[Lotus]

During fetal development a surge of testosterone (or lack there of) determines sexual differentiation (sexuality/gender). When it (sex differences) shows up in life is the question, reports say it starts between 2-4 yrs old, however, I think it shows up at anytime. Additionally, I think the liver and the hypothalamus have a lot to do changing ones sexuality/gender, (further info is posted in the X thread). 


Sexual differentiation of the human brain: relation to gender identity, sexual orientation and neuropsychiatric disorders.

Abstract
During the intrauterine period a testosterone surge masculinizes the fetal brain, whereas the absence of such a surge results in a feminine brain. As sexual differentiation of the brain takes place at a much later stage in development than sexual differentiation of the genitals, these two processes can be influenced independently of each other. Sex differences in cognition, gender identity (an individual's perception of their own sexual identity), sexual orientation (heterosexuality, homosexuality or bisexuality), and the risks of developing neuropsychiatric disorders are programmed into our brain during early development. There is no evidence that one's postnatal social environment plays a crucial role in gender identity or sexual orientation. We discuss the relationships between structural and functional sex differences of various brain areas and the way they change along with any changes in the supply of sex hormones on the one hand and sex differences in behavior in health and disease on the other.
Copyright © 2011 Elsevier Inc. All rights reserved.
PMID 21334362 [PubMed - indexed for MEDLINE]

https://www.ncbi.nlm.nih.gov/m/pubmed/21334362/?i=4&from=/12575757/related