As Boudreaux explains:
The percentage of American households with what might commonly be regarded to be middle-class incomes is indeed falling – but so, too, is the percentage of American households with what are surely “poor” incomes.  So once-middle-class Americans are not becoming poor.  Instead, they’re becoming rich; their real monetary incomes are rising.  The percentage of American households earning high incomes ($100,000 annually and above) is on the rise – and impressively so.  Let me emphasize: A much greater percentage of households today (compared to 1975) have annual incomes of $100,000 or more (in 2013 dollars).....
 the average number of persons per American household today (2013) is 13.6 percent fewer than in 1975, so each real dollar of household income is today shared by fewer people than in 1975 – meaning that the increase in “per-person-in-household” annual real incomes is even more impressive than these data show....these data are pre-tax yet post-cash transfers (such as Social Security payments),
Nick Gillespie from November on three more illuminating charts from Congressional Budget Office data on American income, income growth, and income inequality."