There are many potential right reasons. One wrong reason might be "I heard it from a Facebook group founded to spread the word that GMOs were created to make a pliable citizenry".
Isn't that equivalent to an ad hominem fallacy? The truth of a fact or the validity of an argument is independent of the characteristics of the person saying it or the forum in which it was posted.
For example, suppose I claim to have found a fatal flaw deep in the most difficult part of Wiles' proof of Fermat's Last Theorem.
If someone says that I'm unlikely to be right because my mother was a hamster and my father smelled of elderberries that would be an ad hominem fallacy.
My mother's species and my father's smell have no bearing on my mathematical claim.
However, if someone says that I'm unlikely to be right because I have not studied any higher math beyond what was covered in classes for my bachelor's degree in math from Caltech and so it is quite unlikely that I have any clue as to what is going on even on the first page of the proof let alone anywhere near the most difficult part (or could even figure out what part is the most difficult part) that would be an ad hominem but not an ad hominem fallacy.
It's not a fallacy because it is questioning whether I have the knowledge to be able to even make a valid argument about details deep in Wiles' proof. That has bearing on the likelihood that my mathematical claims are correct.
