Proceedings of the National Academy of Sciences
A study published in the "Proceedings of the National Academy of Sciences" on gender identity reports brains do not really fit into "male" or "female" categories. Tel Aviv University's Daphna Joel's research team, in analyzing the MRI scans of some 1,400 individuals, found only a very small number of the brains studied had features that were entirely male, female, or intermediate between the two. The vast majority had a mosaic.
Sources: Daphna Joel The Proceedings of the National Academy of Sciences The Washington Post