Nothing magic about it. You do know how these tests are calibrated, right? Do you see what you are missing there? Hint: you can't post comparative measurements of any of that stuff on the people taking IQ tests. In the absence of measurement, we bet on likelihood: My money says 1970s black children in the US had significantly higher average exposures to lead than 1970s Chinese children, American white children, and European Jewish children. Would you take that bet - risk actual money on your delusions? Here's another bet: if exposure to lead does not impair Chinese children's brain development as much as it does other people's, on average, the reason is genetic. Meanwhile: the thread topic.