Biology

A synopsis of an old "Law and Order" episode ended up on my YouTube feed which was taken right out of the headlines of the past. Two identical twin boys were born where one of them experienced a botched circumcision resulting in the doctor recommending sex reassignment. 

Of course this was the exact case of David Reimer and the expert in question at the time would have been sexologist John Money who ultimately and unwittingly proved that a gender identity cannot just be the result of socialization. There must be something else involved which likely lies in biological predisposition. 

In the episode, the reassigned twin stops taking hormones and reverts to living as male once they receive the news saying "I knew it, I knew I wasn't a girl". 

How children are able to tell what gender they are is difficult to analyze because the mix of nature and nurture isn't immediately obvious but the cases of David Reimer and Abby Stein both show how you can have circumstances outside of the norm and you come away thinking there is more than exposure to social factors involved. 

One had an identity imposed on them while the other understood something viscerally and instinctively without any exposure to the outside world whatsoever.

Of course one can bend gender norms without any internal identity disconnect, which is a slightly different issue within the larger framework of gender variance.