On the contrary, there is very strong evidence of the biological origins of transgender identities.  From  Chung and Auger, European Journal of Physiology, 2013: http://tinyurl.com/hbvmgmm

"Gender-dependent differentiation of the brain has been detected at every level of organization -- morphological, neurochemical, and functional -- and has been shown to be primarily controlled by sex differences in gonadal steroid hormone levels during perinatal development."

Transgender identities appear to be a genuine mismatch between primary sexual characteristic and neurological phenotypes during prenatal development.

Or you could try reading this

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3654067/