Reference: 
Bao A-M, Swaab DF. 2011. Sexual differentiation of the human brain: relation to gender identity, sexual orientation and neuropsychiatric disorders. Frontiers in Neuroendocrinology 32:214–26.

Link to the text of the bill in question:
http://www.ncleg.net/Sessions/2015E2/Bills/House/PDF/H2v4.pdf