Understand the idea
An external effect arises when a decision affects others in ways not fully captured by the decision-maker's contracts or prices. Private incentives can then encourage too much of a harmful activity or too little of a beneficial one. Policy can change incentives through prices, quantities, standards, or institutions. [S38] The key is to identify the mechanism and who bears the effect rather than label every disliked outcome a market failure.
Worked example
An invented factory pays for fuel but not the noise disturbing nearby students. A noise charge, operating-hour rule, or negotiated insulation agreement changes different margins. Compare how each affects incentives, monitoring needs, and the distribution of costs.
Where the idea stops
Identifying an externality does not prove one policy is best. Measurement, enforcement, bargaining power, administrative costs, and distributional consequences matter. [S38]
Connect it
Culture and society: governance and collective effects.
Go deeper
Why is identifying a market failure only the start of policy analysis?
Sources
CORE Econ, The Economy 1.0 · Research reference · Date: 2017, textbook
Explains external effects and institutional/policy responses.
Link checked 2026-10-17 · Public webpage accessed; no specialist peer review of this brief.