Here's an analysis that purports to explain the phenomenon as a demographic artifact: Solving the Mystery of the Disappearing U.S. Middle Class From the PEW report: Political Calculations confirms that the aging boomers are enjoying high incomes. Generation X is also doing well, but this was a smaller cohort, as shown in another chart. The Millennials are another large group, but they are just starting out and their incomes are still relatively low. http://3.bp.blogspot.com/-1eXQAKUT7zE/Vml0RDywRZI/AAAAAAAAMyg/pLOmJSqoTnw/s1600/us-registered-births-1909-2004.JPG So the evaporation of the middle class may be explained by many boomers doing well and getting Social Security on one end and many young people starting out their careers on the other end.The biggest winners since 1971 are people 65 and older. This age group was the only one that had a smaller share in the lower-income tier in 2015 than in 1971. Not coincidentally, the poverty rate among people 65 and older fell from 24.6% in 1970 to 10% in 2014.