externalitet

An externalitet, in economics, is a cost or benefit arising from an economic activity that affects third parties who are not directly involved in the decision and is not reflected in market prices. Externalities can be negative (costs imposed on others) or positive (benefits conferred on others). They often arise because markets fail to internalize external effects.

Because externalities cause the social costs or benefits to differ from private ones, resource allocation can be inefficient. Producers or consumers may ignore external effects, leading to overproduction of polluting goods or underproduction of beneficial activities such as vaccination or research spillovers. Externalities can be generated by production or consumption and may be local or widespread, even global (e.g., climate change).

Policy responses aim to align private incentives with social outcomes. Tools include Pigouvian taxes or subsidies, regulation, and tradable permits. The Coase theorem argues that if property rights are well-defined and transaction costs are low, private bargaining can achieve efficient outcomes regardless of initial allocation. In some cases government provision or financing of public goods is preferred.

Measuring externalities is challenging due to attribution, magnitude, and time horizons. Analysts often use cost-benefit analysis, social marginal costs/benefits, and empirical estimates to assess policy options. The concept is central to welfare economics and the analysis of market failures.