optimum.

What an externality is

The key to every externality diagram is the gap between private and social curves. The social optimum is where marginal social benefit equals marginal social cost; the free market produces where marginal private benefit equals marginal private cost. Whenever the two differ, there is a welfare loss.

The four externalities

Negative externality of production

A coal-fired power station emits pollution (poor air quality, climate change) borne by third parties. The firm counts only private costs, so supply reflects MPC, which lies below MSC. The free market produces where $MPC = MPB$ at $Q_1$, beyond the social optimum $Q^*$ where $MSC = MSB$. The economy over-produces, and the welfare loss is the triangle between MSC and MSB from $Q^*$ to $Q_1$.

Negative externality of consumption

Sugary drinks impose external costs (obesity, NHS treatment). Here MSB lies below MPB. The market over-consumes at $Q_1$, above the optimum $Q^*$, creating a welfare loss. The policy response is a sugar tax to shift consumption toward the optimum.

Positive externality of production

A firm that trains workers, or invests in research that others can copy, creates external benefits, so MSC lies below MPC (the true social cost is lower). The market under-produces relative to the optimum, the rationale for research subsidies and apprenticeship support.

Positive externality of consumption

Vaccination and education generate external benefits (herd immunity, a more productive society). MSB lies above MPB, so the free market under-consumes at $Q_1$, below the optimum $Q^*$. The missing output is a welfare loss, the rationale for free schooling and subsidised vaccination.

The social optimum and welfare loss

Examples in context

• Carbon emissions. The classic negative production externality. The UK Emissions Trading Scheme caps emissions and lets firms trade allowances, putting a price on the external cost.
• Vaccination. During the COVID-19 rollout, the external benefit of herd immunity meant the social optimum exceeded what individuals would privately choose, justifying free, heavily promoted provision.
• Education. A positive consumption externality: a more skilled, productive and law-abiding society benefits everyone, the rationale for compulsory, state-funded schooling.

Try this

Q1. Explain why a negative externality of production leads to a welfare loss. [4 marks]

• Cue. MSC exceeds MPC, so the market over-produces beyond the optimum, creating a welfare-loss triangle.

Q2. Using an example, explain why a positive externality of consumption leads to under-consumption. [4 marks]

• Cue. MSB above MPB (for example vaccination), so the market consumes less than the optimum where $MSB = MSC$.