Concept Architecture
Concept
Theoretically, Externality is an unintended cost or benefit arising from the production or consumption of a good or service that affects third parties and is not reflected in market prices. Externalities occur because decision-makers do not bear the full social costs or receive the full social benefits of their actions, leading to market failure. In health economics, externalities are particularly important in vaccination, communicable disease control, antimicrobial resistance, environmental health, and public health interventions.
Mathematically, externalities are represented by the divergence between private and social costs or benefits. Negative externalities arise when marginal social cost exceeds marginal private cost, whereas positive externalities arise when marginal social benefit exceeds marginal private benefit. Efficient resource allocation is achieved where marginal social benefit equals marginal social cost.
In practice, externalities are quantified using epidemiological, environmental, and economic data to estimate spillover effects beyond directly affected individuals. They are incorporated into economic evaluations, public health analyses, and policy models to assess interventions such as vaccination programmes, smoking restrictions, pollution control, and infection prevention measures. Government interventions, including taxes, subsidies, and regulation, are commonly evaluated to internalise externalities.
Purpose
Used to quantify spillover costs and benefits affecting third parties, supporting analyses of market failure, public health interventions, resource allocation, and economic policy.
Mathematical Formulae
Primary Formula
Social equilibrium:
MSB = MSC
where:
- MSB = marginal social benefit
- MSC = marginal social cost
Supporting Formulae
Negative externality:
MSC = MPC + MEC
Positive externality:
MSB = MPB + MEB
where:
- MPC = marginal private cost
- MPB = marginal private benefit
- MEC = marginal external cost
- MEB = marginal external benefit
Related Mathematical Methods
- Welfare economics
- Cost-benefit analysis
- Pigouvian taxation
- Subsidy analysis
- Social welfare optimisation
- Economic surplus analysis
Example
A vaccination programme costs �40 per person and provides a private benefit valued at �55. Reduced disease transmission generates an additional external benefit valued at �20.
The marginal social benefit is:
MSB = 55 + 20 = �75
Since the marginal social benefit exceeds the programme cost, inclusion of the positive externality strengthens the economic justification for vaccination.
Excel Implementation
| Function | Example Formula | Health Economics Application |
|---|---|---|
| SUM | =SUM(B2:C2) | Calculates marginal social benefit by adding private and external benefits. |
| SUM | =SUM(D2:E2) | Calculates marginal social cost by adding private and external costs. |
| IF | =IF(F2>G2,""Intervention justified"",""Not justified"") | Compares social benefits and costs for policy evaluation. |
| SUMPRODUCT | =SUMPRODUCT(B2:B20,C2:C20) | Estimates aggregate external benefits or costs across a population. |
VBA (Optional)
Automate estimation of social costs and benefits by incorporating external effects into economic evaluation models and policy simulations.
Sources
- Pigou AC. The Economics of Welfare.
- Varian HR. Intermediate Microeconomics: A Modern Approach.
- Pindyck RS, Rubinfeld DL. Microeconomics.
- Folland S, Goodman AC, Stano M. The Economics of Health and Health Care.
- Drummond MF, et al. Methods for the Economic Evaluation of Health Care Programmes. Oxford University Press.
Related Concepts (3)
Frequently Asked Questions (6)
What is an externality?
A cost or benefit from an economic activity that falls on a third party and is not reflected in the activity's market price.
Source: Pigou 1920
Who developed the concept of externalities?
The idea grew from the work of Alfred Marshall and was developed by Arthur Pigou in the early twentieth century, who analysed how the private cost or benefit of an activity can diverge from its cost or benefit to society. Pigou argued that where such a gap exists, private decisions taken in a market will not reach the outcome that is best for society as a whole. This reasoning underpins the case for taxes and subsidies on activities with spillover effects. Pigou (1920) set out the analysis.
Source: Pigou 1920
What is the difference between a positive and a negative externality?
A negative externality is a cost imposed on third parties, such as pollution, not reflected in the price, so the activity is overproduced relative to what is socially best. A positive externality is a benefit conferred on third parties, such as vaccination reducing others' risk of infection, not reflected in the price, so the activity is underproduced. In both cases the market price omits the effect on third parties, leading to a quantity that differs from the social optimum.
Source: Pigou 1920
Why do externalities cause market failure?
Externalities cause market failure because decision makers weigh only their private costs and benefits, ignoring the effects on third parties, so the market quantity does not match the socially efficient quantity. Activities with external costs are overproduced, since producers do not bear the full cost, and activities with external benefits are underproduced, since producers are not paid for the benefit to others. The gap between private and social cost or benefit is the source of the inefficiency.
Source: Pigou 1920
How can externalities be corrected?
Externalities can be corrected by making decision makers face the full social cost or benefit of their actions. Pigou proposed taxes on activities with external costs and subsidies for those with external benefits, set to the size of the external effect, so private incentives align with social ones. Other remedies include regulation, assigning property rights so parties can bargain, and public provision. Each aims to close the gap between private and social cost or benefit.
Source: Pigou 1920
What externalities arise in health care?
Health care generates important externalities, especially in communicable disease. Vaccination and treatment of infections confer positive externalities by reducing the risk others face, so private demand falls short of the socially efficient level, justifying subsidy or public provision. Conversely, activities that spread disease or antimicrobial resistance impose negative externalities. These external effects are a central reason why health care and public health are not left wholly to the market, since private choices would underprovide protection that benefits others.
Source: Pigou 1920
Trust Record
Verified by Dr Darrin Baines
British health economist
Professional identity: darrinbaines.org
Verification date: 10 Sep 2025
Content version: 1.0.0
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- Persistent URI
- https://healtheconomics.wiki/concept/externality
- Term code
- HE-EE-ME-023
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