Negative Externality
What is Negative Externality?
A negative externality is a cost imposed on a third party who is not part of a market transaction, such as pollution.
Because producers ignore these external costs, the market overproduces relative to the socially optimal quantity, creating deadweight loss. The marginal social cost exceeds the marginal private cost. Governments correct it with taxes, regulation, or tradable permits.
Negative Externality: a worked example
A cement plant faces demand P = 90 - 3Q and marginal private cost MPC = 30 + 2Q, and each ton releases dust that costs nearby households $15. The market ignores the dust: 90 - 3Q = 30 + 2Q gives Q = 12 tons at a price of $54. Marginal social cost is MSC = 45 + 2Q, so efficiency requires 90 - 3Q = 45 + 2Q, or Q = 9 tons. The plant makes three tons too many. At 12 tons marginal social cost is 45 + 24 = $69 against marginal social benefit of $54, a $15 gap that shrinks to zero back at 9 tons. Deadweight loss is 0.5 x 3 x 15 = $22.50. Total external damage is far larger at 15 x 12 = $180, but only the $22.50 counts as lost surplus.
The mistake students make with negative externality
The deadweight loss gets drawn as a rectangle. Because the dust costs $15 on every ton produced, students shade $15 times the market quantity and report that whole damage bill as the efficiency loss. The reasoning feels right since the damage is genuinely paid by somebody. Only the units between the efficient quantity and the market quantity destroy surplus, and across those units the gap between marginal social cost and marginal social benefit starts at zero and widens. The loss is a triangle, always smaller than total external cost.
Negative Externality questions
Why does a negative externality cause overproduction?
Firms decide how much to make by comparing their own marginal cost to the price buyers pay, and harm falling on third parties never appears on that invoice. Because the cost the firm sees is lower than the cost society bears, output continues past the quantity where marginal social cost equals marginal social benefit. Those extra units cost society more than they are worth, which is exactly where deadweight loss comes from.
What is the difference between a production and a consumption negative externality?
Production externalities come from making the good, so marginal social cost sits above the supply curve and the policy targets producers. Consumption externalities come from using it, such as secondhand smoke from a cigarette, so marginal social benefit sits below demand instead. Both leave the market quantity above the efficient one and both show the same style of deadweight loss triangle, but the shifted curve differs, and graders check that the correct one moved.
How do you find the socially optimal quantity with a negative externality?
Set marginal social cost equal to marginal social benefit and solve for quantity. Build marginal social cost by adding the marginal external cost to the supply curve, then locate where that higher curve crosses demand. The answer always lies to the left of the market quantity, and the horizontal distance between the two is the overproduction a free response question will ask you to label.
Formula / Example
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Common comparisons
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