Is there any evidence to say this takes place more to people on the spectrum? I'm not saying that isn't the case mind you, just pointing out that it may not be the case, I know more neurotypical self-harmers than autistic self-harmers. I self-harmed (cutting and burning with cigarettes) from ages 14-24, with a recent desire to cut due to depression. I've yet to meet another person on the spectrum who self-harms via cutting, hitting, burning, etc. but there seem to be a far few who turn to alcohol or drugs to deal with negative emotions.
There are lots of reasons why people would harm themselves, but looking specifically at "self-harm" it's a result of a lacking normal healthy coping strategies - two suggestions I'd put across is that we're more prone to self-harm because while everyone else was learning coping strategies we were busy learning just to be 'normal', and other people have friends as support. I imagine when neurotypical people feel unhappy about something they spend time with friends and that's how they cope, we don't have that so we end-up turning to unhealthy coping methods such as cutting, alcohol abuse or drug abuse. Those of us on the spectrum are more prone to depression and anxiety, we're also a lot more prone to bullying, lacking in self-esteem, and facing prejudice or intolerance.
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Bloodheart
Good-looking girls break hearts, and goodhearted girls mend them.