I think it is an old prejudice. People assume that a person with slow body movement also moves slow inside the brain.
Maybe angry little men started that myth.
I don't truly know, but a thought I had just now was that people growing up with physical impairments or deformities or anything outside of 'normal' will absolutely be perceived and treated differently in school by students and teachers, and that can impact their learning experience for the worse - so is it nature or nurture?
Unless you're an racist eugenist, no, that's actually a trope. I think it's a criticism/depiction of the lower class society, especially at older times, because they tended to work too much and lack education/critical thinking (just like me, except I don't work). I don't like that trope, especially in cartoons meant for children, because it may suggest that relation, which is entirely messed up
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