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).
I became an atheist fairly early, but it took me longer to realize there was no Santa Claus. The idea didn't make sense, but the presents appeared under the tree, and my parents denied being responsible, so clearly they'd gotten there somehow. I concluded that I just didn't understand some important part of how the world worked.
One year, we'd just moved into a new house. For the first time, we had a real fireplace, made of brick. I excitedly spoke of how this would make visiting much easier on Santa, but wondered how he could make it down a chimney at all, and began making plans to string a net of dental floss across the opening in an attempt to see how Santa dealt with the obstacle.
I had been leaning on the brickwork, looking up the flue, as I said these things, and as I turned around I intercepted a look my parents were giving each other. Translated into English, it might have said something like "Isn't this precious?"
In that moment, I intuited that there was no Santa Claus, and that my parents had been lying to me because they thought my belief was cute.
I had already learned that not everyone was my friend. I already knew that some people who weren't my friends actively wished to harm me. But that was the first time I really grasped the idea that my parents had goals and preferences of their own that they would choose over my welfare, that I couldn't rely on them not to harm me for their own benefit.
Before that time, I took for granted without thinking about it that people's stances toward things could be easily derived from what they said and did. Enemies were obvious; so were friends. Only afterwards did I really understand not only that appearances were deceiving but that people would actively create false appearances.
Instead of relying on my first impressions, I began to withhold judgment and (although I lacked the words to describe it at the time) actively seek new evidence to test my beliefs.