I had the same idea. I figured it might well be an intellectual disability, which is most often invisible. MM in particular referred to Arthur in very derogatory terms, which another poster suggested she would not have used if he actually had an intellectual disability. I agree with that -- in a way. If MM actually
knew a person had an intellectual disability, she probably would not speak of him or her using opprobrious epithets. But I suspect she and most others in their circle would not necessarily
recognize intellectual disability in a young adult who doesn't have concomitant physical issues, such as sometimes occur with autism, spina bifida, etc. (conditions that may or may not include intellectual disability but which generally have observable physical characteristics).
Although an adult with intellectual disabilities in the mild to moderate range may look quite normal, speak well in everyday situations, he or she will often have trouble fitting in with peers because of the very real developmental differences, of which peers may only be partially aware. They may ascribe the individual's lack of social competence or difficulties in certain situations as due to laziness, stupidity (ordinary stupidity, not a handicap) or just being "weird."
Although the Supreme Court (R. vs. D.A.I.) ruled that people with cognitive handicaps cannot be excluded from being witnesses, especially when they need to speak to abuse or assault, they have to be interrogated carefully and with no "trick" questions or verbal subtleties. In general, they are not usually called as witnesses where they are not directly involved, because their limited comprehension also limits the value of their evidence. If a person with an intellectual disability is called as a witness, he or she is presumed competent to testify unless that competence is challenged, in which case the matter is settled by a
voir dire before the judge prior to the witness being called.
Some of this is explained here:
https://en.wikibooks.org/wiki/Canad...monial_Evidence/Competence_and_Compellability