Why is it that a mentally handicapped person looks that way? Why is the condition mental and physical? How long until someone is diagnosed as mentally challenged? Can you tell at birth? Or are they the same as everyone else at birth, but develop slower than others, or simply peak with mental development early? Lastly, how much self awareness do they have over their disability?
A mentally handicapped person may look that way because their status as mentally handicapped is just one symptom of a larger genetic problem.
Some syndromes (such as Down syndrome) may have trademark physical aspects, and that will probably be evident at birth. More evident would be congenital problems such as heart problems or digestive issues. Besides, all babies look funny. 
They usually diagnose someone with a developmental disability when they see that the child isn’t progressing to milestones at a rate that other children would (they crawl later, etc.) With some more severe syndromes, it may be apparent earlier. Also, some of the other related symptoms might be evident, and the doctors/parents might be aware that there is a high chance the child will have a developmental disability.
Self-awareness: There is a wide range in those who are mentally handicapped. Some are quite aware. I went to a seminar where a phD who had a child with Down syndrome spoke. This kid plays piano, does geometry, plans on going to college, and competes in the Special Olympics. Yet, she knows she’s different. Self-awareness might vary by IQ, or it might be something that can’t ever be detected, if the person has trouble communicating.
Looks what way? People with Down’s syndrome “look that way” because it does outwardly affect the features as well as cause a mental handicap. Other sources of mental handicaps do not necessarily have any physical manifestations. With regards to self awareness, do you know that there are people with greater or less intelligence than yourself? Why do you expect anything different from people with a mental disability?
My younger brother is mentally retarded and he looks quite normal.
When I went to a NARPA convention as a delegate concerned about the treatment of people diagnosed “mentally ill”, I ran into some advocates who were labeled “mentally retarded” and they were there to organize for their rights. I will confess to astonishment. (I would not have been astonished if they had been picked by social workers or program directors to represent a group or a professional-run organization, but they appeared to be there strictly as individuals, one of them telling us about how the group home didn’t want them to attend this event)
One problem with this question is selection bias. The OP has certainly run into many people who were “mentally retarded” and didn’t realize it at all. The definition for mental retardation varies but an IQ score of 70 or lower is one common definition.. That includes about 2% of all people. Do you think that one in 50 people (who actually are retarded by this definition. many look it but aren’t) look it outwardly?
I know two people (one male and one female) from wealthy families that had a very difficult childbirth and their oxygen supply was cut off for a short while. They are retarded by that definition yet they look normal, dress well by any standard, have jobs, and personalities that are vibrant enough where you wouldn’t know there was a problem unless you spent more than 10 minutes in conversation with them. They do have problems but you would never know having a short interaction with them.
Like others said, the people that look obviously retarded have a syndrome that affects the body as well as the brain in most cases. In other cases, brain damage can cause a change in looks as well even if it occurs later in life. People are very sensitive towards facial expressions and brain damage can cause changes that others can easily pick up on.
Besides the fact mentioned above that things like genetic disorders and fetal alcohol syndrome can cause characteristic facial features in addition to mental retardation, your brain is responsible for things other than thinking. It’s very common for the same brain damage that causes retardation to affect a person’s motor functions. This can cause (among other problems) slackness of facial muscles and difficulty swallowing that can make someone “look retarded.”
Got a 40 year old friend who got a bad case of virile encephalitis (sp) and it changed him totally, his look, walk, stance, expression, everything.
It does not show much in people that are just low IQ a lot of times, just look at Hollywood. Manny beautiful peoples with single digit IQ’s.
The seem to gather there for some reason.
I’ve been wondering about this very thing lately. Certainly, half of the the people you meet will have below average IQ’s, and some will be way low. Even so, I do not notice most of the time. As somone who has a fairly high IQ, I find this interesting, and I think it’s because all in all IQ isn’t the defining characteristic that we sort people out on.
Of course, I usually associate with smarter people due to my job as an engineer. Other people, such as the guy who I carpool with (who is an officer in the Army National Guard) meets folks from a much wider cross section of people, and he can pick out the idiots pretty fast. Not that they are useless, but that they need much more supervision, which is his job to look for as the officer.
The point is that most ‘stupid’ people are not so stupid as to be immediately noticeable. Hiding in plain sight, so to speak. (and all in all, not as stupid as you might think). As for the severely impared, perhaps they way these people ‘look’ is a sign to everyone that they have special needs. If the way they ‘look’ is enough to get you to reconsider how you interact with them, then it works.
