I read another study on families---I think by Kay Hymowicz, but I could be mistaken on the author---that said that poorer mothers, even "good ones," will interact with a kid differently than more educated mothers (though not necessarily more affluent). For ex., a poorer mother who even reads to her kid will read the word "dog" and the kid will repeat "dog." And that is the end of it, where as more educated mothers will say, "And what sound does the dog make? And what sound does the cat make?" and start a whole learning conversation.
The Bell Curve nailed it and a lot of people knew it. It was just an “inconvenient truth” at the time.
I guess it still is.
Maybe if they released it again with the title, “A Real Inconvenient Truth”...
I had a teacher from New Zealand when I was a Special Education major. He would get picture books with no words and ask kids to tell HIM the story, each time from a different character in the book. Just an amazing technique.