Question

When externalities exist, economic efficiency is achieved whenever marginal private benefit equals marginal private cost

Answer 1

We will consider the oft quoted case of a firm which discharges its waste products into a river. Such a firm would be treating the environment as a free resource, and would be imposing a cost on society as a whole, rather than just on the consumers of the good. The price charged to consumers would not therefore, in this instance, reflect the true cost of the product; if the firm were compelled to install equipment which could treat its effluent and render it harmless to the environment, its production costs and prices would rise and consumers would, as a consequence, reduce their demand for the product in question. Resources would then be reallocated to other lines of production.