from wikipedia[1]: Moving between quintiles is more frequent in the middle quintiles (2-4) than in the lowest and highest quintiles. Of those in one of the quintiles 2-4 in 1996, approximately 35% stayed in the same quintile; and approximately 22% went up one quintile or down one quintile (moves of more than one quintile are rarer). However, 42% of children born in the bottom quintile are most likely to stay there, and another 42% move up to the second and middle quintile[3]. On the opposite end of the spectrum, 39% of those who were born into the top quintile as children in 1968 are likely to stay there, and 23% end up in the fourth quintile[3]. Children previously from lower-income families had only a 1% chance of having an income that ranks in the top 5%[4]. On the other hand, the children of wealthy families have a 22% chance of reaching the top 5%[4].

[1]http://en.wikipedia.org/wiki/Economic_mobility#United_States