I'm not sure if I have a flat affect per se, but my therapist has noted that when she first started seeing me, she did find me "slightly perturbing". That was in response to my facial expressions, or lack thereof. Apparently, according to her, my responses are okay when I'm talking - it's when I'm not talking that's the issue. Apparently I can go from somewhat animated [when talking] to zero [when not talking], with nothing in between, but non-spectrum people tend to fade in and out with facial expressions. I had no idea what she was talking about, so she had to demonstrate, heh.
Other people have said similar things about my facial expressions. My boyfriend tells me that oftentimes, it's hard to tell whether I'm joking or not because my face doesn't change much. When I was younger, my mum used to tell me to "smile more" often.
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Said the apple to the orange,
"Oh, I wanted you to come
Close to me and
Kiss me to the core."
Think you're ASD? Get thee to a professional!