1. Top X% of what? The top 1% of earners in the community? The nation?
2. Does income data for a "child" reflect back on all the locations they previously lived or does it reflect solely on the county they currently live in? e.x. If I am a child who moved many times, and at age 26 live in a place where I did not grow up, which locations' income differences are affected by my income?
1. Annual income of 500k in the 80's and 90's using 2012 dollars 2. County kids born from '80 to '86 living in during the 80s and 90s.
Basically, if you don't fit the profile of a family that never moved from the same house in the suburbs, you aren't included. Also, take a look at the national map. It would seem the plains are the best place for upward mobility, but I get the feeling they forgot to control for some confounding variable.
Also, the breakdowns show some Simpsons paradox, where individual counties might be very good for boys or girls, but are on average much worse. I'm sure a racial breakdown would show similar issues.