There’s been a lot of reporting recently about a new Pew Research Center report misleadingly titled “The American Middle Class Is Losing Ground” and even more misleadingly subtitled “No longer the majority and falling behind financially.” But here’s a picture of Pew’s own data on the ‘decline’ of America’s middle class. 
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And here’s a further picture of this same ‘decline’ offered, using Census Bureau data, by Mark Perry.
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Some ‘decline.’ (Note that Mark rightly relies also upon work by Scott Shackford [4] and Warren Meyer [5].)