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

We present an understanding that the spectrum of gender identity is partially biological and variably canalized in cognitive-affective-social signalling moderated by the reproductive axis. This neuro-endocrine system underpins the development of salient male and female reproductive behaviours across mammalian species. We propose that, in humans, neurophysiological differences in the reproductive axis underpins the small number of stable and reliable psychological sex differences observed in behavioural and mental abilities. We further propose that the early core component for our inner sense of gender identity lies within the nexus of these differences in the reproductive axis. This interlinked axis is further influenced throughout life by various socio-environmental factors.