What are these studies? The metadata studies I've heard about say otherwise. That is, some studies say one thing, other studies say the opposite, and so it's impossible to really conclude anything by choosing a subset of the literature.
They talk about male babies who were sexually reassigned at birth and given female hormones. However many of them decided to reidentify as males later on life, showing that even with female hormones and a female upbringing, it was not enough to make them adopt a female gender identity.
I wouldn't say this is conclusive evidence. But I think it's enough to show that people should keep an open mind about this subject.
For a recent lay presentation on this topic, see http://freethoughtblogs.com/lousycanuck/2014/02/03/ftbcon2-e... . At the top is a link to a Google document ( https://docs.google.com/document/d/1slJbQpPTlg_m6cKgsarzGLqY... ) with full citations.