[1] https://www.bloomberg.com/news/articles/2016-08-23/how-to-st...
[0]http://www.fa-mag.com/news/most-millionaires-self-made--stud...
$1mil isn't what it used to be with housing rising as it has over the past couple decades.
(It doesn't actually reference the study, so I can't look at their data.)
I expect that "millionaire" now reflects the top tiers of middle class people without an accompanying increase in purchasing power -- they just inflated across a boundary and now suddenly there's a bunch of "millionaires", without that term meaning what it used to 40-50 years ago.
My point was that a middle class family (say, 120-150k/yr) is going to have around a million dollars in assets by retirement age. But that's not what most people think of when they hear "millionaire", because our cultural expectations are a generation or two old -- they don't reflect the effects of the last couple decades on housing value or inflation.
So instead of looking at families with assets of $1mil, it probably makes more sense to look at families with assets of $10mil if you want to discuss the mobility into "wealthy" instead of "upper middle class".
In a country of 300 million people, 8 million households are are millionaire households.
Even if most millionaires are self-made, many of them are still born to affluent parents that gave them the opportunities to get there.