A mix of options 1 and 2. To the extent it exists, it's obviously a mental disorder, but I still suspect that it is often the result of parents, teachers, or society telling kids in formative years that it is a possibility, which makes them internalize the feelings that they are the other gender, even though they are not. But, it's not a real, legitimate thing, and changing the culture of that would go a long way.
There's a ton of research on this subject that suggests otherwise. So why don't you prove how you're right? Otherwise, admit your views are fueled simply by bigotry.
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