All animals initially start out as female. During the intial stages of development, the XY fetus is morphed into a male.
In the August 2005 Journal of Psychoneuroendocrinology, researchers suggest a genetic link. As I recall, the genes responsible for rewiring the fetal XY brain may be on one of the same chromosome responsible for introducing new variations into the human genome. The speculation is that the gene responsible for reprograming the brain is altered through normal genetic variability. If the gene is altered, the enzymes responsible for morphing the brain are built incorrectly. This prevents the brain from being rewired from female to male.
This explains the results published in the Vol 85, No 5 Journal of Endocrinology and Metabolism. There, researchers found that MTF transexuals have a female brain and FTM transexuals have a male brain.
Chaunte