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 think I began as a rationalist when I read this story. (This was before I had run across anything Eliezer wrote.) I had rationalist tendencies before that, but I wasn't really trying very hard to be rational. Back then my "pet causes" (as I call them now) included things like trying to make all the software transparent and free. These were pet causes simply because I was interested in computers. But here, I had found something that was sufficiently terrible and sufficiently potentially preventable that it utterly dwarfed my pet causes.
I learned a simple lesson: If you really want the things you really want, then you need to think carefully about what those things are and how to accomplish them.