(Background: I have a "non-verbal" sister with autism who is about to become a legal adult, and I worked with her special education classmates, including other autistic children, while I was in grade school. My answer focuses on autism and not really on IDD.)
Originally written December 2017. Updated June 2019 with some additional information. Some stuff has been crossed out but nothing has been removed.)
For reference, this page has the DSM-V criteria for a diagnosis of Autism Spectrum Disorder. (Note that this has changed since the DSM-IV since ASD is still relatively new and we still don't know a lot about it.) A simplified version:
- 3 out of 3 social differences/deficits:
- Missing or excessive components of social reciprocity/back-and-forth interaction
- Missing understanding of nonverbal communication (e.g. gestures) and/or missing/excessive use of them
- personal note: all 3 of these criteria have variations by culture, but this criteria probably has the most variation
- Missing understanding or persistent misunderstanding or no interest in learning about how relationships are developed and maintained
- 2 out of 4 physical/sensory/interest differences/deficits:
- Repetitive physical movements (e.g. fidgeting); also known as stimming
- Very strong need for sameness (e.g. rigid adherence to routines) and a lot of trouble adapting/being flexible (often clearest in childhood before the child learns about or is socially conditioned to hide distress)
- Restricted, very strong interest(s); can be in something generic (animals) or something really specific (the lore and world of Legend of Zelda) or something that seems unusual (fans--as in the kind that blows air at you); also known as a special interest
- Atypical sensory processing, whether oversensitive or undersensitive or both (more info later); also sometimes known as sensory processing disorder
- Present from a very early age, possibly birth
- may not be apparent until later, when finally overwhelmed
- may be hidden later due to learned coping mechanisms or hiding techniques
- Significantly affects life (even if other people don't see it)
- Behaviors/etc are not better explained by solely an intellectual developmental disorder or solely a global developmental delay (autism and IDD are frequently comorbid, though)
The story demands that he be far from being able to function normally
(otherwise, he would have been next in line to the throne).
If this is the case, you may benefit more from finding accounts written by family members/people close to autistic people rather than narratives written by autistic people themselves. A lot of those with autism who write their own narratives are closer to "high-functioning"; it's hard to find narratives written by those who are "far from being able to function normally." Someone who can write (or dictate) their own narrative with a lexicon beyond elementary school but who cannot function normally is more likely to be someone with a physical disability rather than an intellectual disability or autism.
That being said, narratives written by people with autism can still be a starting place, and those narratives are likely to have stories of how the narrator was treated differently in positive and negative ways, how their differences were accepted or rejected, how they were accommodated or bullied, etc. which could still help? Especially when it comes to understanding other people, as many autistic people, including those who are "high-functioning", process social interactions differently. (Refer to the DSM-V's social criteria for ASD.)
I've tried to look for information on the web, but most of what I find
is either very general ("all autistic people are different")
Unfortunately this is true since "autism" is a giant spectrum. For information about the behavior of an autistic person who would not be the heir to the throne:
I almost want to tell you that you should look for the really old stereotypes of autistic people, because those stereotypes stemmed from the atypical behavior of people who were likely on the more severe end of the spectrum. Might be better to look for the stereotypes of autistic people from the last few decades, when more "low-functioning" autistic children were diagnosed and became visible to society. Society was pretty quick to judge them.
Would it be reasonable to make him have autism, and use a letter board
Sounds like he'd do just fine as a mute king. (Unless mute kings aren't a thing.) How far is "far" in "far from being able to function normally"?
- If my sister wants something, then she will look for it on her own, but she usually will not ask for it unless someone asks her "What do you want?" (We've been trying to get her to initiate requests for years with some degree of success.) Previously she didn't even look for what she wanted on her own. This includes essentials such as food, water, and the bathroom (we treat her well, so perhaps that's part of the reason that she didn't ask for them--because she knew we'd give them to her within a reasonable timeframe. But we noticed very early on that unlike me, she never looked for food or water between meals, and she would also immediately use the bathroom after an outing in which she did not use the bathroom. We've been trying to get her to tell us if she wants to use the bathroom when we're out of the house. She has no problem using the bathroom whenever she wants to in our house or at school, so we think the culprit is the uncertainty of an unfamiliar environment.)
