To begin with, the top 20 percent includes people not commonly regarded as rich. Another problem with such analysis is that the groups are not fixed. We may compare the bottom 20 percent of today with the bottom 20 percent of 10 years ago. But we'd better keep in mind that the statistical group does not contain the same people. There is tremendous mobility in this society.
People move up-and down.
Many studies indicate that the vast majority of people in the bottom 20 percent at any given time will move up as their lives proceed. Mobility produces an oddity when comparing the incomes of these groups over time. As people in the bottom 20 percent gain experience and income, they move into high groups, leaving the bottom group's income little changed. But the people in the highest group can't move any higher. So the group's income will balloon.