If we assume that the producer is interested in maximising profits - then they will only take into account the private costs and private benefits arising from their supply of the product. We can see from the diagram below that the profit-maximising level of output is at Q1. However the socially efficient level of production would consider the external costs too. The social optimum output level is lower at Q2.

This leads to the private optimum output being greater than the social optimum level of production. The producer creating the externality does not take the effects of externalities into their own calculations. We assume that producers are only concerned with their own self interest.

In the diagram above, the private optimum output is when where private marginal benefit = private marginal cost, giving an output of Q1. For society as a whole though the social optimum is where social marginal benefit = social marginal cost at output Q2.The failure to take into account the negative externality effects is an example of market failure.  This leads to the good or service being over-produced relative to the social optimum.