Biologizing Gender: The canalized reproductive axis of gender, not sex.

While sex and sexual orientation are widely accepted as emanating from genetic and hormonal templates, the biological basis of gender identity remains controversial. We present our understanding that the spectrum of gender identity is partially biological and variably canalized in cognitive-affective-social signaling moderated by the reproductive axis, a neuro-endocrine system which underpins the development of salient male and female reproductive behaviors across mammalian species. We propose that, in humans, differences in the canalization of this system underpins the small number of psychological sex differences reliably observed in behavioral, mental-ability and self-report measures. We argue that this nexus constitutes the very early core component for the development of our inner sense of gender identity.