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).
When I was 5 or 6, I wanted to be a palaeontologist. I ended up with a small collection of fossils and whatnot, as well as an awareness of both evolution and how old the Earth is. As the infallibility of the Torah was obvious, I assumed that its interpretation, which I wasn't yet old enough to learn, explained how everything could be reconciled. In any case, I had much more serious things to worry about. For instance, the boys and girls I knew showed no inclination to date or marry - therefore the human race would shortly end, unless the Moshiach came. Having tagged 'God exists, Judaism is right' as being obviously true, I simply didn't think about questioning it for a long while. As time went on, I did realise, slowly, that just because I'd decided something was correct, like inexorable human extinction, didn't make it so, and that I was not infinitely smart.
When I was 13, I had my Bar Mitzvah and was surprised that I didn't start observing the strictures of Orthodox Judaism, such as not manipulating electricity on Saturday. Now that I was responsible before God for my actions, I should have been much more compunctious, but I couldn't believe that I would be punished for such things, while non-Jews wouldn't be. I became obsessed with the idea that people like me existed who didn't believe in the infallibility of Judaism. They believed in something else, like the Koran or the New Testament. Why was I Jewish, except that my parents were? I started to think of reasons, but somehow they all depended on the infallibility of the Torah... OK, so suppose for the sake of argument that I don't believe in Judaism; what arguments would convert me to Judaism and not, say, Islam? I realised two things at that moment: first, that I would never have believed the truth of the Torah if I'd learned science first, and second, that Buddhists didn't even believe in God, never mind the Bible. From then on I was provisionally Deist, having no better explanation for the beginning of the universe. I wasn't a very strong Deist, because of what I knew about physics and evolution, but a Creator seemed logically necessary. My idea of looking at my 'deepest beliefs' from a neutral standpoint seemed so much smarter to me than just about every other thought I'd had, that I began to do it all the time. So, at this point I'm a Deist and starting to think rationally a lot more often.
I thought a great deal about God in the next months. When I tried to work out what sort of God could create the universe, I (eventually) realised that the most I could say was that it could create the universe. I'd become very well accustomed to bad logic after a few months of reconversion attempts, so it was obvious that I'd just proven the tautology "If a Creator exists, it exists." My intuition had led me astray: there was no compelling argument for an intelligent Creator on the tip of my tongue, just an unquestioned assumption that the universe had to be caused. That was the end of my Deism.