Most people with really severe Autism can't speak, or if they can they can't speak to other people properly because they're too shy or have no concept of any social interaction whatsoever. (Us high-functioning Autistics/Aspies have at least some concept of social interaction).
I've met 2 different people with severe Autism (both boys) in my life, and they were diagnosed with low-functioning, and neither of them could speak. They would just make noises (perhaps they used those noises as their only way to communicate, like an animal. I don't know). I'm no psychriatrist - all I know from my experience is about 77 percent of low-functioning Autistics can't speak (that was what I was always told anyway). IQ doesn't come into it - I'm talking about the social interaction part.
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Female