Overview
Externalities represent the unpriced consequences of economic transactions that spill over to affect uninvolved third parties. These can manifest as either costs (negative externalities) or benefits (positive externalities) not accounted for in the market price of goods or services. The classic example is environmental pollution, where the cost of degraded air or water quality is borne by society, not solely by the polluter or consumer. First systematically explored by [[alfred-marshall|Alfred Marshall]] in the late 19th century and later detailed by [[arthur-pigou|Arthur Pigou]] in the 1920s, the concept highlights market failures where private costs or benefits diverge from social costs or benefits. Understanding and addressing externalities is crucial for achieving economic efficiency and societal well-being, often through mechanisms like [[pigouvian-tax|Pigouvian taxes]] or regulations.