In the same way saying that no one serious believes in perpetual motion machines is also a True Scotsman argument.
Yes, I would say that it suffers from basically the same same problem. Do you agree that there are people out there who believe, in good faith, that perpetual motion machines are possible? Do you agree that there are people who invest a lot of personal time and energy into constructing perpetual motion machines?
Has adequate knowledge of human physiology and is not trolling :
Okay, 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?
And let's look at the girls in the two videos they linked. Although they did not make general statements about the causes of obesity, they did seem to believe at the beginning that their obesity was caused by slow metabolism, agreed? Would you agree that those two girls were not trolling? And in your opinion, did they have adequate knowledge of human physiology? If not, how do you know it?
In short, are those two girls "reasonably serious"?
Here I mean "serious" in the external-observer sense -- to denote people whom others consider serious = worthy of attention and time. I do NOT mean "serious" in the internal perception sense (as in e.g. "I'm not joking, I'm serious").
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.