In terms of data, here is the graph showing how SES effects achievement for school kids:
http://dl.dropbox.com/u/1655696/SES_and_school.jpg
It's a little difficult to read, but basically what you can see is that poor kids and rich kids learn roughly the same amount in school. The reason there is such a big gap is because A) there is a large gap that already exists when they get to Kindergarten and B) while rich kids are learn over the summer and get smarter, the poor kids are actually forgetting what they've learned the previous year.
If you want to learn about why there is already a 2 year gap before the kids get to kindergarten, you should read the book Meaningful Differences In The Everyday Experience of Young American Children.
The finding about within school differences vs. between school differences comes from the Coleman report, which is one of the largest social surveys ever conducted, and still one of the most important to date. I also have a blog post here explaining a bit about why within school differences are important:
http://alexkrupp.typepad.com/sensemaking/2009/02/the-most-im...
If I remember correctly efficacy of preschool is mixed, and it depends a lot on the type of preschool. However, even the best preschools can never be as effective as good parenting at imparting language schools, for reasons that the Meaningful Difference book explains. (However, preschools may be good for other reasons.)
"The disadvantages that poverty imposes on children aren't primarily about material goods. True, every poor child would benefit from having more books in his home and more nutritious food to eat (and money certainly makes it easier to carry out a program of concerted cultivation). But the real advantages that middle-class children gain come from more elusive processes: the language that their parents use, the attitudes toward life that they convey. However you measure child-rearing, middle-class parents tend to do it differently than poor parents; and the path they follow in turn tends to give their children an array of advantages. As Lareau points out, kids from poor families might be nicer, they might be happier, they might be more polite; but in countless ways, the manner in which they are raised puts them at a disadvantage in the measures that count in contemporary American society."
Undoubtedly?
> The issue is not so much, I believe, in the "goodness" of the parenting, rather, it is in the richness of the environment.
In the US at least, there are free yet rich environments. If you're poor, you do have to get out of the house and seek them out.
So, what definition of "better parenting" are you using?
I would argue that pre-school attendance could be a very strong indicator of parenting quality, if we can even try to quantify such a thing.
On the other hand, lack of pre-school could indicate a parent taking the pre-schooling responsibility upon themselves and providing a rather spectacular "home pre-school" environment.