Pigouvian tax and subsidy
/ pig-OO-vee-un /
Suppose a factory pollutes because dumping waste is free for it, even though the harm to neighbours is real. The simplest fix is to make polluting cost the factory money — exactly as much as the harm it causes. Now the factory, chasing its own profit, will pollute less because it 'feels' the harm in its own pocket. This idea — taxing a harmful spillover to fold its cost back into the price — is named after the economist Arthur Pigou, who proposed it in 1920.
A Pigouvian tax is a tax on an activity that generates a negative externality, set ideally equal to the external cost it imposes — the gap between marginal social cost and marginal private cost. Done right, it raises the producer's cost until the market quantity falls to where marginal social cost equals marginal social benefit, restoring efficiency. Its mirror image is the Pigouvian subsidy: a payment for activities with positive externalities (vaccination, education, rooftop solar), set equal to the external benefit, encouraging more of a thing the market under-supplies. Either way, the goal is to 'internalize the externality' — make the decider face the full social consequences.
Pigouvian taxes are the textbook remedy for pollution, congestion, smoking, and sugar — carbon taxes and congestion charges are real examples — and economists admire them because they fix the price signal rather than ban activity outright, letting people who value it most still do it (just at the true cost). The honest difficulties: setting the tax exactly equal to the harm requires knowing a number nobody can measure precisely; the tax may hit the poor hardest (it can be regressive); and politics, not economics, usually sets the final rate. They reduce a distortion, but rarely achieve the textbook ideal.
A carbon tax of, say, 50 dollars per tonne of CO2 makes burning coal more expensive in rough proportion to the climate harm it causes. Power firms then switch toward cleaner sources not out of virtue but because the dirty option now costs them — the externality has been priced in.
Pricing the harm so the market self-corrects.
The textbook tax 'equals the external harm', but no one can measure that harm exactly, and the tax can fall hardest on the poor — so real Pigouvian taxes are educated approximations shaped by politics, not perfect fixes.