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Spillover costs are the negative effects of an economic activity that are passed on to the people who are not directly involved in any market transaction related to a good or service. These are external costs borne by an external party, and thus spillover cost is called a negative externality.

For example, the air pollution created by a factory puts external costs on people living nearby in terms of bad health or lung problems from the smoke.

Spillover benefits are the positive effects of an economic activity that are passed onto people who are not directly involved in any market transaction related to a good or service. These are external benefits enjoyed by an external party who did not make any payment. Thus, spillover benefit is called a positive externality.

For example, a higher educated person will result in greater production and awareness in the society that will benefit others as well who did not pay for his education.

In the following diagram, the initial market equilibrium is achieved at point 鈥渁鈥� where the supply curve S intersects with the demand curve D. However, the output level Q is not the optimal level as the external costs are not reflected in the supply curve. The intersection of the supply curve S1 and the demand curve results in an optimal level equilibrium at point 鈥渂鈥�, where the efficient output level is Q1.

The difference between Q and Q1 is the overproduction caused due to negative externality, and triangle abc gives the efficiency loss.

An imposition of a tax t (Pigouvian Tax) increases the marginal cost of a firm. As a result of this higher cost of production, the firm鈥檚 supply curve shifts back to S1, and the equilibrium quantity decreases to Q1, which is an optimal level. The tax corrects the problem of overproduction by including the external costs in the form of the cost of production.

In the following diagram, the initial market equilibrium is achieved at point 鈥渁鈥� where the supply curve S intersects with the demand curve, D. However, the output level Q is not the optimal level as the external benefits are not reflected in the supply curve. The intersection of the supply curve S2 and demand curve results in an optimal level equilibrium at point 鈥渄鈥�, where Q2 is the efficient level of output.

The difference between Q and Q2 is the underproduction caused due to positive externality, and the efficiency loss is given by triangle ade.

A subsidy is given to producers reduces the marginal cost of a firm. As a result of this lower cost of production, the firm鈥檚 supply curve shifts forward to S2, and the equilibrium quantity increases to Q2, which is an optimal level. The subsidy corrects the problem of underproduction by including the external benefits in the form of reduced cost of production.

A subsidy to consumers increases the demand and shifts the demand curve forward, whereas a subsidy to producers increases the supply and shifts the supply curve forward. The end effect leads to the production of an optimal level of output Q2.

The effect of a subsidy to consumers is explained using the diagram given below:

A subsidy to consumers reduces the price for consumers. This increases demand and shifts the demand curve from D to D1. A new equilibrium is achieved at point 鈥渜鈥�, where the new demand curve (D1) intersects the supply curve S. As a result, the output level increases to reach its optimal level Q2.