Timothy Taylor writes,

For example, for all those born into the bottom quintile, 44% are still in that quintile as adults. About half as many, 22%, rise to the second quintile by adulthood. The percentages go down from there. … Similarly, those born into the top income quintile are relatively likely to remain in the top. Among children born into the top quintile, 47% are still there as adults. Only 7% fall to the bottom quintile. The experiences of those born into the middle three quintiles are quite different. The distribution among income quintiles as adults is much more even for those born in these three middle groups, suggesting significant mobility for these individuals. … This pattern has led researchers to conclude that the U.S. income distribution has a fairly mobile middle, but considerable “stickiness at the ends” …”

This result is nearly an arithmetical certainty. Suppose that everyone faces three equally-probable outcomes:

–their income as adults puts them in the same quintile as their parents
–their income as adults rises enough to move up a quintile
–their income as adults falls enough (in relative terms) to move down a quintile

If this were the case, then people in the top would have a 2/3 chance of remaining at the top, because those who get lucky have nowhere to go but up within the top quintile. Similarly, people would have a 2/3 chance of remaining at the bottom, because those who get unlucky have nowhere to go but down within the same quintile. People in the middle quintiles would have only a 1/3 chance of remaining in their original quintile, because they can move in either direction. This pattern would lead researchers to conclude that the U.S. income distribution has a fairly mobile middle but considerable stickiness at the ends, even though by construction everyone in all quintiles has the same probability of moving up or down the income scale.