More years in school can add substantially to a person's lifetime income. A study of the earnings and education of identical twins indicates that, on the average:

• Each year of education at any stage, from grade school through graduate school, adds 16 percent to an individual's lifetime earnings.

• Thus a four-year college degree can be expected to increase an individual's earnings by almost two-thirds.

There could be some unmeasured factors causing the 16 percent per year figure to be on the high side. It is also conceivable that the remarkably high average return on education is a transitory phenomenon. However, other research suggests that the demand for skilled workers will grow in the 1990s and that investment in schooling will continue to have a high return.

Source: Orley Ashenfelter and Alan Krueger, Princeton University, unpublished working paper, cited in Peter Passell, "Twins Study Shows School Is Sound Investment," New York Times, August 19, 1992