Once upon a time, education really was the key to opportunity and advancement in America. Although we did not collect comprehensive statistics throughout most of the era in which this was true, we do have some statistics from the tail end of that era, which I've summarized below. As you can see, non-high school graduates gained a small amount of ground from 1967 to 1973, while high school graduates and above gained 10% or more. "All" workers did better by over 10%, too, because the mix of workers was becoming more and more education.
Over the next 17 years, however, only those with a college degree or more showed any improvement (as a group, obviously individuals gaining experience and seniority did better, while some also did worse). High school drop-outs faced a drastic drop in income. Meanwhile, the super-high income groups--the top 0.1% and top 0.01%, who had been in the doldrums from 1967 to 1973, saw their incomes start to skyrocket:
(Note: p90 means 90th income percentile = top 10%. p95, 95th percentile = top 5%, etc.)
These changing trends only intensified in the years 1991 to 2008, although things were relatively good during the 1990s. Altogether, however, people with advanced degrees saw their incomes drop significantly over this time period--83.2% down for those with professional degrees, and 92.2% down for those with doctorates. Meanwhile, those in the top 1% and above showed strong to astronomical income growth:
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