I used to think that overweight was caused by slow metabolism, i.e. that generally speaking fat people are people who have slow metabolisms and thin people are people who have fast metabolisms.
I believed this because (1) it is the conventional wisdom; (2) it is consistent with the observation that some people seem to be thin even though they stuff their faces; and (3) it makes sense from a thermodynamic perspective that someone with a slow metabolism would be prone to putting on weight and someone with a fast metabolism would be prone to staying thin.
Putting aside the fact that this belief was wrong, there does seem to be a certain degree of irrationality about it given the observation that people don't vary all that much in terms of body temperature. Therefore they must not vary all that much in terms of metabolism.
Therefore they must not vary all that much in terms of metabolism.
I don't think that follows, or at least not without a lot of other explanation, even if you grant that temperature doesn't vary in any significant way between people (which I'm not sure I do). The body has multiple mechanisms for maintaining temperature, of which metabolic rate is only one. It seems entirely plausible to me that people run their metabolisms at different rates and adjust their peripheral vasodilation and sweating rate to balance it all out near 37 C/98 F. Core temperature might vary between people by only a few degrees, but surface temperature varies much more widely.
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.