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).
Since it would be impossible to disentangle and explain all the different factors, and all the studies say people are terrible at determining what events influence them anyway, I'll just tell the event in my transition to rationalism that makes for the best story:
When I was around five, my kindergarten teacher decided to initiate my friends and I into the Great Miracle Of Life by bringing an incubator full of chicken eggs into the classroom. I watched them hatch, loved the little chicks, and (after some time and other events including a bad experience at a meat-filled Asian restaurant) decided to become a vegetarian and eat neither meat nor eggs.
When I was about eleven, I got quite into politics, and like most people in my area ended up as a typical liberal. And so I was of course pro-choice: why should we respect the rights of fetuses when they're just a collection of cells and not even really alive?
It took me a while to realize that I was simultaneously refusing to eat eggs because potential-chickens were valuable living beings who deserved respect, and condoning abortion because potential-humans weren't.
If I'd been a little older and a little cleverer, I'd have made up some typical political excuse why it was really about freedom or human rights or society or something (or else just learned the difference between fertilized and unfertilized eggs!)
But I was young and innocent enough to take a moment to think "Maybe my brain is just telling me what it thinks I want to hear in each situation, instead of really thinking things through. I should find a way to stop that."