&gt; Sure, but if people were moving up on aggregate, we would see the middle class growing as children of the poor made their way up.<p>Eh, what you <i>really</i> want to look at is a series of cohorts moving on up as people grow older: a lot of US population growth is from immigration (legal and otherwise) who tend to have less income (which is why they immigrate), and from children born to recent immigrants (who have substantially more children than rich people in this country).