Transsexualism a condition that develops in the womb. All these studies prove that transsexuality is not a schizotypal disorder or delusional disorder and gender dysphoria is real. Trans women have feminized brains, trans men have masculinized brains.
But even these facts do not necessarily cause gender dysphoria (steriotypical passive gays experience feminization, but without gender dysphoria). The mechanism of gender identity is currently unknown.

Androgen Receptor Repeat Length Polymorphism Associated with Male-to-Female Transsexualism
https://www.biologicalpsychiatryjournal.com/article/S0006-3223(08)01087-1/abstract
https://www.abc.net.au/science/articles/2008/10/27/2401941.htm
A polymorphism of the CYP17 gene related to sex steroid metabolism is associated with female-to-male but not male-to-female transsexualism
https://www.fertstert.org/article/S0015-0282(07)01228-9/fulltext
There is a high chance that a transgender person's identical twin will also be trans.
https://www.tandfonline.com/doi/abs/10.1080/15532739.2013.750222
FIRST study about transsexuals (1995). A sex difference in the human brain and its relation to transsexuality
https://www.researchgate.net/profile/Jiang-Ning-Zhou/publication/15722390_A_sex_difference_in_the_human_brain_and_its_relation_to_transsexuality/links/02bfe5122cb5acb524000000/A-sex-difference-in-the-human-brain-and-its-relation-to-transsexuality.pdf?origin=publication_detail&_tp=eyJjb250ZXh0Ijp7ImZpcnN0UGFnZSI6InB1YmxpY2F0aW9uIiwicGFnZSI6InB1YmxpY2F0aW9uRG93bmxvYWQiLCJwcmV2aW91c1BhZ2UiOiJwdWJsaWNhdGlvbiJ9fQ
Male-to-Female Transsexuals Have Female Neuron Numbers in a Limbic Nucleus
https://academic.oup.com/jcem/article-abstract/85/5/2034/2660626?redirectedFrom=fulltext&login=false
The biology of human psychosexual differentiation
https://www.sciencedirect.com/science/article/abs/pii/S0018506X06001462
Sexual Differentiation of the Bed Nucleus of the Stria Terminalis in Humans May Extend into Adulthood