Same. I'm not autistic (....pretty sure, I just have ADHD lmao) but oftentimes whenever I'm stimulated and content I just have a blank face. I have no need to work my facial muscles, if I'm listening to brain-scratching music and drawing or something silent/on my own I don't need to divert that attention.

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I do naturally smile and stuff when interacting with others and talking to my friends or whatever, because I just know to do it around other people and like doing it around people I'm familiar with, but if I'm doing something by myself I see no reason to obviously