A negative externality is a type of market failure in which an individual does not bear the full cost of a good or service. A good with a negative externality imposes external costs on society beyond those paid for by the consumer. In this example, the socially optimal point is point , where marginal social cost MSC is equal to marginal social benefit MSB. At this level, units of the good are sold for a price . However, the market operates at point because individuals are concerned with their marginal private costs MPC rather than the social costs. We see that the cost per unit for the consumer is lower than the cost per unit to all of society, and the result is over-consumption of the good at level along with deadweight loss DWL, which is the loss to the economy due to transactions that do not occur because a market is not operating at its optimal point. By taxing the good, the government increases the marginal private cost to lower the level of consumption down to the socially optimal .