Externalities & the social cost

An externality is a cost or benefit from an activity that falls on someone who isn't party to the transaction. With a negative externality like pollution, the buyer and seller weigh only their own private costs and ignore the harm to bystanders, so the true social cost is higher than the private cost and the market over-produces. The standard remedy is a Pigovian tax — a charge equal to the external cost of one more unit — which makes the polluter face the full social cost, so they cut back to the efficient level on their own, with no quotas or bans needed. The mirror applies to positive externalities, like vaccination or education, where the market under-provides and a subsidy corrects it. The core idea is aligning private incentives with social welfare by putting a price on the side effect, so people account for costs and benefits they would otherwise ignore.