Recent legislative struggles around public restrooms and transgender rights have been so contentious they made Caitlyn Jenner have to pee. She posted a video of herself walking into the women’s restroom at Trump Tower. “Not anymore!” she says as she passes the men’s room.

Needing a bathroom break is basic biology—Caitlyn even posted her video with the hashtag #everyonehastopee. But as the science of transgenderism advances, it seems that being transgender is basic biology, too.

See Also: Transgender 101: 6 Essential Questions and Answers

Usually, when we ask questions along the lines of “why are people the way they are,” the answer falls somewhere between “nature” and “nurture.” Being transgender, however, seems not to be one of these things. While the research is still in its infancy, so far, the answer is overwhelmingly: nature. How do we know? While the biological basis of transgenderism is still being uncovered, there are some leads hotter than Lea T's outfits.

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One recent finding is that anatomical sex, gender identity, and sexual orientation are all determined in the womb. Everything is set before birth and in sequence. Sexual anatomy happens first, in the first six weeks of development. But once anatomy is settled, there’s a big time lag—until about six months gestation—before the brain masculinizes or feminizes. At that point, if exposed to a testosterone surge, a fetal brain’s nerve cells develop in a male direction—a male gender identity. In the absence of such a surge, the brain develops in a female direction—female gender identity. And last but not least, sometime between six months and delivery, sexual orientation is set in the brain through an as-yet unknown combination of genetics, hormones, and the uterine environment.

So why don’t genitals always match the brain? The definitive answer is still beyond the reach of science, but there are three factors that may determine where one falls on the gender identity spectrum:

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