One explanation is that for every "person" who leaves the middle class, one goes down a notch and two go up. I say "person" because these are aggregate measures

One explanation is that for every "person" who leaves the middle class, one goes down a notch and two go up. I say "person" because these are aggregate measures so it's an abstraction of "person". Example: in energy, technicians move up the scale and perform less physical labor than comparable (if even possible) workers thirty years ago. The lower classes are hard up against relative diminishing returns on labor. A textile worker, say, still makes what they'd have made in the 1950s in absolute terms. This does not keep up with a general rising standard of living in the US but it works fine in PacRim countries - until it doesn't.