Perfect example. Somehow we see it as a people who "choose to act stupid," as if they can just will their brains into being more capable than they are.
You're leaning very hard into absolute genetic determinism of any and all actions, as if an otherwise intelligent person couldn't just be careless or not be paying attention and be hit by a bus (to use your example).
"Mentally handicapped" is generally considered to be where someone's capabilities are hindered to the point that they can't function in a normal autonomous way and perform basic life tasks. "Acting stupid" applies to people who don't have such a level of hindrance but are instead not reasonably utilizing the adequate faculties they do have.
So, back to your example: if you lack the capability to cross a street on your own, or to even understand what that means and why it can be dangerous, no one faults you if you are harmed. If you are capable of crossing a street and appreciate the danger but choose not to take proper care when doing so (e.g., you step out onto the street while texting and are struck by a bus), people fault you.
You seem to be making unsupported assumptions and conflating all sorts of things when it's really not complicated: was the person capable of understanding the situation and acting on that understanding?
You're right that smart people can sometimes act stupid, but there are so many situation where we blame people for their inherent intellectual failings.
For example, smart people frequently describe the lottery as "a tax on stupid people" in a sneering way. When people get taken by an obvious scam, we often place as much if not more blame on the victim as we do on the scammer. Or look at the Darwin Awards for numerous examples of people basically saying that others deserve death for being dumb.
Sure, there are examples where a smart person is negligent, we describe that as "stupid," and we rightfully blame them for the consequences. But there are many more examples where a dumb person doesn't understand that what they're doing won't work, it fails predictably, and we say they deserve it. Lotteries, scams (but I repeat myself), avoiding insurance or banks, and much more tend to fall into this category.
If someone got robbed because they had a bad leg and couldn't run away, we'd sympathize. If they got robbed because they had a bad brain and couldn't detect that this smooth-talking stranger was scamming them, we'd tell them they should stop being so stupid.
It's a fact that people sometimes choose to act stupid. I have chosen to act stupid in the past; and when I was reprimanded, I knew exactly why, and how I could've avoided it.
I'm not talking about people who are less capable in their mental faculties. Of course nobody should blame someone who is actually mentally handicapped for "stupid actions", or for "acting stupid". That's exactly the difference I was talking about.
Why do you assume that a fairly stupid person is choosing to act stupid, but an extremely stupid person is not? "Mentally handicapped" is an arbitrary line we draw in the continuum of intelligence.
I didn't realize an IQ of 90 was "fairly stupid" nowadays. I actually meant that as an example for a "smart enough" person. Of course one has to draw the line somewhere, and of course there will be false positives and false negatives; but what else can one do? It's the same for e.g. illness; at which point does a doctor allow a patient to stay home from work? He has to draw the line somewhere. There will be cases when he's too strict and a genuinely ill person is forced to work or use a vacation day; and there will be cases when he's too lenient and a perfectly healthy, simulating person can relax at home. I wouldn't argue that we should give everyone unlimited sick days without a proper diagnosis, which, again, might include false positives/negatives.
100 is average, and the average person is pretty dumb.
Why does there have to be a line drawn anywhere? We don't do this for other disabilities. We don't blame someone with a mild limp but sympathize with someone missing a leg. Why do we blame dumb people for their dumbness, but only if they're not too dumb?
Again, I'm not trying to set myself apart here. I do it too. I just don't really get why.
But we can blame a person with perfectly functioning legs if they pretend to have a limp and e.g. cause a congestion in a subway station. The line should be between "having trouble walking" and "being able to walk just fine", not somewhere on the spectrum between a limp and a missing leg. Same with intelligence, there should be a line above which a person should be able to act normally (as opposed to stupidly). I'm not trying to make a distinction according to the severity of mental disabilities, but between having one and not having one; between purposefully or negligently acting "stupid", and having no other choice. Which, again, can never be done without error; so some people will be unfairly blamed or the other way around. But the alternative is to just accept any kind of stupid behaviour, even from people who have the option to behave better.
>Why do we blame dumb people for their dumbness, but only if they're not too dumb?
Adding to what I wrote above: Ideally we shouldn't blame "dumb" people at all, just relatively smart, or normal, people who act dumb. I will agree though that we (myself included) often do exactly what you described, blaming dumb people, which is wrong in my opinion. "Dumb people" should be understood, educated and/or supported, not blamed.
Train to do what? There are tasks people with severe mental disabilities can still be trained to do, and there are tasks that anyone who isn't a six-sigma genius can't be trained to do.
"Mentally handicapped" is generally considered to be where someone's capabilities are hindered to the point that they can't function in a normal autonomous way and perform basic life tasks. "Acting stupid" applies to people who don't have such a level of hindrance but are instead not reasonably utilizing the adequate faculties they do have.
So, back to your example: if you lack the capability to cross a street on your own, or to even understand what that means and why it can be dangerous, no one faults you if you are harmed. If you are capable of crossing a street and appreciate the danger but choose not to take proper care when doing so (e.g., you step out onto the street while texting and are struck by a bus), people fault you.
You seem to be making unsupported assumptions and conflating all sorts of things when it's really not complicated: was the person capable of understanding the situation and acting on that understanding?