Sexual Hormones and the Brain: An Essential Alliance for Sexual Identity and Sexual Orientation Garcia-Falgueras A, Swaab DF Endocr Dev. 2010;17:22-35
The fetal brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the female direction through the absence of this hormone surge. In this way, our gender identity (the conviction of belonging to the male or female gender) and sexual orientation are programmed or organized into our brain structures when we are still in the womb. However, since sexual differentiation of the genitals takes place in the first two months of pregnancy and sexual differentiation of the brain starts in the second half of pregnancy, these two processes can be influenced independently, which may result in extreme cases in trans-sexuality. This also means that in the event of ambiguous sex at birth, the degree of masculinization of the genitals may not reflect the degree of masculinization of the brain. There is no indication that social environment after birth has an effect on gender identity or sexual orientation.Note though that - Bisexuals may have an apparent change of sexual orientation, and the Bigendered may have an apparent change of gender identity (though I wish they'd call it sex identity). It's neither a strict binary, nor a strict trinary.
Severely Cross-sexed brains compared with appearances will be apparent when young. Milder cases, or cases where the psyche is strong enough to withstand severe societal pressure to conform without cracking under the strain of maintaining the pretence, will manifest later.
But in summary... I could have told them that. I must read it though to see if they have some new data on it. Hopefully so. While it would be nice to do more than just gather yet more confirming evidence, to do something more original, it's still needed at this stage. Yet another meta-study is also good, though not as good as having more data.
What's visible in the free preview is tantalising:
Boys and girls behave in different ways and one of the stereotypical behavioral differences between them, that has often been said to be forced upon them by upbringing and social environment, is their behavior in play. Boys prefer to play with cars and balls, whereas girls prefer dolls. This sex difference in toy preference is present very early in life (3–8 months of age) . The idea that it is not society that forces these choices upon children but a sex difference in the early development of their brains and behavior is also supported by monkey behavioral studies. Alexander and Hines , who offered dolls, toy cars and balls to green Vervet monkeys found the female monkeys consistently chose the dolls and examined these ano-genitally, whereas the male monkeys were more interested in playing with the toy cars and with the ball....What do you call a monkey that plays with a pink Rolls-Royce that shoots missiles, rather than dolls that just sit there, not interacting? Whose reaction to a doll that pees realistically is to try to take it apart to see how that's done? Or would have been, if it didn't look so much like the baby she wanted to have...