Labor is, of course, not the only input into production. Capital is the other major input. According to theory, the right measure of productivity for determining real wages is the marginal product of labor--the amount of output an incremental worker would produce, holding constant the amount of capital. With the standard Cobb-Douglas production function, marginal productivity (dY/dL) is proportional to average productivity (Y/L), which is what we can measure in the data. (See Chapter 3 of my intermediate macro text for a discussion of the Cobb-Douglas production function.) Keep in mind, however, that the Cobb-Douglas assumption of constant factor shares is not perfect. In recent years, labor’s share in income has fallen off a bit. (Between 2000 and 2005, employee compensation as a percentage of gross domestic income fell from 58.2 to 56.8 percent.) From the Cobb-Douglas perspective, this means that the marginal productivity of labor has fallen relative to average productivity. This modest drop in labor’s share is not well understood, but its importance should not be exaggerated. The Cobb-Douglas production function, together with the neoclassical theory of distribution, still seems a pretty good approximation for the U.S. economy.