A few years back there was a reddit AMA on "people with autism, how would you describe it to people without?". I read it out of curiosity, and with a growing sense of unease that most of the things people described I identified with myself! At the time I largely shook off -- I'm an introverted nerd, of course I like spending time on my own and focussed on an interest -- but a pretty common thing mentioned was "it feels like everyone else got the instruction manual of how to behave and socialise and I missed out", and I felt that one in my soul. More recently my brother was exploring the possibility he might be (ND friends had suggested it) and he passed on the embrace-autism.com website which I again identified pretty strongly with, and more pertinently, absolutely aced the self-assessment tests. It still would have been just a now-strong-possibility, but otherwise not that relevant, but after attending a particularly loud party with my girlfriend I had a shutdown episode. I'm pretty sure I've experienced it before but probably not in front of someone like that, and that was when I decided to investigate more formally.