Immigration and the Median Worker
The supposed decline of the poor and middle class is exaggerated even more by the dynamics of population growth. When people look at the "poor" in any two years, they think they're looking at the same people. That's rarely true, especially over longer periods of time.
Since 1998, the U.S. population has increased by over 20 million. Nearly half of that growth has come from immigration, legal and illegal. Overwhelmingly, these immigrants enter at the lowest rungs on the income ladder. Statistically, this immigrant surge not only reduces the income of the "average" household, but also changes the occupants of the lowest income classes.
To understand what's happening here, envision a line of people queued up for March Madness tickets. Individuals move up the line as tickets are purchased. But new people keep coming. So the line never gets shorter, even though individuals are advancing.
Something similar happens with the distribution of income. People keep entering the distribution line from the bottom. Even though individuals are moving up the line, the middle of the line never seems to move. Hence, an unchanged -- or even receding -- median marker could co-exist with individual advancement.