An externality is when a public good (properties of public
good: non-excludable and non-rival) is “dumped” on to third parties outside the
market, see here for more. They occur from the consumption and production of goods and services.
Those receiving the externalities are not compensated for in any way.
Externalities can be a form of market failure because market failure occurs when the wrong
quantity of a good/service is provided at the wrong price.
A negative externality is when the…
marginal social cost
> marginal private cost
The extra cost borne by society resulting from the last unit
of output consumed/produced is greater than the extra cost to the
individual/firm.
The socially optimum level of output (where MSB=MSC) is Q1, and price P1.
The privately optimum level (where MPC=MPB) of output is Q2 and price P2.When
there is a negative externality, the market produces at the privately optimum
level at point X, therefore there is over-production. The shaded area represents
the welfare loss and the MEC.
If a firm, a factory for example, produces electricity, they
also create a negative externality which is pollution. If the firm fails to
recognise and act against reducing the pollution, market failure occurs. The
incentive function of price breaks down (see word of the day) because the firm
is only charging consumers for the output of the good produced in the factory
and not the output produced as a negative externality. Therefore the good is
under-priced, over-consumed and over-produced.
A positive externality is when the…
marginal social benefit
> marginal private benefit
The extra benefit borne to society resulting from the
last unit of output consumed/produced is greater than the extra benefit to the
individual/firm.
If a factory produces a positive externality, for example
increased fish stocks in a lake that result from more warm water being discharged
into it, fisherman are able to exploit the fish without having to pay the
factory owner. The fisherman free rides.
Market failure occurs because the good is under-priced and under-produced.
See here for more on public goods and the free rider problem.
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