My Google-fu is failing me at the moment. It was from an absolutely fascinating article that suggested that the middle-class arose because of downward social mobility in post-Plague Europe. With the serfs mostly killed off by Black Death, the 3rds and 4th sons of the nobility typically couldn't support a feudal household, and so they had to take jobs as merchants or artisans in the emerging market economies. Along the way, they brought with them some of the values of the nobility (most notably, not having kids until they could support the lifestyle they were accustomed to as children), which allowed Europe to escape the Malthusian trap of the Dark Ages.
IIRC, the genealogical records (of most present-day Europeans and Americans being descended from the nobility) was presented as fact, but the causal link with the rise of the middle class was more tenuous.
I'll look again after I've gotten some actual work for $realjob done.
Sounds like _A Farewell to Alms_ by Clark. Though as far as I can remember the data showed the "merchant class" crowding out the lower classes, not the nobility. Nobles only had about an average amount of surviving children while rich merchants had an above average one. Makes more sense that way: medieval nobility doesn't sound like a great base for middle-class values...
I'm not totally convinced about the theory, but the book was a surprisingly good read.
IIRC, the genealogical records (of most present-day Europeans and Americans being descended from the nobility) was presented as fact, but the causal link with the rise of the middle class was more tenuous.
I'll look again after I've gotten some actual work for $realjob done.