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I have other questions though; why is it that most people in the world are not retarded?
They say there are no stupid questions... But that question certainly contains a rather stupid presumption.
"Retarded" is a relative metric. It means slower, much slower actually, than average. Asking why everyone isn't below average, is retarded.
Anyways, US academic scores, on things like reason and mathematics, have been in decline, even accounting for immigration and economics. Hard to say whether that's actually measuring intelligence, but I tend to think so.