As someone who nearly got pigeon-hold into a diagonsis of autism, I am quite literate on the subject.
Autism is a syndrome. We still don't know what causes it. It is and always has been diagnosed according to behavior patterns. All children in the spectrum exhibit these behaviors.
My son just appears a little nerdy to people who know him now. However, when no one is around he allows himself to express his compulsive behavior. We have called it his "skippidy doodahs" since he was little. He will get excited by something online, or on TV or simply in his head and he will start jumping up and down, making hissing sounds and flapping his hands. IQ is part of the difference in the different parts of the spectrum. My son has a high IQ so he is able to understand his condition and control it in public enough to get by as normal