No one knows for sure, just as no one knows exactly what causes any gender identity. It must have a basis in genetics, but occurrences of transgendered persons cannot not be accounted for through any application of Mendelian genetics. Alternate genetic paths are a good candidate, along with errors or differences in fetal development. Lastly, there is three to nine months of intensive data uploading, where every child’s brain is programmed to start the life ahead.

In the end, it is all in the head. All identity, all decisions, all responses to hormones must be processed by the brain.  And, of course, unless caused by some kind of physical or substance abuse, it is all natural.