Externalities, public goods, information gaps and the merit and demerit good distinction, and how each causes the market to misallocate resources.

What this dot point is asking

Edexcel wants you to explain the main types of market failure, distinguish private from social costs and benefits, draw and analyse externality diagrams, and explain why public goods, information gaps and merit and demerit goods lead the free market to over-produce or under-produce relative to the social optimum.

What market failure means

Edexcel distinguishes complete market failure (a missing market, as with pure public goods) from partial market failure (a market exists but produces the wrong quantity, as with externalities and merit goods).

Externalities

Negative externality of production

A coal-fired power station imposes pollution costs (poor air quality, climate change) on third parties. The firm counts only its private costs, so the supply curve reflects MPC, which lies below MSC. On a diagram with quantity on the horizontal axis and cost or benefit on the vertical, the free market produces where $MPC = MPB$ at $Q_1$, which is greater than the social optimum $Q^*$ where $MSC = MSB$. The economy over-produces, and the welfare loss is the shaded triangle between MSC and MSB from $Q^*$ to $Q_1$. The UK internalises this with carbon taxes and the UK Emissions Trading Scheme.

Negative externality of consumption

Cigarettes impose external costs (passive smoking, NHS treatment costs). Here MSB lies below MPB. The market over-consumes at $Q_1$, above the optimum $Q^*$, generating a welfare loss. UK tobacco duty shifts consumption toward the optimum.

Positive externality of consumption

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

Public goods

A quasi-public good (a beach, an uncongested road) has these features only partially: it can become rival when congested or excludable through tolls. Because the market under-provides public goods, they are typically funded by the state from taxation.

Information gaps and merit or demerit goods

An information gap (imperfect or asymmetric information) means buyers or sellers lack full information, so they make decisions that are not in their long-term interest. Asymmetric information underlies adverse selection (used-car "lemons", where good cars leave the market) and moral hazard.

• A merit good (education, healthcare, pensions) is under-consumed because individuals under-value its long-term private and external benefits, often through present bias.
• A demerit good (tobacco, junk food, gambling) is over-consumed because individuals under-value its long-term private and external costs.

The merit and demerit classification rests on a value judgement about what consumption "should" be, so it is normative.

Examples in context

• Carbon emissions. The classic negative production externality. The UK Emissions Trading Scheme caps emissions and lets firms trade allowances, pricing the external cost (around $\pounds 35$ to $\pounds 50$ per tonne of CO2 in recent years).
• 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.
• Pensions. UK auto-enrolment (from 2012) tackled the information gap and present bias behind under-saving, raising workplace pension participation from around 55 per cent to over 88 per cent of eligible employees.
• Flood defences. A textbook public good: non-rival and non-excludable, so the Environment Agency funds them from general taxation because the market would supply none.

Try this

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

• Cue. Marginal social cost exceeds marginal private cost, so the market over-produces beyond the social optimum, creating a welfare-loss triangle.

Q2. Explain why the free market fails to provide a pure public good. [3 marks]

• Cue. Non-excludability creates the free-rider problem, so firms cannot charge and earn no revenue, so the good is not supplied.