Do you have a source for the claim that fat people don't generally have slow metabolisms?
I assume you mean some kind of formal reference as opposed to common sense arguments or general observations. If so, are you seriously skeptical of the claim? If the answer is "yes," then I will try to dig something up.
If so, are you seriously skeptical of the claim?
I am sceptical of that claim, too.
I don't expect all fat people to have slow metabolisms, but I expect slow metabolism to be more prevalent among fat people as compared to thin people.
For example, there is a correlation between people's metabolism slowing down as they age and people gaining weight as they age (we're talking about the progression from, say, the 20s into the middle age, not old age). I am not ready to pronounce it a causal relationship, but the correlation is there.
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.