Dr. Marc Breedlove

Well, this suggested that gonadal hormones were not in charge of sexual differentiation.

And so what I wanted to do and never figured out quite how to do was to take a male brain and put it on a female body and vice versa.

to ask what sexual differentiation will be like.

Will the whole body listen to the brain and become male?

Or will the gonads?

It was a crazy experiment where if I could dissociate what the sex of the gonad was from what the sex of the brain was and what would happen.

But it turned out, you know, I...

I didn't have the good hands that you would do, for example.

I never learned how to do that experiment.

But one resolution of it is, one of the things we now know is going on in birds, is that in some cases it's the brain itself that's making the hormone and is masculinizing itself.

So in birds, there's pretty good evidence that brain sexual differentiation happens because the genetic sex of the brain

determines how much hormone, including testosterone and estrogen, is getting made locally, and that that then is driving sexual differentiation.

And that's why you can have, you know, one side of the brain is male and the other side is females, and that just doesn't happen in mammals.

That's the mechanism.

Once you start using gonadal hormones to direct sexual differentiation, you can end up with that sort of...

mosaicism, which is maybe too bad.

I mean, there are carnival acts where supposedly one side of the body is male and the other side is female, and those have been around for quite a while, but that's a different thing.

And we know that is true.

Because back in the previous century, Julian Davidson at Stanford, actually, in the physiology department there, was among the first to do these double-blind placebo-controlled studies in men who had lost their testes for one reason or another, accident or cancer, things like that.

And so double-blind means neither the man nor the physician who was interacting with him had any idea whether he was getting the placebo or the testosterone.