Here are some I think are interesting. http://www.isna.org/node/564 http://en.wikipedia.org/wiki/David_Reimer

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.