I don't really think anyone is genuinely stupid.... (not many, anyway, maybe unless there's an error in their brains...)
It only depends on what kind of information you have, which again depends on where/when you've been brought up, by whom. How you act on the information you have, depends on your culture, and how you are, or think you are, expected to act. Like many (girls, earlier, and now) pretend to be stupid enev thoght they're not, but because they are, or think they are, expected to act like that.
But no matter how ppl really are, this issue here is just plain silly.