There are many factors that have contributed to this diversity. The diversity of gender classifications and sexualities depends upon cultural norms. In many parts of the world, particularly Africa, Middle East, and the Far East, this diversity is not as common and many times socially unacceptable. This diversity found in North America, South America and Europe can be attributed to the direction the development of the culture took in the 20th Century. To get to the root of the answer, a study into the culture's of different nations and regions has to be conducted. This should cover relevant topics like family relationships, marriage, treatment of homosexuals, divorce rates, religion, moral values. The list is not exhaustive yet provides insight into where parts of the answer to your question may be found.
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