To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).
The distant: I am diagnosed with Asperger Syndrome (an autism spectrum disorder). I was unpopular at school, I understood poorly how to fit in, but I understood well how to get smarter. I took high school completion mathematics exams in primary school, university exams in high school, and while I never bit a teacher like in HPMoR, I did punch a student one time and demand that the teachers back me up.
As I remember it, said student was talking about a "tirus" which was supposedly like a next-generation virus which would eat up your computer unless you washed your motherboard properly with soap and water. Being a nerd with some technical aptitude, I told him that he was a bullshitting liar and he was to show me what he was on about or stop it immediately. He continued, so I told him to shut up or I'd hit him. He still continued, so I punched him in the stomach, which winded him and made him shut up. The student was surprised that I had carried through my threat, the teachers were surprised that I was unapologetic, and I was surprised that the teachers (this was at a religious school) were putting a distinctly man-made and exception-deformed rule about limits to hitting over a clear and obvious divine commandment against lying.
In retrospect I think I was somewhat lucky here. I was a youth in an argument turning violent, which is hardly a rational state to be in at the best of times, and in the process of trying to excuse myself for committing violence, I happened to take a stupidly defiant stance on the ground of "He was lying!" and got this bound up with my identity, the boy who really hates lies, for the next few years.
The local: I was arguing on Civilization Fanatics' Center in the Off Topic forum some years ago when a poster named Integral gave me a link to 'applause lights' at Overcoming Bias. I then read OB a lot, read the posts that would eventually get moved to Less Wrong, and ended up here.