What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.
And how exactly do I do that?
Why not?
And if someone asserts or implies that generally speaking obesity is caused by low metabolism, how do I know if they have adequate knowledge of human physiology and are not trolling?
Oh, dear . If you really have no idea -- none at all whatsoever -- how to find out whether people you're listening to are credible, please go figure out how to do this. This is going to be a much better use of your time than posting on LW.
Because as far as I can see (which, as I mentioned, isn't very far) they were just fat girls specifically selected for having a particular false belief so that this belief could be debunked on video.