I do it too. There are times when you can't shut me up and times when I barely speak a word. Most of the times, I am quiet in situations where I am the outsider or dealing with depression. I like to observe people in a new situation to try and figure them out first before engaging in much conversation. Get me into a conversation about a subject in which I am well-versed though and it is hard to keep me quiet.
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Uncertain of diagnosis, either ADHD or Aspergers.
Aspie quiz: 143/200 AS, 81/200 NT; AQ 43; "eyes" 17/39, EQ/SQ 21/51 BAPQ: Autistic/BAP- You scored 92 aloof, 111 rigid and 103 pragmatic