Further points for consideration:
Institutionalization leads to certain forms of dress and behaviour that are recognized as ‘acting retarded’.
Parents tend in some cases to dress their children and young adult children in ways that mark them as different. Additionally, some parents will limit the chances for socialization for their children which ends up with the child not learning how to look or act normal for their age.
The above lead to a certain ‘geeky’ appearance that is societally recognized as ‘retardation’.
People with these conditions will also be at increased risk of being medicated. The side effects of these medications include- drooling, mouth movements, altered modes of walking, strange arm and bodily movements, grimacing, tics, dullness, obesity, lethargy. All of the above are associated with the stereotypes of madness and stupidity and act as signs that the person is different.
If the person is forced to spend much of their time (in residential or work facilities) then they will tend to learn abberrant behaviours from other service users and conform even more closely to the stereotype for ‘being retarded’.
It could also be the way some of them dress. I met a guy and his mother at the bowling alley once, I would never have guessed he was retarded if he wasn’t wearing his jumper tucked into his track pants.
I don’t think I’m prejudiced here, and I don’t claim to be able to tell all such unfortunates by their physical appearances, but I think I understand what the OP was getting at. And it’s not clothes. I used to work near a center that cared for people with mental disabilities and gave them meaningful work to do that, I’m sure, served as therapy. I’d frequently pass groups of them taking a walk on break, and I couldn’t help but be struck by how all of them seemed to have facial irregularities – crooked teeth, lopsided looks, walleyes, and the like. It’s not the kind of thing that they could help (you can have bad teeth from lack of care, and my own teeth aren’t perfectly straight, but theirs were severely out-of-line).
I’m going along with what Paranoid and others have suggested:
I have a mentally retarded sister. She “looks” retarded. I cannot remember if she looked that way at birth. All people with her disability share the same facial features, it is a characteristic of the syndrome*. So while some retarded people will look like normal people, others won’t. It all depends on the syndrome (which will usually be genetic when accompanied by the “retarded” appearance).
Down’s kids, I find, are overly represented in the retarded population, and have the mongoloid appearance.
Random mental retardation link.
*Which includes a heart defect, so not just the brain is affected. Her muscles are spastic also, she can walk, but never “normally”.
Geez, nobody saw that episode of South Park? I thought the fact mental retardation is a symptom that can be caused by one of many conditions, and that Downs Syndrome is the most common one that manifests itself the most physically, had entered general pop-cultural knowledge. The funniest gag in that SP episode was when Cartman’s mom told the Special Olympics secretary, falsely, that her son was “retarded”. She asked his mother how he was retarded, and ran off a list of common conditions, and Ms Cartman sputtered and said “Uh. I think he’s just retarded”. I thought this thread would have been over by the second post, but the tentativeness of some of these responses are surprising. ::shrug::
Are you sure they weren’t just English tourists? 
Anyway, to the OP, genetics, sure. Down’s syndrome and the like.
But also behavioural clues, I suppose, like awkward social behaviour, tends to be a big clue for me.
One of the problems I (and probably many people of my generation and older) have with the “ghetto” fashion styles is that when I was growing up, if I saw someone wearing their ball cap sideways or backwards, with oversized baggy clothes and untied shoelaces, I would have been 100% correct to think that this was someone who was significantly mentally handicapped. I’m sure that this colours my impressions of teens and young adults I see today wearing this look as a deliberate fashion style.
While we’re fighting ignorance on this topic, help me fight some of my own.
When I’m out and about and run across kids with downs syndrome I have noticed that they are usually with “older” parents. That is, the kid looks maybe about 18-20 but the parents look to be in their 60s.
Is their a corellation between downs syndrome and women having kids past their mid 40s or is there another reason I observe this. Like maybe the kids mother was a alcholic or drug user and the kid ended up being raised by his granparents?
Please fight my ignorant observations.
The wild, wreckless scramble of rationalization never fails to entertain.
I don’t have a cite handy at the moment, but the chances of down syndrome begin to significantly rise when a woman turns 35.
Hampshire: Yes, the risk of a woman’s child having Downs Syndrome gradually increases with the woman’s age at conception and sharply increases at the age of 35. I believe that the father’s age also has an influence, although not as clear-cut. It is important to note that even women in their teens and 20s can have children with Downs Syndrome.
More info here: http://www.nichd.nih.gov/publications/pubs/downsyndrome/down.htm