It’s odd that “The Bell Curve” keeps on coming around. If you examine the assumptions in the book, it’s ASSUMED that “X” percentage of reduced test scores relative to the average are due to “environmental” factors, but all the remaining percentage is due to genetics.
The issue with that was twofold, first, if you wanted to find yourself a pure pure African to do this testing with, you’d be looking pretty strictly at recent immigrants. Thus, you’d be claiming that some kind of percentage thing was in effect, or that “bad” genes drive out “good” genes – which is not born out by anything in science.
To make that even worse, recent immigrants WHO HAVE NOT BEEN EXPOSED IN YOUTH to US social conditions do better in both testing and in actual outcomes over time. So you get into a really twisty mess trying to claim genetics is the inevitable predictor of success. So twisty that it’s ridiculous to make the claim.
And the reason the claim won’t die is really simple, if you can shift blame to “they were born that way” then there’s nothing that can be done and you don’t have any obligation to spend money to improve conditions.
If you want to read the book, it’s easy to find as a pdf. As for the statements of fact above: