A couple of thoughts:
Test prep for anything that's supposed to be an "ability" test, in my opinion, invalidates the legitimacy of the outcome. As someone who is trying to understand my child, it's a frustration of mine because when I try to look at her results in the context of other peoples' results, I don't know how much to account for scores that have resulted from extensive preparation. Understanding how rare (or not) my child's abilities are is important to me not because I want her to be the "best" (whatever that means), but because I don't know how to evaluate her needs and progress without some sense of how usual or unusual she is. Should I trust the assurances I'm given by the school? Is it likely that there really are scads of academic peers available--or does that mean the school doesn't get my child? Is the progress she's making what I should expect? Or is it underacheivement? Is she learning or going through the motions?
Bottom line, our education programs have to get better at meeting kids where they're at and giving them rich, challenging experiences. The fact that families feel a need to compete for an appropriate educational experience is unacceptable on so many levels. There shouldn't be a sense that you have to "get into" school X or your child won't get what they need. Parents shouldn't feel like they have to deprive their child of a joyful, play and interest based childhood so that they can prep them (at age 3 and 4!) to score at a certain level on some high stakes test. Ick. Testing at that age should, IMHO, be done because the child is truly surprising to their parent or teacher and there is a strong need to UNDERSTAND the child vs. a need to QUALIFY for something. I have many complaints about our local school system, but I am incredibly grateful not to be living in a place where children are routinely tested and placed before they've even had the opportunity to go to school. I can't imagine the pressure that this must place on families. It must be very, very difficult to be a parent of a high average or moderately gifted child and maintain a non-competitive child rearing approach. I'd like to think that I still would have let my children's interests and wants direct their pre school age activities and time, but it would have caused me a lot more anxiety!
This competitive approach to placement creates too many problems---not the least of which is that the tests stop being able to reveal rarity reliably because outcomes are always less rare when large groups of people invest a lot of time practicing in order to reach those outcomes. That's unfortunate, because in order for us to understand our children it's helpful to have a sense of relativity. A child who recieves rare scores without preparation absolutely has different needs than the child who prepares in order to get those scores. Unfortunately, as long as scores of "prepared" and "unprepared" children are pooled together, that reality is often going to be invisible to the families or the schools or both.
As to the question of rarity within or between populations (by race, ethnicity, neighborhood, etc).....
Even if you take the various studies out there at face value (which I personally do not), the identification discrepancies far exceed the scope of of any supposed differences. Further, since it is impossible to fully disaggregrate race and class, and because we have such an uneven race/class distribution in this country, access to test prep can only exacerbate the problem.
