Personally I feel like the big thing to come out of rationalism is the insight that, in Scott Alexander's words [0] (freely after Julia Galef),

> Of the fifty-odd biases discovered by Kahneman, Tversky, and their successors, forty-nine are cute quirks, and one is destroying civilization. This last one is confirmation bias - our tendency to interpret evidence as confirming our pre-existing beliefs instead of changing our minds.

I'm mildly surprised the author didn't include it in the list.

[0] https://www.astralcodexten.com/p/book-review-the-scout-minds...