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.
False belief: That in the U.S. the death penalty was cheaper than life in prison.
Believing this wasn't rational. I didn't take such basic steps as looking up the costs surrounding executions or life imprisonment. Executions get much more appeals, trials and legal attention.
False belief: That in the U.S. deaths by firearm are generally homicides, not suicides.
Believing this also wasn't rational. I didn't take such basic steps as looking up available death statistics.
Actually, looking through things potentially on the list for me, a lot of them seem to have the following general form:
1: Something is asserted.
2: I think: 'Yeah, that sounds plausible.'
3: I don't bother to look up any data about it, I just move myself to the believe column.
4: Later, someone else reports data about it.
5: I'm surprised that my earlier beliefs were wrong.
I've since became more skeptical of believing things based on just assertions, (I can even recall a recent instance where an assertion popped up on TV which my wife believed, but which I was skeptical of and which upon looking it up we found data didn't support it and that they were massively overstating their case)
But I can definitely recall beliefs that I have had in the past that were fundamentally just assertion based and the followed the above pattern.
I would have expected accidents to lead that metric. A quick check of the actual data says it's negligible. Time to rescind my support for gun lock laws (except perhaps to reduce the likelihood that people purchase guns in the first place).