Contrary to popular opinion, social mobility in the U.S. is as great today as it was 20-30 years ago. See an article in The New Yorker for more. A careful look at the author's reasoning suggests he tries to minimize upward mobility, focusing mostly on how few of the poor who manage to become rich.
He speaks of quintiles, or fifths of the population. COTTonLINE believes that someone who moves from quintile 5 to quintile 4 has made a non-trivial improvement in his/her life. I'm guessing that kind of improvement isn't uncommon today.
