Chicago economist Raghuram Rajan offers the following explanation for the long-term stagnant real incomes of Americans at the 50th percentile of the income distribution (compared to their compatriots at the 90th):

bq. A number of factors are responsible for the growth in the 90/50 differential. Perhaps the most important is that technological progress in the US requires the labor force to have ever greater skills. A high school diploma was sufficient for office workers 40 years ago, whereas an undergraduate degree is barely sufficient today. But the education system has been unable to provide enough of the labor force with the necessary education. The reasons range from indifferent nutrition, socialization, and early-childhood learning to dysfunctional primary and secondary schools that leave too many Americans unprepared for college.

I really find this difficult to believe. My guess is that, in terms of the real skills objectively needed to do the job, a high school diploma is more than adequate for most office work. Of course, it may be that, because of competition for those jobs, you need a higher level of qualification to get one. But that’s a different story.