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This will be proven statistically, as in times past, there were very, very few people with gender reassignment issues; but today it is a much more common dysfunction. How common might be the product of additive chemicals in our diet.
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Granted there is more exposure to endocrine disruptors today, many of them are actually iatrogenic. However, one factor in an apparent incidence of transsexualism, is the fact medical treatment (including SRS) is now available. The individuals, who are afflicted with strong transgender feelings, now feel free to seek out physicians and surgeons for treatment.
I have read some old texts on human sexuality, and they mention some cases, which would, today, most certainly be considered transsexualism. One scenario, which happened frequently, was suicide by a male, who dressed in a wedding dress, before committing suicide.
The indication is those individuals were very unhappy, so discouraged, they committed suicide, to escape the pain.
You guys can mess around with those theories all you like.
The cause of same-sex attraction disorder is molestation in the pre-adult years.