The theory some have is that the reason for growing wage inequality is that the market is rewarding those with more skill. David Card says it's more complicated than that, and to that I have to say, "duh! of course it's more complicated than that." This David Card fellow seems brighter than your average economist.
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.
Obviously the change had to do with women choosing to acquire more valuable skills because society encouraged women in this direction during the 1970s and 1980s. However, many women still choose to acquire less valuable skills because they place less value on money. Furthermore, in fields dominated by women, the average salary is depressed because women place less value on earning money thereby depressing the collective bargaining power of all the workers in that field.
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.
Now there's something I never knew before! Maybe, because of a combination of H1-B visa technical workers and outsourcing of labor to foreign countries, wages for technical workers are being depressed. Humanities graduates are probably working in marketing related functions, and this lends evidence to my long held contention that the United States is moving towards a marketing economy.
This doesn't mean that the average engineering or computer science graduate doesn't still earn more than the average humanities graduate, just that the gap has been declining.