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ECON 103

Who counts the outside cost?

Compare private decisions with social costs and benefits, then explore taxes, subsidies, permits and bargaining. Solve 10 questions with worked answers.

Private and social costs · Step 1 of 12

A private trade can affect someone else

Mossleaf lanterns: private and social cost
Price ($ per unit)
QmPmMDS
Quantity (units)
D; S. Marked point M.
D: private valueS: private cost
Market: 40 units at $60
At the private outcomeAmount
Lanterns sold40
Harm per lantern$20
Total outside harm$800
All examples here are invented. Mossleaf lantern makers sell to willing buyers, and their smoke can harm neighbours who receive no compensation. When that harm is positive, the uncompensated effect on a bystander is a negative externality; at zero harm this model has no external cost.
Try it: Change the harm per lantern. The neighbours' total harm changes even though the private trade stays the same.
External cost per lantern$20
Market units
40
Outside harm
$800
D: P = 100 − 1 × Q. S: P = 20 + 1 × Q.
Qmarket = (100 − 20) ÷ (1 + 1) = 40 units.
Outside harm = (20 × 40) = $800.
Check yourself
Which effect is an externality?

Exam coming up and social cost versus private cost still tangles?

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