Could be that there are kinds of nonverbal communication that autistic people typically miss, and kinds that we don't necessarily miss, and that it's the kind that we miss that gets noticed. Similar pattern could exist expressively.
Also, some autistic people can read more standard nonverbal communication as long as they're not the one directly involved in the interaction, or else can understand it but only after a long delay (kind of like lag on the net).
Temple Grandin writes about how she can understand tone of voice, but if she does, then she can't understand the words, so she concentrates on the words instead. So for a lot of autistic people it could be more that they can only interpret one thing at once, rather than just an inability to interpret voice tone/body language/etc.
Note that if I do have any kind of charisma, it's not whatever kind makes you... known for your great NT-style social skills or something. I still constantly experience all the typical gaffes and so forth, and so do all the autistic people I can think of that are perceived as charismatic.
And, yeah, the ability to perceive charisma would point at that being perceiving some kind of nonverbal or implied communication, even if you're not perceiving, say, someone's emotional state while you're talking to them.
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