第13章正の外部性と公共財

The investment in anything, whether it is the construction of a new power plant or research in a new cancer treatment, usually requires a certain upfront cost with an uncertain future benefit. The investment in education, or human capital, is no different. Over the span of many years, a student and her family invest significant amounts of time and money into that student’s education. The idea is that higher levels of educational attainment will eventually serve to increase the student’s future productivity and subsequent ability to earn. Once the student crunches the numbers, does this investment pay off for her?

Almost universally, economists have found that the answer to this question is a clear “Yes.” For example, several studies of the return to education in the United States estimate that the rate of return to a college education is approximately 10-15%. Data in Table 13.2, from the U.S. Bureau of Labor Statistics’ Usual Weekly Earnings of Wage and Salary Workers, Fourth Quarter 2021, demonstrate that median weekly earnings are higher for workers who have completed more education. While these rates of return will beat equivalent investments in Treasury bonds or savings accounts, the estimated returns to education go primarily to the individual worker, so these returns are private rates of return to education.

What does society gain from investing in the education of another student? After all, if the government is spending taxpayer dollars to subsidize public education, society should expect some kind of return on that spending. Economists like George Psacharopoulos have found that, across a variety of nations, the social rate of return on schooling is also positive. After all, positive externalities exist from investment in education. While not always easy to measure, according to Walter McMahon, the positive externalities to education typically include better health outcomes for the population, lower levels of crime, a cleaner environment and a more stable, democratic government. For these reasons, many nations have chosen to use taxpayer dollars to subsidize primary, secondary, and higher education. Education clearly benefits the person who receives it, but a society where most people have a good level of education provides positive externalities for all.

Although technology may be the most prominent example of a positive externality, it is not the only one. For example, vaccinations against disease are not only a protection for the individual, but they have 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 homes' value, but other property values 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.

Figure 13.3 shows the market for flu shots. The market demand curve D_Market 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 Q_Market and the price of flu shots is P_Market. However, spillover benefits exist in this market because others, those who chose not to purchase a flu shot, receive a positive externality in the form of 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 D_Social. Because the MSB is greater than MPB, we see that the socially optimal level of flu shots is greater than the market quantity (Q_Social exceeds Q_Market) and the corresponding price of flu shots, if the market were to produce Q_Social, would be at P_Social. Unfortunately, the marketplace does not recognize the positive externality and flu shots will go under-produced and under-consumed.

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 one could use to purchase only a flu shot and, if the voucher were exactly equal to the per-unit spillover benefits, would increase market equilibrium to a quantity of Q_Social and a price of P_Social where MSB equals MSC (which equals MPC given the assumption that there are no spillover costs in producing the vaccine). Suppliers of the flu shots would receive payment of P_Social per vaccination, while consumers of flu shots would redeem the voucher and only pay a price of P_Subsidy. When the government uses a subsidy in this way, it produces the socially optimal quantity of vaccinations.