gender is psychological, not anatomical, and it's too intrinsic to each individual person to identify a source of. simply put, we just know. it's in the wiring of the human brain. your brain matches your body so you don't ever have to give conscious thought to your gender identity. ours don't, so we get stuck with a mismatch that we then have to deal with in whichever way we choose to.

science doesn't yet have a definitive answer as to why that mismatch exists for some people, but it's theorized that since the brain develops before the genitals, the hormone levels that a fetus is exposed to can effect how the brain is structured. what's most important, though, is that we definitively exist. we are real and valid and we're not going anywhere.