I'm sorry this is so long. It's just a topic that I'm passionate about. Nobody should be made to feel like they're stupid when the people doing it could instead be making that person smarter. It boils my blood.
Here's how I think of it: sure, some people really are incapable of learning, but they're pretty few and far between. Most people who are written off as stupid aren't, they just don't know things, which is very, very different. They slip through the cracks maybe, they don't have good teachers, they have a horrible home life, or even just that when they were a teenager, they were lazy and never paid attention because we all remember, I hope, how fucking weird the world was during that stage of our lives. It could be anything, but the point is that these people are sometimes written off as stupid because they lack the book-learnin'.
I have at least two separate friends who think of themselves as really stupid, and they're not. They're both bright people, they're switched on, they just don't know things that a lot of people consider common knowledge. They get mocked for being stupid. They are tough enough that it doesn't bother them, but they believe it and it drives me up the fucking wall when it happens.
I'll just use one of them as an example: from an early age she was placed into a career as a dancer. You might know somebody like this, dancing since they were 3 years old, forced bulimia, allowed almost no time for schoolwork, every waking moment dancing or doing something dance related. As is often the case with someone really talented she scraped through primary and high school because they were willing to overlook the fact that she never had time for coursework and was constantly exhausted in class given she was such a good dancer. As is also often the case, she sustained a serious injury and had to stop dancing permanently.
So what was she left with? A high school graduate with a career selected for her essentially at birth that she now can't pursue, who's been belittled from an early age for not being good enough because some dance school teachers (not all, I assume and hope) are the fucking devil, and she never really had an education. She has no idea what Communism is, for example, because when would that even come up? And because she doesn't know stuff like that, little stuff that everyone knows because they pick it up during their time in school, a lot of people just assumed she was dumb, and so did she. Never mind she's a wonderful person with a quick mind and incredible compassion for others, she's dumb, end of story.
She's 24 now, and I and her other friends (of which she has many) managed to convince her to give college a go, at a course she's wanted to do for years and years, but never felt like she was smart enough to get into. Of course she got into it, of course she's doing well now. She's never going to be the absolute tip top of the class, but she's doing just as well as anyone there, because she's smart, she was just never taught.
People aren't dumb. They just don't know things. Shame on anyone who uses an opportunity to teach as an opportunity for scorn, and thank goodness for those who do the opposite.