6.2

Externalities

AP Microeconomics

What Is an Externality?

Negative Externalities

In Production

In Consumption

Correcting Negative Externalities

1234567891012345678910111213Quantity$/unitMarket Q (too much)Optimal Q (with tax)MPC (Supply)MSC (MPC + tax)D = MPB = MSB

Pigouvian tax = external cost → shifts supply up to MSC → reduces output to social optimum

Positive Externalities

In Production

In Consumption

Correcting Positive Externalities

123456789101234567891011121314Quantity$/unitMarket QOptimal Q (with subsidy)S = MPC = MSCMPB (D)MSB (MPB + subsidy)

Subsidy = external benefit → shifts demand up to MSB → increases output to social optimum

The Coase Theorem

Conditions Required 1. Well-defined property rights 2. Low transaction costs 3. Small number of parties involved

Example

Limitations

Summary Table

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