I have a hard time with the idea of any level of biological determinism because my "training" as an anthropologist emphasized again and again how wildly flexible culture can be, and how expressions of masculine and feminine rolls can be very different in differing cultures. So I kind of just trust people when they tell me that they always knew they were X or Y or Q or whatever. I think we're still a long way from understanding fully how chromosomes, genetics, culture, and individual experience combine to manifest as gender. Because it does seem to be a mash up of different things, with most people being somewhere on the two humps of the bell curve for man and woman, but then there's all kinds of outliers and unexpected things popping up that don't necessarily fit the model.

Infinite diversity in infinite combinations.

On a related note, I went to college back in '03ish, and I observed at the time that a lot of people were much more flexible in their sexuality and gender presentation than I'd ever seen in highschool. I long held a theory that there were a lot more not-straight people than commonly believed, and it came out in college because people were in a safer place, away from their parents and communities, where they could experiment and express themselve for the first time. And then when the left college and went back in to the real, more socially violent and repressive world, they'd clamp down on those differences and perform the expected cisheteronormative roles. And I think that 21% number supports what I was seeing. I'm so glad younger people feel that they can be open about it and don't have to hide just to survive as much.