Sex vs Gender: A biological basis for gender, not sex.

Definitions of gender most always define it as a person's sense of core self and centered in the brain, but none venture to suggest where in the brain? While bodily sex and sexual orientation are accepted as emanating from genetic and hormonal templates, gender’s existence has so far either emanated from strictly social origins, a nebulous ‘somewhere’ in the brain, some combination of sources, or been rejected outright. We embrace evolutionary principles to examine contemporary research on sex and gender, review differences in neural and endocrine development, differences in the reproductive burden on ciswomen compared to cismen, combined with research into the trans community. We present an understanding based on early stable dimorphic sex differences in cognitive signaling related to the reproductive axis and reproduction. We propose the neural networks associated with these few stable and significant sex-linked behaviors provide a biological template for gender identity rather than sex.