Other Examples of Positive Externalities
Although technology may be the most prominent example of a positive externality, it is not the only one. For example, being vaccinated against disease is not only a protection for the individual, but it has the positive spillover of protecting others who may become infected. When a number of homes in a neighborhood are modernized, updated, and restored, not only does it increase the value of those homes, but the value of other properties in the neighborhood may increase as well.
The appropriate public policy response to a positive externality, like a new technology, is to help the party creating the positive externality receive a greater share of the social benefits. In the case of vaccines, like flu shots, an effective policy might be to provide a subsidy to those who choose to get vaccinated.
This figure shows the market for flu shots. The market demand curve DMarket for flu shots reflects only the marginal private benefits (MPB) that the vaccinated individuals receive from the shots. Assuming that there are no spillover costs in the production of flu shots, the market supply curve is given by the marginal private cost (MPC) of producing the vaccinations.
The equilibrium quantity of flu shots produced in the market, where MPB is equal to MPC, is QMarket and the price of flu shots is PMarket. However, spillover benefits exist in this market because others, those who chose not to purchase a flu shot, receive a positive externality in a reduced chance of contracting the flu. When we add the spillover benefits to the marginal private benefit of flu shots, the marginal social benefit (MSB) of flu shots is given by DSocial. Because the MPB is greater than MSB, we see that the socially optimal level of flu shots is greater than the market quantity (QSocial exceeds QMarket) and the corresponding price of flu shots, if the market were to produce QSocial, would be at PSocial. Unfortunately, the marketplace does not recognize the positive externality and flu shots will go under produced and under consumed.
So how can government try to move the market level of output closer to the socially desirable level of output? One policy would be to provide a subsidy, like a voucher, to any citizen who wishes to get vaccinated. This voucher would act as “income” that could be used to purchase only a flu shot and, if the voucher was exactly equal to the per-unit spillover benefits, would increase market equilibrium to a quantity of QSocial and a price of PSocial where MSB equals MSC. Suppliers of the flu shots would receive payment of PSocial per vaccination, while consumers of flu shots would redeem the voucher and only pay a price of PSubsidy. When the government uses a subsidy in this way, the socially optimal quantity of vaccinations is produced.
The Market for Flu Shots with Spillover Benefits (A Positive Externality)