I think it really depends. I think there are happy stims, anxious/nervous stims, stims that help with focus, and stims that are automatic and don't seem to serve a recognizable purpose. I have all of the above.
Now, when I say "automatic", I really mean stims that I am unaware of. ALL of my stims are automatic in that I don't consciously choose to do them. However, I'm usually always aware that I'm doing them.
My one "automatic" stim is toe wiggling. I bend and wiggle my toes when I'm completely at ease...when my mind is calm, like when I'm reading a book or watching a movie. I guess I don't know I'm doing this one because I'm engrossed in whatever I'm doing. I only know I do it because my boyfriend has taken to pointing it out (at least he thinks it's cute!).
Pacing is my 'focus' stim. I do it when I'm trying to formulate my thoughts or when I'm talking on the phone.
Bouncing up and down and flapping my hands is my 'excited' stim. I don't flap my hands any other time...and excitement is a rare emotion for me, so I guess it's accompanied by my most exaggerated stim because I'm not used to that level of emotion.
The rest of my stims are what I would call nervous or anxious stims. These happen when I'm doing one thing when I'd rather be doing another. These include leg/foot shaking or bouncing, chewing my nails or the skin around my nails, flicking a pen or pencil against a hard surface, etc.
Oh, of course, there is rocking, which is usually accompanied by an intense feeling of distress.
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Aspie Quiz: AS - 141/200, NT - 77/200 (Very likely an Aspie)
AQ: 34/50 (Aspie range)
EQ: 32 / SQ: 68 (Extreme Systemizing / AS or HFA)
Diagnosed with AS and Anxiety Disorder - NOS on 03/21/2012