It comes from William Emmons, but I cannot find the presentation, which is referenced here. I got as far as I did by following a pointer from Tyler Cowen.
Emmons shows median real income for households headed by college graduates roughly constant from 1991 to 2012, with median real income over that same period falling over 15 percent for households headed by those without college degrees.
Some remarks:
1. I would guess that the share of households headed by someone with a college degree has gone up, so that perhaps overall median household income has gone up. And mean incomes have probably gone up even more, because the mean includes high-salary individuals, successful investors, and entrepreneurs.
2. This looks like workers without college degrees becoming ZMP.
3. Other factors of production, namely capital and foreign workers, are putting downward pressure on American wages.
Professor:
I don’t follow your point #2. It smells like arguing from a price change.
Max L.
Here is the Census historical data on percent of households and educational attainment. From 1991 to 2013 the share of households headed by someone with a college degree rose from 21.2% to 31.7%.
https://www.census.gov/hhes/socdemo/education/data/cps/historical/index.html
Correction: The percentages are of people, not households.
So an alternative story is: Positive Marginal Product workers segrated themselves more heavily into the “college-educated” bucket
It seems that at any time there is some ratio of jobs that require college (or require proven intelligence and diligence) to jobs that do not require college and so if you produce more college grads you get nothing for it, that the bottom (or least lucky) college grads they end up doing just what they would have been doing without a college degree that is being the best blue collar workers.