- My sister doesn't really have a first language. She's brilliant and exhibits reasoning/deduction skills, pattern recognition, etc. and she can learn by observation. But she didn't really learn a first language, so now she has no grammar for any language that would allow her to really read and understand information about the world. (I suspect she still doesn't know what a president is, and she definitely doesn't understand hygiene. She can learn through observation, but how can you observe germs if your hands don't look dirty?) This probably has the biggest impact on how she functions every day, although it's an indirect impact. Without language, she probably can't understand what's going on in the world. Her world might be limited to the places she's been.
- Her autistic classmates, both those who were also non-verbal and those who were closer to "high-functioning" (but not far enough to not be in special education), seemed to be unable to understand or have a harder time understanding or remembering unspoken social rules, including simple ones such as if someone is standing up and speaking into a microphone and everyone else is sitting, then you should sit, too. I imagine this would be a much bigger deal in a royal family!
Does your character have a first language? How much does your character understand? Then go from there.
How would a person with autism respond to a situation with a crowd,
colours and noise, such as a royal wedding?
I think the majority of people with autism are more sensitive physically than non-autistic people, so they can get overstimulated with too many colors and noises. I think a lot of them also don't like crowds, although I don't know if it's because something about crowds overstimulates their senses (body odor? too much touching?) or if it's because of something else, such as a preference for being alone or with fewer people.
Paragraph above still kind of stands, but here's a better version:
A lot of people with autism have different sensory processing than other people. They may be oversensitive/sensory-avoidant or undersensitive/sensory-seeking. So depending on which autistic person you encounter, if you throw them into a royal wedding, you are likely to get one of these two reactions:
- No! Too much! I need to run away from the colors and noise and people!
- Yes! More colors! More noise! More people! Still not enough!
And if you don't get one of those reactions, you may get a mixture of the two, as oversensitivity and undersensitivity isn't total; you can be oversensitive to most things but undersensitive to a few things, or vice versa, or you could even have an equal split. So your character could crave visual stimuli and love all the colorful decorations and flashy outfits while simultaneously hating all the noise and chatter and music and footsteps.
Personal examples of oversensitivity: My sister and at least one of her autistic classmates put their hands over their ears when they hear something that's really loud. If there's room and the loud noise doesn't stop, my sister will move further away from the source. (My sister also happens to be hearing impaired, so if it gets really bad, she just turns off her hearing aids.)
I myself also have done very poorly with loud noises (I almost cried during my elementary school band's concerts because I hate how powerful the booms are in person) and have only recently discovered that my sensory processing is apparently atypical (possibly Sensory Processing Disorder) and actually akin to the oversensitivity of some individuals with autism. One of my smaller quirks that I share with some autistic people is that I can't stand sock/stocking/etc seams anywhere but the top of my foot.
Examples of how you might see undersensitivity: seemingly high pain tolerance (possibly linked to self-harming stims, such as hand-biting and head-banging), inability to sleep without a heavy/weighted blanket, constantly needing to touch things (can be everything in general, can be specific textures)
Atypical sensory processing is very common among individuals with autism and is listed as one of the possible criteria in the DSM-V, but note that it is not a required criteria (it's in the "2 out of 4" section)
What should I avoid by all means if I don't want to offend, but to
give a positive representation?
This is tricky to answer. My answer is in the form of what you might want to try rather than what you should avoid.
If you want to give a positive representation, show this character excelling at something. It doesn't even have to be a "productive" thing that lets him directly contribute to society, although that would definitely be great; it can be something as simple as being really good at organizing (maybe he sorted the entire royal library in a day and now everyone who goes there can find things way more easily), or maybe he's a great gardener, or maybe he's a fantastic painter and has an amazing eye for details*, or maybe he's very sensitive to people's emotions and always discreetly lets his sibling know when that sibling is pissing someone off by tugging at the sleeve...
* On one end of the spectrum, we have Temple Grandin and her amazing eye for detail. On the other end of the spectrum, we have my sister and her amazing eye for detail.