I am, rather more than most around here, pretty well versed in psychology. At least given my interested amateur standing; do not take anything I say as a professional medical opinion, because it assuredly isn't.
That being said, yes, some of them are mentally ill, but no, most of them are not. I have observed what I believe to be signs of schizophrenia in a rare few of them, but there are a lot of other factors at hand. Some are ignorant; I would, in fact, say that most are ignorant. Well, we're all ignorant of something--of many things, even. The issue becomes deeper when the ignorance becomes willful.
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Gillian
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"