Externality

An externality is a cost or benefit that falls on someone who was not party to the transaction that caused it — a factory's downstream pollution, a vaccinated person's protection of others, a noisy party's effect on neighbours. Left alone, a transaction with a negative externality happens too often, because the price the parties negotiate omits the cost imposed on everyone else; a positive one happens too rarely, for the mirror reason.

Two classic fixes disagree about where the correction belongs. A Pigouvian tax puts the missing cost into the price directly — tax the pollutant by the harm it does, and the market re-optimises around the corrected price. Coasean bargaining, from Ronald Coase's argument, holds that if property rights are clear and the parties can negotiate cheaply, they will reach the efficient outcome themselves regardless of who holds the right, without any regulator setting a number at all.

Which one works is decided by transaction costs, not by which theory is more elegant. Coasean bargaining needs few enough parties to identify and negotiate with — two neighbouring factories, not a global atmosphere and eight billion people breathing it. Once the affected group is large and diffuse, bargaining collapses into the same Collective Action problem it was meant to route around, and a tax or a quota, however blunt, becomes the only mechanism that can act on the group's behalf.

The genuinely hard part is measurement, not theory. Pricing a harm with no market of its own — a tonne of carbon, a species, a quiet street — requires an estimate, often little better than a Fermi Estimation, and that estimate is contestable by construction. Making the harm countable enough to price at all is its own act of simplification, the one described at Legibility; getting the number wrong reintroduces the distortion the tax was meant to remove, which makes whoever sets it a Principal-Agent Problem in miniature, exposed to the same Rent-Seeking and Regulatory Capture pressures a regulator faces.