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Let’s talk about the distinction between gender and sex.

Virginia (nee Charles) Prince, founder of Transvestia magazine, famously said that “gender is what’s above the neck and sex is what’s below the neck.” Gender is meta-sex—it’s what we make of the difference in our bodies and their reproductive anatomy and capabilities. My female body is made to give birth and to nurture. Your male body is constructed to seed me and to protect our offspring. From an evolutionary perspective, our common goal is to ensure that our children survive until they can reproduce themselves and thus transmit our genes to the next generation. Gender is the cultural tapestry that we weave from those fundamental facts.

But gender differences cannot be rooted in culture alone, because my body (what’s below my neck) and my brain (what’s above my neck) are not divided by some kind of biological Berlin Wall. The body and the brain are an open city, built on the constant exchange of information. Just after my mother’s egg and my father’s sperm united, each contributing an X chromosome to my female genotype, skeins of DNA began to uncoil and replicate. Messages traveled between the rapidly multiplying cells that had not yet differentiated into specific organs and tissues, switching genes on and off under instructions from the master template, guiding my development. In the sixth week of pregnancy, the process of sexual differentiation began. The androgynous embryo, which possesses both mullerian and wolffian ducts and thus has the potential to develop either a male or a female reproductive anatomy, accepted its genetic fate, and an exquisitely choreographed dance began, performed by a company of steroid hormones. Because I am an XX person, midway through the second month of pregnancy, the primordial gonad developed into egg-bearing ovaries. My nascent wolffian ducts began to wither away, and the mullerian ducts differentiated into the gothic architecture of the female reproductive system— fallopian tubes, uterus, cervix, vagina. Within a few weeks, ultrasound images revealed a recognizably female external anatomy. Evidence suggests that my brain was prenatally “sexed” as well, though the mechanism by which this process is carried out is less clearly understood. Animal research has provided ample evidence of the organizing effects of hormones on the sexual differentiation of the brain, but the extent to which the animal data can be extrapolated to human development remains hotly contested. The sexual differentiation of the brain is completed after birth, as I learn what sorts of attitudes, behavior, and role my culture expects of me as a female.

In an XY fetus, a different set of chemical messages begins circulating in the second month of pregnancy, based on instructions encoded in the Y chromosome. “Male!” the Y chromosome shouts, and a gene called SRY directs the primitive gonad to form testicles, rather than ovaries. The testicles soon begin to produce androgens, which will masculinize both genitalia and brain. One of the chemical messengers produced by the testicles, mullerian-inhibiting substance (MIS), begins circulating throughout the rapidly dividing cells, barking out orders to arrest the development of a female reproductive anatomy. Testosterone and MIS ensure that the fissure that would otherwise develop into a vagina fuses together to form a scrotum, and that the primary instrument of sexual pleasure (glans penis) develops outside the fleshy mound of the pubis, rather than hidden within it (glans clitoridis). In males, the hormone-driven sexing of the brain is known to continue into the weeks immediately following birth, when the testicles pump out a flood of testosterone at levels that will not be matched until puberty. By that time, the male child will have learned what behaviors and attitudes his family and culture expect him to display; these are based on the presence of male genitals.

The process of prenatal sexual differentiation is complex and multi-faceted.