David Card on Skill-Based Technical Changes and Wages
David Card is one of the best economists around at looking closely and carefully at the data we have available to us; his specialty is what's happening in labor markets. Over the last 10 - 15 years, a primary hypothesis for explaining growing inequality in wage and salary income has been the Skill-Based Technical Change hypothesis, which argues that much of the growing inequality (e.g., the rising gap between wages of college-educated workers and others) is a growing skill gap. In an interview by the Minneapolis Federal Reserve Bank and published in The Region, Card makes the following observations about the SBTC hypothesis:
John DiNardo (of the University of Michigan) and I were troubled by the fact that there are a lot of patterns and trends in the labor market that don't fit in very well with a skill-biased technical change explanation. We were motivated to embark on a Don Quixote mission, a noble cause that wasn't going to go anywhere [laughs].
One thing we pointed out, for example, is that women are lower skilled than men, if you take the fact that they have lower wages as evidence of their skill. The SBTC theory says that people with lower skills should have slower wage growth than people with higher skills. But over the 1980s, women did much better than men. It's also the case that over the 1990s, women's relative wages were fairly stable again. So there was a long period of stability of women's relative wages, then a period of convergence of women relative to men that ended in 1991-92, and then stability again. That's an important set of trends that SBTC doesn't address. SBTC might be consistent with it; it might not be, but the theory needs a lot of auxiliary hypotheses to work.
The same thing is true with respect to the black/white wage gaps. Blacks earn less than whites, and many people believe that the reason they do so is because they're less skilled. Nevertheless, during the 1980s, the black/white wage differential was stable. It didn't widen as people had predicted it might.
Another trend that didn't fit with the SBTC hypothesis concerns the relative wages of people with different bachelor's degrees. There are a couple of different data sets that collect starting salaries for newly minted B.A.s. What these data show is quite remarkable. Everyone knows that the average wage of young college graduates went up over the 1980s. It wasn't the case, however, that the gains were most pronounced in engineering or science. They were actually greater for graduates in the humanities, which doesn't seem consistent with the idea that there is increasing demand for technically proficient, computer-savvy people.
...A final puzzle concerned the age structure of the increases in the relative wages of college versus high school graduates. Wages of young college-educated workers rose relative to young high school workers, but for people over age 40 or so, there really wasn't any change in the high school/college premium.
The moral for economists is that theory is a useful thing, but the real issue is whether the theory is in agreement with reality. If it isn't, then it's the theory that needs to be revised.