One explanation is that for every &quot;person&quot; who leaves the middle class, one goes down a notch and two go up. I say &quot;person&quot; because these are aggregate measures so it&#x27;s an abstraction of &quot;person&quot;.<p>Example: in energy, technicians move up the scale and perform less physical labor than comparable (if even possible) workers thirty years ago.<p>The lower classes are hard up against relative diminishing returns on labor. A textile worker, say,  still makes what they&#x27;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&#x27;t.