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).
Christianity drove me to rationalism. I went to a Catholic college where we had to take 2 semesters of theology and 3 semesters of philosophy.
We studied Aquinas for weeks. Knowing that Aquinas was generally regarded as the smartest person in the Middle Ages, I was stunned by the stupidity of his arguments. Aquinas could not have been stupid. Therefore, social pressure was capable of warping the minds of the smartest people on the planet for a thousand years. Therefore, it could be warping my mind right now.
Another thing we did was to study a parallel edition of the gospels. That means that it has one column each for Matthew, Mark, Luke, and John. Laying them out side-by-side, I saw many places where Matthew, Mark, and Luke had identical sentences. They couldn't have come up with the same grammar and word choice independently. At least 2 of them had copied from someone else. I had studied the Bible all my life, and was surrounded by hundreds of people who also studied the Bible regularly. Some of them had read it every day for decades. And none of them had ever noticed this; or if they did, they didn't mention it. (It is universally known to Biblical scholars; but most churches take a dim view of Bible scholars.) I realized that they couldn't see it, no matter how smart they were or how much they read the Bible, because their preconceptions prevented them from looking for it.
I was astonished that most Christians have never read the entire Bible. If you believe that God wrote one book in which he said everything he wanted to say to the world, you would read that book. Yet even being aware of this, I found it hard to read. (To this day I suspect I may not have read Haggai.) I knew that this meant my rationality was broken (although at the time I attributed it to sin).
But now I remember an earlier event: I was about five. I was in the car, on a long trip with my family. Traffic stopped. There was an accident ahead. I saw a little dog walking away from the accident, down the road. I said that it was probably from one of the cars in the accident, and that I wanted to get out and pick it up before someone hit it. My parents said that was foolish, and that the dog could be dangerous. Then someone hit it. And I realized that I had been right - and that the fact that there were hundreds of grown-ups around me in their stopped cars, and none of them had done anything, didn't mean a thing.