I used to believe that almost nobody was really interested in anything. This was because (a) I had never been really interested in anything and (b) "Passion" was a mandatory signal, required for getting into college. When I saw people who appeared to be genuinedly interested in things (sports, music, running the school newspaper, building robots, whatever), I assumed they were just better than me at sending the required signals. When I got to college and saw people who continued to appear interested in these things, even though extracurriculars were no longer valuable, I realized I had been wrong.
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.