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since the top and bottom 5-10% will always be pulling away from the pack in oppoisite directions

Almost, but remember the bottom can only "pull away" a little bit, or else they hit 0 and aren't even counted anymore. While the top theoretically has no limit to how much they can go up.

Which is why the top will almost ALWAYS go up the most. The bottom is pegged to the minimum of zero, or more accurately the minimum income you would accept before you quit your job and gave up. The middle if they increase too much fall into the higher brackets, and only the top gets to rise without limit.

The actual movement of quintiles is very complex, because individuals move up AND down the scale, and so when you compare "quintiles" over time you aren't comparing the same workers.

for example, suppose there are 5 people. So 1 in each quintile. They make 1,2,3,4, and 5 dollars. Now, give the middle class a 100% raise, while holding everybody else constant.

Now your wagest are 1,4,6,8,5, which is 1,4,5,6,8. The increases by quintile were: 0%,100%,66%,50%,60%. Notice that even though your top quintile got NOTHING, after you were done the STATISTICS show the "top qintile" did BETTER than the 4th quintile. And only 20% show a 100% increase, when in fact 60% got one.

So you shafted the rich, and yet it looks like the rich got ahead on the backs of the upper middle class.

Here is the actual graph from the cbo report:

There are good things and bad things here. The good is that the earnings level of this group has indeed gone up. It rose steadily during the 90s. This is what you'd expect on a booming economy. It peaked in 2000 and started to fall off. This is what you'd expect when a recession hits. The problem is what happens next. As the economy recovers, the earnings level does not. It has stayed pretty much flat along it's 1998-1999 level.

I understand what you are saying CharlesWayneCT, but you are misreading the study. Your example shows that people can move among which “quintile” they are in. The study specifically removes that possibility by tracking the families from the quintile they were in when the data was first collected. In your example, the study would show the quintiles, after the 100% increase for the middle class, as 1,4,6,8,5. It would correctly show all 3 middle quintiles getting a 100% increase.