It seems, anecdotally, to be quite common. I certainly did as a teen, and have had the urge in times of extreme stress, although I've been able to resist almost completely since I was out of High School. I have read that one of the reasons Aspergians self-harm is to create an emotional response in themselves - because external stimuli so seldom have as great an effect on Aspergians as they do on NTs, but we still *need* to experience certain things, when we're hurting or in emotional pain, to accentuate that feeling and allow it to be experienced fully, we'll do something that increases the experience. For instance, for years, nomatter how badly I was hurt emotionally, I couldn't cry without adding physical pain into the mix, which I did by cutting or scratching. Sometimes, you need to cry to get the emotion out, and the increase in the feelings by bringing in the physical made that a lot easier.
There's also the stimming aspect (which is also sort of stress related), and I'd guess just good-old self-loathing for some of us, as well as a destructive loss of control during meltdowns. Lots of reasons. I think the physical manifestation of an internal urge is really what it's all about at its heart, though.
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