Why would there be a brain gender binary? I think what she was saying about human brains being unique mosaics of male and female characteristics is very plausible. She also says it makes more sense to talk about high versus low, long versus short (re: dendritic spines or whatever else she is alluding to).
This may be a bad analogy, but about mosaics— sex chromosome karyotypes can be mosaics. For example, people with Klinefelter's syndrome can either have a karyotype of 47XXY or a mosaic of 46XY and 47XXY. The degree of mosaicism influences how much testosterone a male fetus produces, which is what shapes a fetus into a male infant. The less testosterone, the less masculinized ("virilzed") the fetus becomes. There can be even more complex mosaics, like 47XXY/48XXXY/46XY—stuff like this can sometimes only be detected if a sufficient number of cells are tested—blood cells and skin cells. Then there are chimeras (something Jack mentioned in another recent thread):
"Chimeras are not the only people who carry different sets of DNA samples in their bodies. Mosaics also have variation in their DNA from one cell to the next.
A mosaic, unlike a chimera, starts out with the same set of DNA in every single cell. You could look at any cell in the body and the DNA inside of it would be exactly the same as the DNA inside a different cell.
At some point during a mosaic's life, though, his or her DNA changes in some but not all of the cells. Now, the DNA in one cell is slightly different from the DNA in the neighboring cell.
This scenario is actually very common. In fact, we are all actually mosaics.
Our bodies are made up of trillions of cells. Most of these cells contain the exact same copy of our DNA. Over the course of our development and lives, however, the DNA in some of these cells can change."
I'm only mentioning this not because having mosaic genes is the same as having a mosaic brain, but because there are already very well-documented ways in which humans can be mosaics of characteristics.
And then there is the issue of mutations. A human zygote with a 46XY ("male") karyotype can have a mutation on the AR (androgen receptor) gene that affects androgen sensitivity and ultimately the gender it gets assigned at birth:
"Mutations in the AR gene cause androgen insensitivity syndrome. This gene provides instructions for making a protein called an androgen receptor. Androgen receptors allow cells to respond to androgens, which are hormones (such as testosterone) that direct male sexual development. Androgens and androgen receptors also have other important functions in both males and females, such as regulating hair growth and sex drive. Mutations in the AR gene prevent androgen receptors from working properly, which makes cells less responsive to androgens or prevents cells from using these hormones at all. Depending on the level of androgen insensitivity, an affected person's sex characteristics can vary from mostly female to mostly male." If a 46XY baby has complete androgen insensitivity (CAIS), they can be indistinguishable from 46XX girl-babies— sometimes CAIS isn't noticed until a girl reaches puberty and never has a period. Then their doctor discovers that they have no uterus or ovaries. https://en.wikipedia.org/wiki/Complete_androgen_insensitivity_syndrome
I'm going to stop here because I'm not a scientist or doctor, and I'm only recently getting familiar with these interesting facts—I don't want to make statements that are idiotic or false (please let me know if I do!). Still, I think it is fun to ponder how we are all mutants, mosaics and maybe even chimeras in our genetics, our bodies and brains. Maybe people's experienced gender identity is a binary-ish side-effect of all these glorious internal genetic and biological complexities interacting with our environments and experiences before birth and after birth.
Or maybe not. Maybe it's simpler.