Essentially, it’s the inability to achieve higher levels of education.
Here’s the latest and greatest economic paper:
http://client.norc.org/jole/SOLEweb/886.pdf
Thesis:
We examine changes in the characteristics of American youth between the late 1970s and the late 1990s, with a focus on characteristics that matter for labor market success. We reweight the NLSY79 to look like the NLSY97 along a number of dimensions that are related to labor market success, including race, gender, parental background, education, test scores, and variables that capture whether individuals transition smoothly from school to work. We then use the re-weighted sample to examine how changes in the distribution of observable skills affect employment and wages. We also use more standard regression methods to assess the labor market consequences of differences between the two cohorts. Overall, we find that the current generation is more skilled than the previous one. Blacks and Hispanics have gained relative to whites and women have gained relative to men. However, skill differences within groups have increased considerably and in aggregate the skill distribution has widened. Changes in parental education seem to generate many of the observed changes. (bolding and underlining added by me)
The problems with lower income stagnation do not arise fundamentally from trade, discrimination, weak labor unions, or even displacing technical technical change.
Less educated people do worse. The unstated implication is that a large part of the drag that keeps average wages down is the sticky bottom level of unskilled people - and of course, adding more unskilled people each year via immigration, both illegal and legal (since we favor family reunification over skills in our immigration policy) only exacerbates the effect and increases the competition for provision of unskilled labor.
To put it another way - it’s not the hedge fund guys at the top causing the problem. (And at any rate, I’m sure the top fell down a few rungs lately - I’m sure that will make the class-envy crowd on the Democratic side quite pleased.)