Lisa Jain Thompson explained it like this:
Conception occurs at the moment sperm fuses with ova. The resulting zygote, not yet an embryo, has no sexual differentiation. This is how all of us begin, even those of us born in test tubes.
The default condition of a human is female. At the moment of conception we have female configured brains and, without an influx of male hormone, we will be born with a female brain and female sexual anatomy. We call this the standard development pattern.
Sometimes, however, the genetic plan for the developing embryo misfires. The brain’s neurobiology configures first, the body’s later. For a boy to be born, the male hormone testosterone must wash over the undifferentiated foetus at two distinct points in the development.
• The first is when the human brain locks into gender identity. If the male hormone is not present at this time, the neurobiology of the brain will remain configured in the female state.
• The second crucial point is when the human body develops sexual organs, internal and eternal. If the male hormone is present, for whatever reason, the sexual organs will be male; if the male hormone is not present, the organs will default to female.
In an HBS man (female to male), testosterone washed over the brain, changing the neurobiology to male, but not over the body so that the person was born with male neurobiology within a female anatomy
In an HBS woman (male to female), testosterone washed over the body, but not the brain so that the person was born with female neurobiology in a body with male anatomy.
You have to think that Mother Nature must be bending over with laughter.
The full article is at http://ts-si.org/content/view/3026/995/