Assume the production of a good causes a negative externality. in the market equilibrium, the marginal consumer values the good

less than the social cost of producing it

Explanation:

A negative externality is a cost that is suffered by a third party as a result of an economic transaction. In a transaction, the producer and consumer are the first and second parties, and third parties include any individual, organisation, property owner, or resource that is indirectly affected. Externalities are also referred to as spill over effects, and a negative externality is also referred to as an external cost.  Some externalities, like waste, arise from consumption while other externalities, like carbon emissions from factories, arise from production. For example, If we consider a manufacturer of computers which emits pollutants into the atmosphere, the free market equilibrium will occur when marginal private benefit = marginal private costs, at output Q and price P. The market equilibrium is at point A. However, if we add external costs, the socially efficient output is Q1, at point B.  At Q marginal social costs (at C) are greater than marginal social benefits (at A) so there is a net loss. For example, if the marginal social benefit at A is £5m, and the marginal social cost at C is £10m, then the net welfare loss of this output is £10m - £5m = £5m. In fact, any output between Q1 and Q creates a net welfare loss, and the area for all the welfare loss is the area ABC.  Therefore, in terms of welfare, markets over-produce goods that generate external costs. In the market equilibrium, the marginal consumer values the good less than the social cost of producing it.